Improve file check speed, apparently Files.exists is slow

.gitignore

@@ -1,74 +1,87 @@
-# Created by https://www.gitignore.io/api/java,gradle,eclipse
-# Edit at https://www.gitignore.io/?templates=java,gradle,eclipse
 
-### Eclipse ###
-.metadata
-bin/
-tmp/
-*.tmp
-*.bak
-*.swp
-*~.nib
-local.properties
-.settings/
-.loadpath
-.recommenders
+# Created by https://www.gitignore.io/api/java,gradle,intellij
+# Edit at https://www.gitignore.io/?templates=java,gradle,intellij
 
-# External tool builders
-.externalToolBuilders/
+### Intellij ###
+# Covers JetBrains IDEs: IntelliJ, RubyMine, PhpStorm, AppCode, PyCharm, CLion, Android Studio and WebStorm
+# Reference: https://intellij-support.jetbrains.com/hc/en-us/articles/206544839
 
-# Locally stored "Eclipse launch configurations"
-*.launch
+# User-specific stuff
+.idea/**/workspace.xml
+.idea/**/tasks.xml
+.idea/**/usage.statistics.xml
+.idea/**/dictionaries
+.idea/**/shelf
 
-# PyDev specific (Python IDE for Eclipse)
-*.pydevproject
+# Generated files
+.idea/**/contentModel.xml
 
-# CDT-specific (C/C++ Development Tooling)
-.cproject
+# Sensitive or high-churn files
+.idea/**/dataSources/
+.idea/**/dataSources.ids
+.idea/**/dataSources.local.xml
+.idea/**/sqlDataSources.xml
+.idea/**/dynamic.xml
+.idea/**/uiDesigner.xml
+.idea/**/dbnavigator.xml
 
-# CDT- autotools
-.autotools
+# Gradle
+.idea/**/gradle.xml
+.idea/**/libraries
 
-# Java annotation processor (APT)
-.factorypath
+# Gradle and Maven with auto-import
+# When using Gradle or Maven with auto-import, you should exclude module files,
+# since they will be recreated, and may cause churn.  Uncomment if using
+# auto-import.
+# .idea/modules.xml
+# .idea/*.iml
+# .idea/modules
+# *.iml
+# *.ipr
 
-# PDT-specific (PHP Development Tools)
-.buildpath
+# CMake
+cmake-build-*/
 
-# sbteclipse plugin
-.target
+# Mongo Explorer plugin
+.idea/**/mongoSettings.xml
 
-# Tern plugin
-.tern-project
+# File-based project format
+*.iws
 
-# TeXlipse plugin
-.texlipse
+# IntelliJ
+out/
 
-# STS (Spring Tool Suite)
-.springBeans
+# mpeltonen/sbt-idea plugin
+.idea_modules/
 
-# Code Recommenders
-.recommenders/
+# JIRA plugin
+atlassian-ide-plugin.xml
 
-# Annotation Processing
-.apt_generated/
+# Cursive Clojure plugin
+.idea/replstate.xml
 
-# Scala IDE specific (Scala & Java development for Eclipse)
-.cache-main
-.scala_dependencies
-.worksheet
+# Crashlytics plugin (for Android Studio and IntelliJ)
+com_crashlytics_export_strings.xml
+crashlytics.properties
+crashlytics-build.properties
+fabric.properties
 
-### Eclipse Patch ###
-# Eclipse Core
-.project
+# Editor-based Rest Client
+.idea/httpRequests
 
-# JDT-specific (Eclipse Java Development Tools)
-.classpath
+# Android studio 3.1+ serialized cache file
+.idea/caches/build_file_checksums.ser
 
-# Annotation Processing
-.apt_generated
+### Intellij Patch ###
+# Comment Reason: https://github.com/joeblau/gitignore.io/issues/186#issuecomment-215987721
 
-.sts4-cache/
+# *.iml
+# modules.xml
+# .idea/misc.xml
+# *.ipr
+
+# Sonarlint plugin
+.idea/sonarlint
 
 ### Java ###
 # Compiled class file
@@ -114,4 +127,4 @@ gradle-app.setting
 ### Gradle Patch ###
 **/build/
 
-# End of https://www.gitignore.io/api/java,gradle,eclipse
+# End of https://www.gitignore.io/api/java,gradle,intellij

.idea/inspectionProfiles/Project_Default.xml

@@ -0,0 +1,36 @@
+<component name="InspectionProjectProfileManager">
+  <profile version="1.0">
+    <option name="myName" value="Project Default" />
+    <inspection_tool class="JavaDoc" enabled="true" level="WARNING" enabled_by_default="true">
+      <option name="TOP_LEVEL_CLASS_OPTIONS">
+        <value>
+          <option name="ACCESS_JAVADOC_REQUIRED_FOR" value="none" />
+          <option name="REQUIRED_TAGS" value="" />
+        </value>
+      </option>
+      <option name="INNER_CLASS_OPTIONS">
+        <value>
+          <option name="ACCESS_JAVADOC_REQUIRED_FOR" value="none" />
+          <option name="REQUIRED_TAGS" value="" />
+        </value>
+      </option>
+      <option name="METHOD_OPTIONS">
+        <value>
+          <option name="ACCESS_JAVADOC_REQUIRED_FOR" value="none" />
+          <option name="REQUIRED_TAGS" value="@return@param@throws or @exception" />
+        </value>
+      </option>
+      <option name="FIELD_OPTIONS">
+        <value>
+          <option name="ACCESS_JAVADOC_REQUIRED_FOR" value="none" />
+          <option name="REQUIRED_TAGS" value="" />
+        </value>
+      </option>
+      <option name="IGNORE_DEPRECATED" value="false" />
+      <option name="IGNORE_JAVADOC_PERIOD" value="true" />
+      <option name="IGNORE_DUPLICATED_THROWS" value="false" />
+      <option name="IGNORE_POINT_TO_ITSELF" value="false" />
+      <option name="myAdditionalJavadocTags" value="wbp.parser.entryPoint" />
+    </inspection_tool>
+  </profile>
+</component>

.idea/misc.xml

@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="EntryPointsManager">
+    <list size="1">
+      <item index="0" class="java.lang.String" itemvalue="com.google.gson.annotations.SerializedName" />
+    </list>
+  </component>
+  <component name="ExternalStorageConfigurationManager" enabled="true" />
+  <component name="ProjectRootManager" version="2" languageLevel="JDK_1_8" default="false" project-jdk-name="11" project-jdk-type="JavaSDK" />
+</project>

.idea/vcs.xml

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="VcsDirectoryMappings">
+    <mapping directory="" vcs="Git" />
+  </component>
+</project>

.vscode/settings.json

@@ -1,9 +0,0 @@
-{
-	"files.exclude": {
-		"**/.classpath": true,
-		"**/.project": true,
-		"**/.settings": true,
-		"**/.factorypath": true
-	},
-	"java.configuration.updateBuildConfiguration": "interactive"
-}

build.gradle

@@ -3,8 +3,11 @@ plugins {
     id 'application'
     id 'com.github.johnrengelman.shadow' version '5.0.0'
     id 'com.palantir.git-version' version '0.11.0'
+    id 'com.github.breadmoirai.github-release' version '2.2.9'
 }
 
+sourceCompatibility = 1.8
+
 dependencies {
 	implementation 'commons-cli:commons-cli:1.4'
 	implementation 'com.moandjiezana.toml:toml4j:0.7.2'
@@ -45,4 +48,25 @@ task copyJar(type: Copy) {
     into "build/libs/"
 }
 
-build.dependsOn copyJar
+build.dependsOn copyJar
+
+if (project.hasProperty("github.token")) {
+    githubRelease {
+        // IntelliJ u ok?
+        //noinspection GroovyAssignabilityCheck
+        owner "comp500"
+        //noinspection GroovyAssignabilityCheck
+        repo "packwiz-installer"
+        //noinspection GroovyAssignabilityCheck
+        tagName "${project.version}"
+        //noinspection GroovyAssignabilityCheck
+        releaseName "Release ${project.version}"
+        //noinspection GroovyAssignabilityCheck
+        draft true
+        //noinspection GroovyAssignabilityCheck
+        token findProperty("github.token") ?: ""
+        releaseAssets = [jar.destinationDirectory.file("packwiz-installer.jar").get()]
+    }
+
+    tasks.githubRelease.dependsOn(build)
+}

src/main/java/link/infra/packwiz/installer/DownloadTask.java

@@ -0,0 +1,245 @@
+package link.infra.packwiz.installer;
+
+import link.infra.packwiz.installer.metadata.IndexFile;
+import link.infra.packwiz.installer.metadata.ManifestFile;
+import link.infra.packwiz.installer.metadata.SpaceSafeURI;
+import link.infra.packwiz.installer.metadata.hash.GeneralHashingSource;
+import link.infra.packwiz.installer.metadata.hash.Hash;
+import link.infra.packwiz.installer.metadata.hash.HashUtils;
+import link.infra.packwiz.installer.ui.IExceptionDetails;
+import link.infra.packwiz.installer.ui.IOptionDetails;
+import okio.Buffer;
+import okio.Okio;
+import okio.Source;
+
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.Paths;
+import java.nio.file.StandardCopyOption;
+import java.util.ArrayList;
+import java.util.List;
+
+class DownloadTask implements IOptionDetails, IExceptionDetails {
+	final IndexFile.File metadata;
+	ManifestFile.File cachedFile = null;
+	private Exception failure = null;
+	private boolean alreadyUpToDate = false;
+	private boolean metadataRequired = true;
+	private boolean invalidated = false;
+	// If file is new or isOptional changed to true, the option needs to be presented again
+	private boolean newOptional = true;
+	private final UpdateManager.Options.Side downloadSide;
+
+	private DownloadTask(IndexFile.File metadata, String defaultFormat, UpdateManager.Options.Side downloadSide) {
+		this.metadata = metadata;
+		if (metadata.hashFormat == null || metadata.hashFormat.length() == 0) {
+			metadata.hashFormat = defaultFormat;
+		}
+		this.downloadSide = downloadSide;
+	}
+
+	void invalidate() {
+		invalidated = true;
+		alreadyUpToDate = false;
+	}
+
+	void updateFromCache(ManifestFile.File cachedFile) {
+		if (failure != null) return;
+		if (cachedFile == null) {
+			this.cachedFile = new ManifestFile.File();
+			return;
+		}
+
+		this.cachedFile = cachedFile;
+
+		if (!invalidated) {
+			Hash currHash;
+			try {
+				currHash = HashUtils.getHash(metadata.hashFormat, metadata.hash);
+			} catch (Exception e) {
+				failure = e;
+				return;
+			}
+			if (currHash != null && currHash.equals(cachedFile.hash)) {
+				// Already up to date
+				alreadyUpToDate = true;
+				metadataRequired = false;
+			}
+		}
+		if (cachedFile.isOptional) {
+			// Because option selection dialog might set this task to true/false, metadata is always needed to download
+			// the file, and to show the description and name
+			metadataRequired = true;
+		}
+	}
+
+	void downloadMetadata(IndexFile parentIndexFile, SpaceSafeURI indexUri) {
+		if (failure != null) return;
+		if (metadataRequired) {
+			try {
+				metadata.downloadMeta(parentIndexFile, indexUri);
+			} catch (Exception e) {
+				failure = e;
+				return;
+			}
+			if (metadata.linkedFile != null) {
+				if (metadata.linkedFile.option != null) {
+					if (metadata.linkedFile.option.optional) {
+						if (cachedFile.isOptional) {
+							// isOptional didn't change
+							newOptional = false;
+						} else {
+							// isOptional false -> true, set option to it's default value
+							// TODO: preserve previous option value, somehow??
+							cachedFile.optionValue = this.metadata.linkedFile.option.defaultValue;
+						}
+					}
+				}
+				cachedFile.isOptional = isOptional();
+				cachedFile.onlyOtherSide = !correctSide();
+			}
+		}
+	}
+
+	void download(String packFolder, SpaceSafeURI indexUri) {
+		if (failure != null) return;
+
+		// Ensure it is removed
+		if (!cachedFile.optionValue || !correctSide()) {
+			if (cachedFile.cachedLocation == null) return;
+			try {
+				Files.deleteIfExists(Paths.get(packFolder, cachedFile.cachedLocation));
+			} catch (IOException e) {
+				// TODO: how much of a problem is this? use log4j/other log library to show warning?
+				e.printStackTrace();
+			}
+			cachedFile.cachedLocation = null;
+			return;
+		}
+
+		if (alreadyUpToDate) return;
+
+		Path destPath = Paths.get(packFolder, metadata.getDestURI().toString());
+
+		// Don't update files marked with preserve if they already exist on disk
+		if (metadata.preserve) {
+			if (destPath.toFile().exists()) {
+				return;
+			}
+		}
+
+		try {
+			Hash hash;
+			String fileHashFormat;
+			if (metadata.linkedFile != null) {
+				hash = metadata.linkedFile.getHash();
+				fileHashFormat = metadata.linkedFile.download.hashFormat;
+			} else {
+				hash = metadata.getHash();
+				fileHashFormat = metadata.hashFormat;
+			}
+
+			Source src = metadata.getSource(indexUri);
+			GeneralHashingSource fileSource = HashUtils.getHasher(fileHashFormat).getHashingSource(src);
+			Buffer data = new Buffer();
+			Okio.buffer(fileSource).readAll(data);
+
+			if (fileSource.hashIsEqual(hash)) {
+				Files.createDirectories(destPath.getParent());
+				Files.copy(data.inputStream(), destPath, StandardCopyOption.REPLACE_EXISTING);
+			} else {
+				// TODO: no more SYSOUT!!!!!!!!!
+				System.out.println("Invalid hash for " + metadata.getDestURI().toString());
+				System.out.println("Calculated: " + fileSource.getHash());
+				System.out.println("Expected:   " + hash);
+				failure = new Exception("Hash invalid!");
+			}
+
+			if (cachedFile.cachedLocation != null && !destPath.equals(Paths.get(packFolder, cachedFile.cachedLocation))) {
+				// Delete old file if location changes
+				Files.delete(Paths.get(packFolder, cachedFile.cachedLocation));
+			}
+		} catch (Exception e) {
+			failure = e;
+		}
+		if (failure == null) {
+			if (cachedFile == null) {
+				cachedFile = new ManifestFile.File();
+			}
+			// Update the manifest file
+			try {
+				cachedFile.hash = metadata.getHash();
+			} catch (Exception e) {
+				failure = e;
+				return;
+			}
+			cachedFile.isOptional = isOptional();
+			cachedFile.cachedLocation = metadata.getDestURI().toString();
+			if (metadata.linkedFile != null) {
+				try {
+					cachedFile.linkedFileHash = metadata.linkedFile.getHash();
+				} catch (Exception e) {
+					failure = e;
+				}
+			}
+		}
+	}
+
+	public Exception getException() {
+		return failure;
+	}
+
+	boolean isOptional() {
+		if (metadata.linkedFile != null) {
+			return metadata.linkedFile.isOptional();
+		}
+		return false;
+	}
+
+	boolean isNewOptional() {
+		return isOptional() && this.newOptional;
+	}
+
+	boolean correctSide() {
+		if (metadata.linkedFile != null) {
+			return metadata.linkedFile.side.hasSide(downloadSide);
+		}
+		return true;
+	}
+
+	public String getName() {
+		return metadata.getName();
+	}
+
+	@Override
+	public boolean getOptionValue() {
+		return cachedFile.optionValue;
+	}
+
+	@Override
+	public String getOptionDescription() {
+		if (metadata.linkedFile != null) {
+			return metadata.linkedFile.option.description;
+		}
+		return null;
+	}
+
+	public void setOptionValue(boolean value) {
+		if (value && !cachedFile.optionValue) {
+			// Ensure that an update is done if it changes from false to true, or from true to false
+			alreadyUpToDate = false;
+		}
+		cachedFile.optionValue = value;
+	}
+
+	static List<DownloadTask> createTasksFromIndex(IndexFile index, String defaultFormat, UpdateManager.Options.Side downloadSide) {
+		ArrayList<DownloadTask> tasks = new ArrayList<>();
+		for (IndexFile.File file : index.files) {
+			tasks.add(new DownloadTask(file, defaultFormat, downloadSide));
+		}
+		return tasks;
+	}
+
+
+}

src/main/java/link/infra/packwiz/installer/Main.java

@@ -1,40 +1,32 @@
 package link.infra.packwiz.installer;
 
-import java.awt.EventQueue;
-import java.awt.GraphicsEnvironment;
-import java.net.URI;
-import java.net.URISyntaxException;
-
-import javax.swing.JOptionPane;
-import javax.swing.UIManager;
-
-import org.apache.commons.cli.CommandLine;
-import org.apache.commons.cli.CommandLineParser;
-import org.apache.commons.cli.DefaultParser;
-import org.apache.commons.cli.Options;
-import org.apache.commons.cli.ParseException;
-
+import link.infra.packwiz.installer.metadata.SpaceSafeURI;
 import link.infra.packwiz.installer.ui.CLIHandler;
 import link.infra.packwiz.installer.ui.IUserInterface;
+import link.infra.packwiz.installer.ui.InputStateHandler;
 import link.infra.packwiz.installer.ui.InstallWindow;
+import org.apache.commons.cli.*;
 
+import javax.swing.*;
+import java.awt.*;
+import java.net.URISyntaxException;
+
+@SuppressWarnings("unused")
 public class Main {
 
 	// Actual main() is in RequiresBootstrap!
-
+	@SuppressWarnings("unused")
 	public Main(String[] args) {
 		// Big overarching try/catch just in case everything breaks
 		try {
 			this.startup(args);
 		} catch (Exception e) {
 			e.printStackTrace();
-			EventQueue.invokeLater(new Runnable() {
-				public void run() {
-					JOptionPane.showMessageDialog(null,
-							"A fatal error occurred: \n" + e.getClass().getCanonicalName() + ": " + e.getMessage(),
-							"packwiz-installer", JOptionPane.ERROR_MESSAGE);
-					System.exit(1);
-				}
+			EventQueue.invokeLater(() -> {
+				JOptionPane.showMessageDialog(null,
+						"A fatal error occurred: \n" + e.getClass().getCanonicalName() + ": " + e.getMessage(),
+						"packwiz-installer", JOptionPane.ERROR_MESSAGE);
+				System.exit(1);
 			});
 			// In case the eventqueue is broken, exit after 1 minute
 			try {
@@ -47,7 +39,7 @@ public class Main {
 		}
 	}
 
-	protected void startup(String[] args) {
+	private void startup(String[] args) {
 		Options options = new Options();
 		addNonBootstrapOptions(options);
 		addBootstrapOptions(options);
@@ -89,7 +81,8 @@ public class Main {
 			ui.setTitle(title);
 		}
 
-		ui.show();
+		InputStateHandler inputStateHandler = new InputStateHandler();
+		ui.show(inputStateHandler);
 
 		UpdateManager.Options uOptions = new UpdateManager.Options();
 
@@ -109,7 +102,7 @@ public class Main {
 		}
 
 		try {
-			uOptions.downloadURI = new URI(unparsedArgs[0]);
+			uOptions.downloadURI = new SpaceSafeURI(unparsedArgs[0]);
 		} catch (URISyntaxException e) {
 			// TODO: better error message?
 			ui.handleExceptionAndExit(e);
@@ -119,26 +112,22 @@ public class Main {
 		// Start update process!
 		// TODO: start in SwingWorker?
 		try {
-			ui.executeManager(new Runnable(){
-				@Override
-				public void run() {
-					try {
-						new UpdateManager(uOptions, ui);
-					} catch (Exception e) {
-						// TODO: better error message?
-						ui.handleExceptionAndExit(e);
-						return;
-					}
+			ui.executeManager(() -> {
+				try {
+					new UpdateManager(uOptions, ui, inputStateHandler);
+				} catch (Exception e) {
+					// TODO: better error message?
+					ui.handleExceptionAndExit(e);
 				}
 			});
 		} catch (Exception e) {
 			// TODO: better error message?
 			ui.handleExceptionAndExit(e);
-			return;
 		}
 	}
 
 	// Called by packwiz-installer-bootstrap to set up the help command
+	@SuppressWarnings("WeakerAccess")
 	public static void addNonBootstrapOptions(Options options) {
 		options.addOption("s", "side", true, "Side to install mods from (client/server, defaults to client)");
 		options.addOption(null, "title", true, "Title of the installer window");

src/main/java/link/infra/packwiz/installer/RequiresBootstrap.java

@@ -1,10 +1,8 @@
 package link.infra.packwiz.installer;
 
+import javax.swing.*;
 import java.util.Arrays;
 
-import javax.swing.JOptionPane;
-import javax.swing.UIManager;
-
 public class RequiresBootstrap {
 
 	public static void main(String[] args) {
@@ -15,10 +13,10 @@ public class RequiresBootstrap {
 		if (Arrays.stream(args).map(str -> {
 			if (str == null) return "";
 			if (str.startsWith("--")) {
-				return str.substring(2, str.length());
+				return str.substring(2);
 			}
 			if (str.startsWith("-")) {
-				return str.substring(1, str.length());
+				return str.substring(1);
 			}
 			return "";
 		}).anyMatch(str -> str.equals("g") || str.equals("no-gui"))) {

src/main/java/link/infra/packwiz/installer/UpdateManager.java

@@ -1,60 +1,54 @@
 package link.infra.packwiz.installer;
 
-import java.io.FileNotFoundException;
-import java.io.FileReader;
-import java.io.FileWriter;
-import java.io.IOException;
-import java.io.Writer;
-import java.net.URI;
-import java.nio.file.Files;
-import java.nio.file.Paths;
-import java.nio.file.StandardCopyOption;
-import java.util.HashMap;
-import java.util.List;
-import java.util.concurrent.Callable;
-import java.util.concurrent.CompletionService;
-import java.util.concurrent.ConcurrentLinkedQueue;
-import java.util.concurrent.ExecutionException;
-import java.util.concurrent.ExecutorCompletionService;
-import java.util.concurrent.ExecutorService;
-import java.util.concurrent.Executors;
-import java.util.stream.Collectors;
-
 import com.google.gson.Gson;
 import com.google.gson.GsonBuilder;
 import com.google.gson.JsonIOException;
 import com.google.gson.JsonSyntaxException;
 import com.google.gson.annotations.SerializedName;
 import com.moandjiezana.toml.Toml;
-
 import link.infra.packwiz.installer.metadata.IndexFile;
 import link.infra.packwiz.installer.metadata.ManifestFile;
 import link.infra.packwiz.installer.metadata.PackFile;
+import link.infra.packwiz.installer.metadata.SpaceSafeURI;
 import link.infra.packwiz.installer.metadata.hash.GeneralHashingSource;
 import link.infra.packwiz.installer.metadata.hash.Hash;
 import link.infra.packwiz.installer.metadata.hash.HashUtils;
 import link.infra.packwiz.installer.request.HandlerManager;
+import link.infra.packwiz.installer.ui.IExceptionDetails;
 import link.infra.packwiz.installer.ui.IUserInterface;
+import link.infra.packwiz.installer.ui.InputStateHandler;
 import link.infra.packwiz.installer.ui.InstallProgress;
-import okio.Buffer;
 import okio.Okio;
 import okio.Source;
 
+import java.io.*;
+import java.nio.file.Files;
+import java.nio.file.Paths;
+import java.util.*;
+import java.util.concurrent.*;
+import java.util.stream.Collectors;
+
 public class UpdateManager {
 
-	public final Options opts;
+	private final Options opts;
 	public final IUserInterface ui;
+	private boolean cancelled;
+	private boolean cancelledStartGame = false;
+	private InputStateHandler stateHandler;
+	private boolean errorsOccurred = false;
 
 	public static class Options {
-		public URI downloadURI = null;
-		public String manifestFile = "packwiz.json"; // TODO: make configurable
-		public String packFolder = ".";
-		public Side side = Side.CLIENT;
+		SpaceSafeURI downloadURI = null;
+		String manifestFile = "packwiz.json"; // TODO: make configurable
+		String packFolder = ".";
+		Side side = Side.CLIENT;
 
-		public static enum Side {
+		public enum Side {
 			@SerializedName("client")
-			CLIENT("client"), @SerializedName("server")
-			SERVER("server"), @SerializedName("both")
+			CLIENT("client"),
+			@SerializedName("server")
+			SERVER("server"),
+			@SerializedName("both")
 			BOTH("both", new Side[] { CLIENT, SERVER });
 
 			private final String sideName;
@@ -80,8 +74,8 @@ public class UpdateManager {
 					return true;
 				}
 				if (this.depSides != null) {
-					for (int i = 0; i < this.depSides.length; i++) {
-						if (this.depSides[i].equals(tSide)) {
+					for (Side depSide : this.depSides) {
+						if (depSide.equals(tSide)) {
 							return true;
 						}
 					}
@@ -92,7 +86,7 @@ public class UpdateManager {
 			public static Side from(String name) {
 				String lowerName = name.toLowerCase();
 				for (Side side : Side.values()) {
-					if (side.sideName == lowerName) {
+					if (side.sideName.equals(lowerName)) {
 						return side;
 					}
 				}
@@ -101,13 +95,14 @@ public class UpdateManager {
 		}
 	}
 
-	public UpdateManager(Options opts, IUserInterface ui) {
+	UpdateManager(Options opts, IUserInterface ui, InputStateHandler inputStateHandler) {
 		this.opts = opts;
 		this.ui = ui;
+		this.stateHandler = inputStateHandler;
 		this.start();
 	}
 
-	protected void start() {
+	private void start() {
 		this.checkOptions();
 
 		ui.submitProgress(new InstallProgress("Loading manifest file..."));
@@ -123,6 +118,11 @@ public class UpdateManager {
 			return;
 		}
 
+		if (stateHandler.getCancelButton()) {
+			showCancellationDialog();
+			handleCancellation();
+		}
+
 		ui.submitProgress(new InstallProgress("Loading pack file..."));
 		GeneralHashingSource packFileSource;
 		try {
@@ -142,24 +142,77 @@ public class UpdateManager {
 			return;
 		}
 
-		if (manifest.packFileHash != null && packFileSource.hashIsEqual(manifest.packFileHash)) {
+		if (stateHandler.getCancelButton()) {
+			showCancellationDialog();
+			handleCancellation();
+		}
+
+		ui.submitProgress(new InstallProgress("Checking local files..."));
+
+		// Invalidation checking must be done here, as it must happen before pack/index hashes are checked
+		List<SpaceSafeURI> invalidatedUris = new ArrayList<>();
+		if (manifest.cachedFiles != null) {
+			for (Map.Entry<SpaceSafeURI, ManifestFile.File> entry : manifest.cachedFiles.entrySet()) {
+				// ignore onlyOtherSide files
+				if (entry.getValue().onlyOtherSide) {
+					continue;
+				}
+				boolean invalid = false;
+				// if isn't optional, or is optional but optionValue == true
+				if (!entry.getValue().isOptional || entry.getValue().optionValue) {
+					if (entry.getValue().cachedLocation != null) {
+						if (!Paths.get(opts.packFolder, entry.getValue().cachedLocation).toFile().exists()) {
+							invalid = true;
+						}
+					} else {
+						// if cachedLocation == null, should probably be installed!!
+						invalid = true;
+					}
+				}
+				if (invalid) {
+					SpaceSafeURI fileUri = entry.getKey();
+					System.out.println("File " + fileUri.toString() + " invalidated, marked for redownloading");
+					invalidatedUris.add(fileUri);
+				}
+			}
+		}
+
+		if (manifest.packFileHash != null && packFileSource.hashIsEqual(manifest.packFileHash) && invalidatedUris.isEmpty()) {
 			System.out.println("Modpack is already up to date!");
 			// todo: --force?
-			return;
+			if (!stateHandler.getOptionsButton()) {
+				return;
+			}
 		}
 
 		System.out.println("Modpack name: " + pf.name);
 
+		if (stateHandler.getCancelButton()) {
+			showCancellationDialog();
+			handleCancellation();
+		}
+
 		try {
+			// This is badly written, I'll probably heavily refactor it at some point
 			processIndex(HandlerManager.getNewLoc(opts.downloadURI, pf.index.file),
-					HashUtils.getHash(pf.index.hashFormat, pf.index.hash), pf.index.hashFormat, manifest);
+					HashUtils.getHash(pf.index.hashFormat, pf.index.hash), pf.index.hashFormat, manifest, invalidatedUris);
 		} catch (Exception e1) {
 			ui.handleExceptionAndExit(e1);
 		}
 
+		handleCancellation();
+
 		// TODO: update MMC params, java args etc
 
-		manifest.packFileHash = packFileSource.getHash();
+		// If there were errors, don't write the manifest/index hashes, to ensure they are rechecked later
+		if (errorsOccurred) {
+			manifest.indexFileHash = null;
+			manifest.packFileHash = null;
+		} else {
+			manifest.packFileHash = packFileSource.getHash();
+		}
+
+		manifest.cachedSide = opts.side;
 		try (Writer writer = new FileWriter(Paths.get(opts.packFolder, opts.manifestFile).toString())) {
 			gson.toJson(manifest, writer);
 		} catch (IOException e) {
@@ -169,14 +222,16 @@ public class UpdateManager {
 
 	}
 
-	protected void checkOptions() {
+	private void checkOptions() {
 		// TODO: implement
 	}
 
-	protected void processIndex(URI indexUri, Hash indexHash, String hashFormat, ManifestFile manifest) {
-		if (manifest.indexFileHash != null && manifest.indexFileHash.equals(indexHash)) {
+	private void processIndex(SpaceSafeURI indexUri, Hash indexHash, String hashFormat, ManifestFile manifest, List<SpaceSafeURI> invalidatedUris) {
+		if (manifest.indexFileHash != null && manifest.indexFileHash.equals(indexHash) && invalidatedUris.isEmpty()) {
 			System.out.println("Modpack files are already up to date!");
-			return;
+			if (!stateHandler.getOptionsButton()) {
+				return;
+			}
 		}
 		manifest.indexFileHash = indexHash;
 
@@ -200,177 +255,246 @@ public class UpdateManager {
 
 		if (!indexFileSource.hashIsEqual(indexHash)) {
 			// TODO: throw exception
+			System.out.println("I was meant to put an error message here but I'll do that later");
+			return;
+		}
+
+		if (stateHandler.getCancelButton()) {
+			showCancellationDialog();
 			return;
 		}
 
 		if (manifest.cachedFiles == null) {
-			manifest.cachedFiles = new HashMap<URI, ManifestFile.File>();
+			manifest.cachedFiles = new HashMap<>();
 		}
 
-		// TODO: progress bar
-		ConcurrentLinkedQueue<Exception> exceptionQueue = new ConcurrentLinkedQueue<Exception>();
-		List<IndexFile.File> newFiles = indexFile.files.stream().map(f -> {
-			if (f.hashFormat == null || f.hashFormat.length() == 0) {
-				f.hashFormat = indexFile.hashFormat;
+		ui.submitProgress(new InstallProgress("Checking local files..."));
+		Iterator<Map.Entry<SpaceSafeURI, ManifestFile.File>> it = manifest.cachedFiles.entrySet().iterator();
+		while (it.hasNext()) {
+			Map.Entry<SpaceSafeURI, ManifestFile.File> entry = it.next();
+			if (entry.getValue().cachedLocation != null) {
+				boolean alreadyDeleted = false;
+				// Delete if option value has been set to false
+				if (entry.getValue().isOptional && !entry.getValue().optionValue) {
+					try {
+						Files.deleteIfExists(Paths.get(opts.packFolder, entry.getValue().cachedLocation));
+					} catch (IOException e) {
+						// TODO: should this be shown to the user in some way?
+						e.printStackTrace();
+					}
+					// Set to null, as it doesn't exist anymore
+					entry.getValue().cachedLocation = null;
+					alreadyDeleted = true;
+				}
+				if (indexFile.files.stream().noneMatch(f -> f.file.equals(entry.getKey()))) {
+					// File has been removed from the index
+					if (!alreadyDeleted) {
+						try {
+							Files.deleteIfExists(Paths.get(opts.packFolder, entry.getValue().cachedLocation));
+						} catch (IOException e) {
+							// TODO: should this be shown to the user in some way?
+							e.printStackTrace();
+						}
+					}
+					it.remove();
+				}
 			}
-			return f;
-		}).filter(f -> {
-			ManifestFile.File cachedFile = manifest.cachedFiles.get(f.file);
-			Hash newHash;
-			try {
-				newHash = HashUtils.getHash(f.hashFormat, f.hash);
-			} catch (Exception e) {
-				exceptionQueue.add(e);
-				return false;
-			}
-			return cachedFile == null || !newHash.equals(cachedFile.hash);
-		}).parallel().map(f -> {
-			try {
-				f.downloadMeta(indexFile, indexUri);
-			} catch (Exception e) {
-				exceptionQueue.add(e);
-			}
-			return f;
-		}).collect(Collectors.toList());
-
-		for (Exception e : exceptionQueue) {
-			// TODO: collect all exceptions, present in one dialog
-			ui.handleException(e);
 		}
 
-		// TODO: present options
-		// TODO: all options should be presented, not just new files!!!!!!!
-		// and options should be readded to newFiles after option -> true
-		newFiles.stream().filter(f -> f.linkedFile != null).filter(f -> f.linkedFile.option != null).map(f -> {
-			return "option: " + (f.linkedFile.option.description == null ? "null" : f.linkedFile.option.description);
-		}).forEachOrdered(desc -> {
-			System.out.println(desc);
+		if (stateHandler.getCancelButton()) {
+			showCancellationDialog();
+			return;
+		}
+		ui.submitProgress(new InstallProgress("Comparing new files..."));
+
+		// TODO: progress bar?
+		if (indexFile.files == null || indexFile.files.size() == 0) {
+			System.out.println("Warning: Index is empty!");
+			indexFile.files = new ArrayList<>();
+		}
+		List<DownloadTask> tasks = DownloadTask.createTasksFromIndex(indexFile, indexFile.hashFormat, opts.side);
+		// If the side changes, invalidate EVERYTHING just in case
+		// Might not be needed, but done just to be safe
+		boolean invalidateAll = !opts.side.equals(manifest.cachedSide);
+		if (invalidateAll) {
+			System.out.println("Side changed, invalidating all mods");
+		}
+		tasks.forEach(f -> {
+			// TODO: should linkedfile be checked as well? should this be done in the download section?
+			if (invalidateAll) {
+				f.invalidate();
+			} else if (invalidatedUris.contains(f.metadata.file)) {
+				f.invalidate();
+			}
+			ManifestFile.File file = manifest.cachedFiles.get(f.metadata.file);
+			if (file != null) {
+				// Ensure the file can be reverted later if necessary - the DownloadTask modifies the file so if it fails we need the old version back
+				file.backup();
+			}
+			// If it is null, the DownloadTask will make a new empty cachedFile
+			f.updateFromCache(file);
 		});
 
+		if (stateHandler.getCancelButton()) {
+			showCancellationDialog();
+			return;
+		}
+
+		// Let's hope downloadMetadata is a pure function!!!
+		tasks.parallelStream().forEach(f -> f.downloadMetadata(indexFile, indexUri));
+
+		List<IExceptionDetails> failedTasks = tasks.stream().filter(t -> t.getException() != null).collect(Collectors.toList());
+		if (!failedTasks.isEmpty()) {
+			errorsOccurred = true;
+			IExceptionDetails.ExceptionListResult exceptionListResult;
+			try {
+				exceptionListResult = ui.showExceptions(failedTasks, tasks.size(), true).get();
+			} catch (InterruptedException | ExecutionException e) {
+				// Interrupted means cancelled???
+				ui.handleExceptionAndExit(e);
+				return;
+			}
+			switch (exceptionListResult) {
+				case CONTINUE:
+					break;
+				case CANCEL:
+					cancelled = true;
+					return;
+				case IGNORE:
+					cancelledStartGame = true;
+					return;
+			}
+		}
+
+		if (stateHandler.getCancelButton()) {
+			showCancellationDialog();
+			return;
+		}
+
+		List<DownloadTask> nonFailedFirstTasks = tasks.stream().filter(t -> t.getException() == null).collect(Collectors.toList());
+		List<DownloadTask> optionTasks = nonFailedFirstTasks.stream().filter(DownloadTask::correctSide).filter(DownloadTask::isOptional).collect(Collectors.toList());
+		// If options changed, present all options again
+		if (stateHandler.getOptionsButton() || optionTasks.stream().anyMatch(DownloadTask::isNewOptional)) {
+			// new ArrayList is requires so it's an IOptionDetails rather than a DownloadTask list
+			Future<Boolean> cancelledResult = ui.showOptions(new ArrayList<>(optionTasks));
+			try {
+				if (cancelledResult.get()) {
+					cancelled = true;
+					// TODO: Should the UI be closed somehow??
+					return;
+				}
+			} catch (InterruptedException | ExecutionException e) {
+				// Interrupted means cancelled???
+				ui.handleExceptionAndExit(e);
+			}
+		}
+		ui.disableOptionsButton();
+
 		// TODO: different thread pool type?
 		ExecutorService threadPool = Executors.newFixedThreadPool(10);
-		CompletionService<DownloadCompletion> completionService = new ExecutorCompletionService<DownloadCompletion>(
-				threadPool);
+		CompletionService<DownloadTask> completionService = new ExecutorCompletionService<>(threadPool);
 
-		for (IndexFile.File f : newFiles) {
-			ManifestFile.File cachedFile = manifest.cachedFiles.get(f.file);
-			completionService.submit(new Callable<DownloadCompletion>() {
-				public DownloadCompletion call() {
-					DownloadCompletion dc = new DownloadCompletion();
-					dc.file = f;
+		tasks.forEach(t -> completionService.submit(() -> {
+			t.download(opts.packFolder, indexUri);
+			return t;
+		}));
 
-					if (cachedFile != null && cachedFile.linkedFileHash != null && f.linkedFile != null) {
-						try {
-							if (cachedFile.linkedFileHash.equals(f.linkedFile.getHash())) {
-								// Do nothing, the file didn't change
-								// TODO: but if the hash of the metafile changed, what did change?????
-								// should this be checked somehow??
-								return dc;
-							}
-						} catch (Exception e) {}
-					}
-
-					// Don't update files marked with preserve if they already exist on disk
-					if (f.preserve) {
-						if (Files.exists(Paths.get(opts.packFolder, f.getDestURI().toString()))) {
-							return dc;
-						}
-					}
-
-					try {
-						Hash hash;
-						String fileHashFormat;
-						if (f.linkedFile != null) {
-							hash = f.linkedFile.getHash();
-							fileHashFormat = f.linkedFile.download.hashFormat;
-						} else {
-							hash = f.getHash();
-							fileHashFormat = f.hashFormat;
-						}
-
-						Source src = f.getSource(indexUri);
-						GeneralHashingSource fileSource = HashUtils.getHasher(fileHashFormat).getHashingSource(src);
-						Buffer data = new Buffer();
-						Okio.buffer(fileSource).readAll(data);
-
-						if (fileSource.hashIsEqual(hash)) {
-							Files.createDirectories(Paths.get(opts.packFolder, f.getDestURI().toString()).getParent());
-							Files.copy(data.inputStream(), Paths.get(opts.packFolder, f.getDestURI().toString()), StandardCopyOption.REPLACE_EXISTING);
-						} else {
-							System.out.println("Invalid hash for " + f.getDestURI().toString());
-							System.out.println("Calculated: " + fileSource.getHash());
-							System.out.println("Expected:   " + hash);
-							dc.err = new Exception("Hash invalid!");
-						}
-						
-						return dc;
-					} catch (Exception e) {
-						dc.err = e;
-						return dc;
-					}
-				}
-			});
-		}
-
-		for (int i = 0; i < newFiles.size(); i++) {
-			DownloadCompletion ret;
+		for (int i = 0; i < tasks.size(); i++) {
+			DownloadTask task;
 			try {
-				ret = completionService.take().get();
+				task = completionService.take().get();
 			} catch (InterruptedException | ExecutionException e) {
-				// TODO: collect all exceptions, present in one dialog
 				ui.handleException(e);
-				ret = null;
+				task = null;
 			}
-			// Update manifest
-			if (ret != null && ret.err == null && ret.file != null) {
-				ManifestFile.File newCachedFile = new ManifestFile.File();
-				try {
-					newCachedFile.hash = ret.file.getHash();
-					if (newCachedFile.hash == null) {
-						throw new Exception("Invalid hash!");
+			// Update manifest - If there were no errors cachedFile has already been modified in place (good old pass by reference)
+			if (task != null) {
+				if (task.getException() != null) {
+					ManifestFile.File file = task.cachedFile.getRevert();
+					if (file != null) {
+						manifest.cachedFiles.putIfAbsent(task.metadata.file, file);
 					}
-				} catch (Exception e) {
-					ret.err = e;
-				}
-				if (ret.file.metafile && ret.file.linkedFile != null) {
-					newCachedFile.isOptional = ret.file.linkedFile.isOptional();
-					if (newCachedFile.isOptional) {
-						newCachedFile.optionValue = ret.file.optionValue;
-					}
-					try {
-						newCachedFile.linkedFileHash = ret.file.linkedFile.getHash();
-					} catch (Exception e) {
-						ret.err = e;
-					}
-				}
-				manifest.cachedFiles.put(ret.file.file, newCachedFile);
-			}
-			// TODO: show errors properly?
-			String progress;
-			if (ret != null) {
-				if (ret.err != null) {
-					if (ret.file != null) {
-						progress = "Failed to download " + ret.file.getName() + ": " + ret.err.getMessage();
-					} else {
-						progress = "Failed to download: " + ret.err.getMessage();
-					}
-					ret.err.printStackTrace();
-				} else if (ret.file != null) {
-					progress = "Downloaded " + ret.file.getName();
 				} else {
-					progress = "Failed to download, unknown reason";
+					// idiot, if it wasn't there in the first place it won't magically appear there
+					manifest.cachedFiles.putIfAbsent(task.metadata.file, task.cachedFile);
+				}
+			}
+
+			String progress;
+			if (task != null) {
+				if (task.getException() != null) {
+					progress = "Failed to download " + task.metadata.getName() + ": " + task.getException().getMessage();
+					task.getException().printStackTrace();
+				} else {
+					// TODO: should this be revised for tasks that didn't actually download it?
+					progress = "Downloaded " + task.metadata.getName();
 				}
 			} else {
 				progress = "Failed to download, unknown reason";
 			}
-			ui.submitProgress(new InstallProgress(progress, i + 1, newFiles.size()));
+			ui.submitProgress(new InstallProgress(progress, i + 1, tasks.size()));
+
+			if (stateHandler.getCancelButton()) {
+				// Stop all tasks, don't launch the game (it's in an invalid state!)
+				threadPool.shutdown();
+				cancelled = true;
+				return;
+			}
+		}
+
+		// Shut down the thread pool when the update is done
+		threadPool.shutdown();
+
+		List<IExceptionDetails> failedTasks2ElectricBoogaloo = nonFailedFirstTasks.stream().filter(t -> t.getException() != null).collect(Collectors.toList());
+		if (!failedTasks2ElectricBoogaloo.isEmpty()) {
+			errorsOccurred = true;
+			IExceptionDetails.ExceptionListResult exceptionListResult;
+			try {
+				exceptionListResult = ui.showExceptions(failedTasks2ElectricBoogaloo, tasks.size(), false).get();
+			} catch (InterruptedException | ExecutionException e) {
+				// Interrupted means cancelled???
+				ui.handleExceptionAndExit(e);
+				return;
+			}
+			switch (exceptionListResult) {
+				case CONTINUE:
+					break;
+				case CANCEL:
+					cancelled = true;
+					return;
+				case IGNORE:
+					cancelledStartGame = true;
+			}
 		}
-		// option = false file hashes should be stored to disk, but not downloaded
-		// TODO: don't include optional files in progress????
 	}
 
-	private class DownloadCompletion {
-		Exception err;
-		IndexFile.File file;
+	private void showCancellationDialog() {
+		IExceptionDetails.ExceptionListResult exceptionListResult;
+		try {
+			exceptionListResult = ui.showCancellationDialog().get();
+		} catch (InterruptedException | ExecutionException e) {
+			// Interrupted means cancelled???
+			ui.handleExceptionAndExit(e);
+			return;
+		}
+		switch (exceptionListResult) {
+			case CONTINUE:
+				throw new RuntimeException("Continuation not allowed here!");
+			case CANCEL:
+				cancelled = true;
+				return;
+			case IGNORE:
+				cancelledStartGame = true;
+		}
+	}
+
+	private void handleCancellation() {
+		if (cancelled) {
+			System.out.println("Update cancelled by user!");
+			System.exit(1);
+		} else if (cancelledStartGame) {
+			System.out.println("Update cancelled by user! Continuing to start game...");
+			System.exit(0);
+		}
 	}
 }

src/main/java/link/infra/packwiz/installer/metadata/EfficientBooleanAdapter.java

@@ -0,0 +1,29 @@
+package link.infra.packwiz.installer.metadata;
+
+import com.google.gson.TypeAdapter;
+import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonToken;
+import com.google.gson.stream.JsonWriter;
+
+import java.io.IOException;
+
+public class EfficientBooleanAdapter extends TypeAdapter<Boolean> {
+
+	@Override
+	public void write(JsonWriter out, Boolean value) throws IOException {
+		if (value == null || !value) {
+			out.nullValue();
+			return;
+		}
+		out.value(true);
+	}
+
+	@Override
+	public Boolean read(JsonReader in) throws IOException {
+		if (in.peek() == JsonToken.NULL) {
+			in.nextNull();
+			return false;
+		}
+		return in.nextBoolean();
+	}
+}

src/main/java/link/infra/packwiz/installer/metadata/IndexFile.java

@@ -1,13 +1,7 @@
 package link.infra.packwiz.installer.metadata;
 
-import java.net.URI;
-import java.nio.file.Paths;
-import java.util.List;
-
-import com.google.gson.annotations.JsonAdapter;
 import com.google.gson.annotations.SerializedName;
 import com.moandjiezana.toml.Toml;
-
 import link.infra.packwiz.installer.metadata.hash.GeneralHashingSource;
 import link.infra.packwiz.installer.metadata.hash.Hash;
 import link.infra.packwiz.installer.metadata.hash.HashUtils;
@@ -15,26 +9,27 @@ import link.infra.packwiz.installer.request.HandlerManager;
 import okio.Okio;
 import okio.Source;
 
+import java.nio.file.Paths;
+import java.util.List;
+
 public class IndexFile {
 	@SerializedName("hash-format")
 	public String hashFormat;
 	public List<File> files;
 	
 	public static class File {
-		@JsonAdapter(SpaceSafeURIParser.class)
-		public URI file;
+		public SpaceSafeURI file;
 		@SerializedName("hash-format")
 		public String hashFormat;
 		public String hash;
-		public URI alias;
+		public SpaceSafeURI alias;
 		public boolean metafile;
 		public boolean preserve;
 
 		public transient ModFile linkedFile;
-		public transient URI linkedFileURI;
-		public transient boolean optionValue = true;
+		public transient SpaceSafeURI linkedFileURI;
 
-		public void downloadMeta(IndexFile parentIndexFile, URI indexUri) throws Exception {
+		public void downloadMeta(IndexFile parentIndexFile, SpaceSafeURI indexUri) throws Exception {
 			if (!metafile) {
 				return;
 			}
@@ -52,14 +47,14 @@ public class IndexFile {
 			}
 		}
 
-		public Source getSource(URI indexUri) throws Exception {
+		public Source getSource(SpaceSafeURI indexUri) throws Exception {
 			if (metafile) {
 				if (linkedFile == null) {
 					throw new Exception("Linked file doesn't exist!");
 				}
 				return linkedFile.getSource(linkedFileURI);
 			} else {
-				URI newLoc = HandlerManager.getNewLoc(indexUri, file);
+				SpaceSafeURI newLoc = HandlerManager.getNewLoc(indexUri, file);
 				if (newLoc == null) {
 					throw new Exception("Index file URI is invalid");
 				}
@@ -69,6 +64,7 @@ public class IndexFile {
 
 		public Hash getHash() throws Exception {
 			if (hash == null) {
+				// TODO: should these be more specific exceptions (e.g. IndexFileException?!)
 				throw new Exception("Index file doesn't have a hash");
 			}
 			if (hashFormat == null) {
@@ -90,16 +86,17 @@ public class IndexFile {
 			if (file != null) {
 				return Paths.get(file.getPath()).getFileName().toString();
 			}
-			return file.getPath();
+			// TODO: throw some kind of exception?
+			return "Invalid file";
 		}
 
-		public URI getDestURI() {
+		public SpaceSafeURI getDestURI() {
 			if (alias != null) {
 				return alias;
 			}
 			if (metafile && linkedFile != null) {
 				// TODO: URIs are bad
-				return file.resolve(linkedFile.filename.replace(" ", "%20"));
+				return file.resolve(linkedFile.filename);
 			} else {
 				return file;
 			}

src/main/java/link/infra/packwiz/installer/metadata/ManifestFile.java

@@ -1,20 +1,45 @@
 package link.infra.packwiz.installer.metadata;
 
-import java.net.URI;
-import java.util.Map;
-
+import com.google.gson.annotations.JsonAdapter;
+import link.infra.packwiz.installer.UpdateManager;
 import link.infra.packwiz.installer.metadata.hash.Hash;
 
+import java.util.Map;
+
 public class ManifestFile {
-	
 	public Hash packFileHash = null;
 	public Hash indexFileHash = null;
-	public Map<URI, File> cachedFiles;
+	public Map<SpaceSafeURI, File> cachedFiles;
+	// If the side changes, EVERYTHING invalidates. FUN!!!
+	public UpdateManager.Options.Side cachedSide = UpdateManager.Options.Side.CLIENT;
 
 	public static class File {
+		private transient File revert;
+
 		public Hash hash = null;
+		public Hash linkedFileHash = null;
+		public String cachedLocation = null;
+
+		@JsonAdapter(EfficientBooleanAdapter.class)
 		public boolean isOptional = false;
 		public boolean optionValue = true;
-		public Hash linkedFileHash = null;
+
+		@JsonAdapter(EfficientBooleanAdapter.class)
+		public boolean onlyOtherSide = false;
+
+		// When an error occurs, the state needs to be reverted. To do this, I have a crude revert system.
+		public void backup() {
+			revert = new File();
+			revert.hash = hash;
+			revert.linkedFileHash = linkedFileHash;
+			revert.cachedLocation = cachedLocation;
+			revert.isOptional = isOptional;
+			revert.optionValue = optionValue;
+			revert.onlyOtherSide = onlyOtherSide;
+		}
+
+		public File getRevert() {
+			return revert;
+		}
 	}
 }

src/main/java/link/infra/packwiz/installer/metadata/ModFile.java

@@ -1,17 +1,14 @@
 package link.infra.packwiz.installer.metadata;
 
-import java.net.URI;
-import java.util.Map;
-
-import com.google.gson.annotations.JsonAdapter;
 import com.google.gson.annotations.SerializedName;
-
 import link.infra.packwiz.installer.UpdateManager.Options.Side;
 import link.infra.packwiz.installer.metadata.hash.Hash;
 import link.infra.packwiz.installer.metadata.hash.HashUtils;
 import link.infra.packwiz.installer.request.HandlerManager;
 import okio.Source;
 
+import java.util.Map;
+
 public class ModFile {
 	public String name;
 	public String filename;
@@ -19,8 +16,7 @@ public class ModFile {
 
 	public Download download;
 	public static class Download {
-		@JsonAdapter(SpaceSafeURIParser.class)
-		public URI url;
+		public SpaceSafeURI url;
 		@SerializedName("hash-format")
 		public String hashFormat;
 		public String hash;
@@ -36,14 +32,14 @@ public class ModFile {
 		public boolean defaultValue;
 	}
 
-	public Source getSource(URI baseLoc) throws Exception {
+	public Source getSource(SpaceSafeURI baseLoc) throws Exception {
 		if (download == null) {
 			throw new Exception("Metadata file doesn't have download");
 		}
 		if (download.url == null) {
 			throw new Exception("Metadata file doesn't have a download URI");
 		}
-		URI newLoc = HandlerManager.getNewLoc(baseLoc, download.url);
+		SpaceSafeURI newLoc = HandlerManager.getNewLoc(baseLoc, download.url);
 		if (newLoc == null) {
 			throw new Exception("Metadata file URI is invalid");
 		}

src/main/java/link/infra/packwiz/installer/metadata/PackFile.java

@@ -1,18 +1,15 @@
 package link.infra.packwiz.installer.metadata;
 
-import java.net.URI;
-import java.util.Map;
-
-import com.google.gson.annotations.JsonAdapter;
 import com.google.gson.annotations.SerializedName;
 
+import java.util.Map;
+
 public class PackFile {
 	public String name;
 
 	public IndexFileLoc index;
 	public static class IndexFileLoc {
-		@JsonAdapter(SpaceSafeURIParser.class)
-		public URI file;
+		public SpaceSafeURI file;
 		@SerializedName("hash-format")
 		public String hashFormat;
 		public String hash;

src/main/java/link/infra/packwiz/installer/metadata/SpaceSafeURI.java

@@ -0,0 +1,88 @@
+package link.infra.packwiz.installer.metadata;
+
+import com.google.gson.annotations.JsonAdapter;
+
+import java.io.Serializable;
+import java.net.MalformedURLException;
+import java.net.URI;
+import java.net.URISyntaxException;
+import java.net.URL;
+
+// The world's worst URI wrapper
+@JsonAdapter(SpaceSafeURIParser.class)
+public class SpaceSafeURI implements Comparable<SpaceSafeURI>, Serializable {
+	private final URI u;
+
+	public SpaceSafeURI(String str) throws URISyntaxException {
+		u = new URI(str.replace(" ", "%20"));
+	}
+
+	public SpaceSafeURI(URI uri) {
+		this.u = uri;
+	}
+
+	public SpaceSafeURI(String scheme, String authority, String path, String query, String fragment) throws URISyntaxException {
+		// TODO: do all components need to be replaced?
+		scheme = scheme.replace(" ", "%20");
+		authority = authority.replace(" ", "%20");
+		path = path.replace(" ", "%20");
+		query = query.replace(" ", "%20");
+		fragment = fragment.replace(" ", "%20");
+		u = new URI(scheme, authority, path, query, fragment);
+	}
+
+	public String getPath() {
+		return u.getPath().replace("%20", " ");
+	}
+
+	public String toString() {
+		return u.toString().replace("%20", " ");
+	}
+
+	@SuppressWarnings("WeakerAccess")
+	public SpaceSafeURI resolve(String path) {
+		return new SpaceSafeURI(u.resolve(path.replace(" ", "%20")));
+	}
+
+	public SpaceSafeURI resolve(SpaceSafeURI loc) {
+		return new SpaceSafeURI(u.resolve(loc.u));
+	}
+
+	public SpaceSafeURI relativize(SpaceSafeURI loc) {
+		return new SpaceSafeURI(u.relativize(loc.u));
+	}
+
+	@Override
+	public boolean equals(Object obj) {
+		if (obj instanceof SpaceSafeURI) {
+			return u.equals(((SpaceSafeURI) obj).u);
+		}
+		return false;
+	}
+
+	@Override
+	public int hashCode() {
+		return u.hashCode();
+	}
+
+	@Override
+	public int compareTo(SpaceSafeURI uri) {
+		return u.compareTo(uri.u);
+	}
+
+	public String getScheme() {
+		return u.getScheme();
+	}
+
+	public String getAuthority() {
+		return u.getAuthority();
+	}
+
+	public String getHost() {
+		return u.getHost();
+	}
+
+	public URL toURL() throws MalformedURLException {
+		return u.toURL();
+	}
+}

src/main/java/link/infra/packwiz/installer/metadata/SpaceSafeURIParser.java

@@ -1,26 +1,24 @@
 package link.infra.packwiz.installer.metadata;
 
-import java.lang.reflect.Type;
-import java.net.URI;
-import java.net.URISyntaxException;
-
 import com.google.gson.JsonDeserializationContext;
 import com.google.gson.JsonDeserializer;
 import com.google.gson.JsonElement;
 import com.google.gson.JsonParseException;
 
+import java.lang.reflect.Type;
+import java.net.URISyntaxException;
+
 /**
  * This class encodes spaces before parsing the URI, so the URI can actually be
  * parsed.
  */
-class SpaceSafeURIParser implements JsonDeserializer<URI> {
+class SpaceSafeURIParser implements JsonDeserializer<SpaceSafeURI> {
 
 	@Override
-	public URI deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context)
+	public SpaceSafeURI deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context)
 			throws JsonParseException {
-		String uriString = json.getAsString().replace(" ", "%20");
 		try {
-			return new URI(uriString);
+			return new SpaceSafeURI(json.getAsString());
 		} catch (URISyntaxException e) {
 			throw new JsonParseException("Failed to parse URI", e);
 		}

src/main/java/link/infra/packwiz/installer/metadata/hash/HashingSourceHasher.java

@@ -4,9 +4,9 @@ import okio.HashingSource;
 import okio.Source;
 
 public class HashingSourceHasher implements IHasher {
-	String type;
+	private String type;
 
-	public HashingSourceHasher(String type) {
+	HashingSourceHasher(String type) {
 		this.type = type;
 	}
 
@@ -15,7 +15,7 @@ public class HashingSourceHasher implements IHasher {
 		HashingSource delegateHashing;
 		HashingSourceHash value;
 
-		public HashingSourceGeneralHashingSource(HashingSource delegate) {
+		HashingSourceGeneralHashingSource(HashingSource delegate) {
 			super(delegate);
 			delegateHashing = delegate;
 		}
@@ -46,9 +46,9 @@ public class HashingSourceHasher implements IHasher {
 			}
 			HashingSourceHash objHash = (HashingSourceHash) obj;
 			if (value != null) {
-				return value.equals(objHash.value);
+				return value.equalsIgnoreCase(objHash.value);
 			} else {
-				return objHash.value == null ? true : false;
+				return objHash.value == null;
 			}
 		}
 
@@ -71,9 +71,12 @@ public class HashingSourceHasher implements IHasher {
 	@Override
 	public GeneralHashingSource getHashingSource(Source delegate) {
 		switch (type) {
+			case "md5":
+				return new HashingSourceGeneralHashingSource(HashingSource.md5(delegate));
 			case "sha256":
-			return new HashingSourceGeneralHashingSource(HashingSource.sha256(delegate));
-			// TODO: support other hash types
+				return new HashingSourceGeneralHashingSource(HashingSource.sha256(delegate));
+			case "sha512":
+				return new HashingSourceGeneralHashingSource(HashingSource.sha512(delegate));
 		}
 		throw new RuntimeException("Invalid hash type provided");
 	}

src/main/java/link/infra/packwiz/installer/metadata/hash/Murmur2Hasher.java

@@ -1,10 +1,10 @@
 package link.infra.packwiz.installer.metadata.hash;
 
-import java.io.IOException;
-
 import okio.Buffer;
 import okio.Source;
 
+import java.io.IOException;
+
 public class Murmur2Hasher implements IHasher {
 	private class Murmur2GeneralHashingSource extends GeneralHashingSource {
 		Murmur2Hash value;
@@ -40,8 +40,7 @@ public class Murmur2Hasher implements IHasher {
 		private byte[] computeNormalizedArray(byte[] input) {
 			byte[] output = new byte[input.length];
 			int num = 0;
-			for (int i = 0; i < input.length; i++) {
-				byte b = input[i];
+			for (byte b : input) {
 				if (!(b == 9 || b == 10 || b == 13 || b == 32)) {
 					output[num] = b;
 					num++;
@@ -54,7 +53,7 @@ public class Murmur2Hasher implements IHasher {
 
 	}
 
-	private class Murmur2Hash extends Hash {
+	private static class Murmur2Hash extends Hash {
 		int value;
 		private Murmur2Hash(String value) {
 			// Parsing as long then casting to int converts values gt int max value but lt uint max value

src/main/java/link/infra/packwiz/installer/request/HandlerManager.java

@@ -1,23 +1,23 @@
 package link.infra.packwiz.installer.request;
 
-import java.net.URI;
-import java.util.ArrayList;
-import java.util.List;
-
+import link.infra.packwiz.installer.metadata.SpaceSafeURI;
 import link.infra.packwiz.installer.request.handlers.RequestHandlerGithub;
 import link.infra.packwiz.installer.request.handlers.RequestHandlerHTTP;
 import okio.Source;
 
+import java.util.ArrayList;
+import java.util.List;
+
 public abstract class HandlerManager {
 	
-	public static List<IRequestHandler> handlers = new ArrayList<IRequestHandler>();
+	private static List<IRequestHandler> handlers = new ArrayList<>();
 	
 	static {
 		handlers.add(new RequestHandlerGithub());
 		handlers.add(new RequestHandlerHTTP());
 	}
 	
-	public static URI getNewLoc(URI base, URI loc) {
+	public static SpaceSafeURI getNewLoc(SpaceSafeURI base, SpaceSafeURI loc) {
 		if (loc == null) return null;
 		if (base != null) {
 			loc = base.resolve(loc);
@@ -35,7 +35,7 @@ public abstract class HandlerManager {
 	// Zip handler discards once read, requesting multiple times on other handlers would cause multiple downloads
 	// Caching system? Copy from already downloaded files?
 
-	public static Source getFileSource(URI loc) throws Exception {
+	public static Source getFileSource(SpaceSafeURI loc) throws Exception {
 		for (IRequestHandler handler : handlers) {
 			if (handler.matchesHandler(loc)) {
 				Source src = handler.getFileSource(loc);

src/main/java/link/infra/packwiz/installer/request/IRequestHandler.java

@@ -1,7 +1,6 @@
 package link.infra.packwiz.installer.request;
 
-import java.net.URI;
-
+import link.infra.packwiz.installer.metadata.SpaceSafeURI;
 import okio.Source;
 
 /**
@@ -9,9 +8,9 @@ import okio.Source;
  */
 public interface IRequestHandler {
 	
-	public boolean matchesHandler(URI loc);
+	boolean matchesHandler(SpaceSafeURI loc);
 	
-	public default URI getNewLoc(URI loc) {
+	default SpaceSafeURI getNewLoc(SpaceSafeURI loc) {
 		return loc;
 	}
 	
@@ -20,8 +19,8 @@ public interface IRequestHandler {
 	 * It is assumed that each location is read only once for the duration of an IRequestHandler.
 	 * @param loc The location to be read
 	 * @return The Source containing the data of the file
-	 * @throws Exception
+	 * @throws Exception Exception if it failed to download a file!!!
 	 */
-	public Source getFileSource(URI loc) throws Exception;
+	Source getFileSource(SpaceSafeURI loc) throws Exception;
 
 }

src/main/java/link/infra/packwiz/installer/request/handlers/RequestHandlerGithub.java

@@ -1,6 +1,7 @@
 package link.infra.packwiz.installer.request.handlers;
 
-import java.net.URI;
+import link.infra.packwiz.installer.metadata.SpaceSafeURI;
+
 import java.util.HashMap;
 import java.util.Map;
 import java.util.concurrent.locks.ReentrantReadWriteLock;
@@ -14,38 +15,41 @@ public class RequestHandlerGithub extends RequestHandlerZip {
 	}
 
 	@Override
-	public URI getNewLoc(URI loc) {
+	public SpaceSafeURI getNewLoc(SpaceSafeURI loc) {
 		return loc;
 	}
 	
 	// TODO: is caching really needed, if HTTPURLConnection follows redirects correctly?
-	private Map<String, URI> zipUriMap = new HashMap<String, URI>();
-	final ReentrantReadWriteLock zipUriLock = new ReentrantReadWriteLock();
+	private Map<String, SpaceSafeURI> zipUriMap = new HashMap<>();
+	private final ReentrantReadWriteLock zipUriLock = new ReentrantReadWriteLock();
 	private static Pattern repoMatcherPattern = Pattern.compile("/([\\w.-]+/[\\w.-]+).*");
 	
-	private String getRepoName(URI loc) {
+	private String getRepoName(SpaceSafeURI loc) {
 		Matcher matcher = repoMatcherPattern.matcher(loc.getPath());
-		matcher.matches();
-		return matcher.group(1);
+		if (matcher.matches()) {
+			return matcher.group(1);
+		} else {
+			return null;
+		}
 	}
 
 	@Override
-	protected URI getZipUri(URI loc) throws Exception {
+	protected SpaceSafeURI getZipUri(SpaceSafeURI loc) throws Exception {
 		String repoName = getRepoName(loc);
 		String branchName = getBranch(loc);
 		zipUriLock.readLock().lock();
-		URI zipUri = zipUriMap.get(repoName + "/" + branchName);
+		SpaceSafeURI zipUri = zipUriMap.get(repoName + "/" + branchName);
 		zipUriLock.readLock().unlock();
 		if (zipUri != null) {
 			return zipUri;
 		}
 		
-		zipUri = new URI("https://api.github.com/repos/" + repoName + "/zipball/" + branchName);
+		zipUri = new SpaceSafeURI("https://api.github.com/repos/" + repoName + "/zipball/" + branchName);
 		
 		zipUriLock.writeLock().lock();
 		// If another thread sets the value concurrently, use the value of the
 		// thread that first acquired the lock.
-		URI zipUriInserted = zipUriMap.putIfAbsent(repoName + "/" + branchName, zipUri);
+		SpaceSafeURI zipUriInserted = zipUriMap.putIfAbsent(repoName + "/" + branchName, zipUri);
 		if (zipUriInserted != null) {
 			zipUri = zipUriInserted;
 		}
@@ -55,20 +59,23 @@ public class RequestHandlerGithub extends RequestHandlerZip {
 	
 	private static Pattern branchMatcherPattern = Pattern.compile("/[\\w.-]+/[\\w.-]+/blob/([\\w.-]+).*");
 	
-	private String getBranch(URI loc) {
+	private String getBranch(SpaceSafeURI loc) {
 		Matcher matcher = branchMatcherPattern.matcher(loc.getPath());
-		matcher.matches();
-		return matcher.group(1);
+		if (matcher.matches()) {
+			return matcher.group(1);
+		} else {
+			return null;
+		}
 	}
 
 	@Override
-	protected URI getLocationInZip(URI loc) throws Exception {
+	protected SpaceSafeURI getLocationInZip(SpaceSafeURI loc) throws Exception {
 		String path = "/" + getRepoName(loc) + "/blob/" + getBranch(loc);
-		return new URI(loc.getScheme(), loc.getAuthority(), path, null, null).relativize(loc);
+		return new SpaceSafeURI(loc.getScheme(), loc.getAuthority(), path, null, null).relativize(loc);
 	}
 
 	@Override
-	public boolean matchesHandler(URI loc) {
+	public boolean matchesHandler(SpaceSafeURI loc) {
 		String scheme = loc.getScheme();
 		if (!("http".equals(scheme) || "https".equals(scheme))) {
 			return false;

src/main/java/link/infra/packwiz/installer/request/handlers/RequestHandlerHTTP.java

@@ -1,22 +1,22 @@
 package link.infra.packwiz.installer.request.handlers;
 
-import java.net.URI;
-import java.net.URLConnection;
-
+import link.infra.packwiz.installer.metadata.SpaceSafeURI;
 import link.infra.packwiz.installer.request.IRequestHandler;
 import okio.Okio;
 import okio.Source;
 
+import java.net.URLConnection;
+
 public class RequestHandlerHTTP implements IRequestHandler {
 
 	@Override
-	public boolean matchesHandler(URI loc) {
+	public boolean matchesHandler(SpaceSafeURI loc) {
 		String scheme = loc.getScheme();
 		return "http".equals(scheme) || "https".equals(scheme);
 	}
 
 	@Override
-	public Source getFileSource(URI loc) throws Exception {
+	public Source getFileSource(SpaceSafeURI loc) throws Exception {
 		URLConnection conn = loc.toURL().openConnection();
 		// TODO: when do we send specific headers??? should there be a way to signal this?
 		// github *sometimes* requires it, sometimes not!

src/main/java/link/infra/packwiz/installer/request/handlers/RequestHandlerZip.java

@@ -1,7 +1,12 @@
 package link.infra.packwiz.installer.request.handlers;
 
+import link.infra.packwiz.installer.metadata.SpaceSafeURI;
+import okio.Buffer;
+import okio.BufferedSource;
+import okio.Okio;
+import okio.Source;
+
 import java.io.IOException;
-import java.net.URI;
 import java.net.URISyntaxException;
 import java.util.HashMap;
 import java.util.Map;
@@ -11,14 +16,9 @@ import java.util.function.Predicate;
 import java.util.zip.ZipEntry;
 import java.util.zip.ZipInputStream;
 
-import okio.Buffer;
-import okio.BufferedSource;
-import okio.Okio;
-import okio.Source;
-
 public abstract class RequestHandlerZip extends RequestHandlerHTTP {
 	
-	protected final boolean modeHasFolder;
+	private final boolean modeHasFolder;
 	
 	public RequestHandlerZip(boolean modeHasFolder) {
 		this.modeHasFolder = modeHasFolder;
@@ -35,14 +35,14 @@ public abstract class RequestHandlerZip extends RequestHandlerHTTP {
 	private class ZipReader {
 		
 		private final ZipInputStream zis;
-		private final Map<URI, Buffer> readFiles = new HashMap<URI, Buffer>();
+		private final Map<SpaceSafeURI, Buffer> readFiles = new HashMap<>();
 		// Write lock implies access to ZipInputStream - only 1 thread must read at a time!
 		final ReentrantLock filesLock = new ReentrantLock();
 		private ZipEntry entry;
 
 		private final BufferedSource zipSource;
 
-		public ZipReader(Source zip) {
+		ZipReader(Source zip) {
 			zis = new ZipInputStream(Okio.buffer(zip).inputStream());
 			zipSource = Okio.buffer(Okio.source(zis));
 		}
@@ -55,14 +55,14 @@ public abstract class RequestHandlerZip extends RequestHandlerHTTP {
 		}
 		
 		// File lock must be obtained before calling this function
-		private Buffer findFile(URI loc) throws IOException, URISyntaxException {
+		private Buffer findFile(SpaceSafeURI loc) throws IOException, URISyntaxException {
 			while (true) {
 				entry = zis.getNextEntry();
 				if (entry == null) {
 					return null;
 				}
 				Buffer data = readCurrFile();
-				URI fileLoc = new URI(removeFolder(entry.getName()));
+				SpaceSafeURI fileLoc = new SpaceSafeURI(removeFolder(entry.getName()));
 				if (loc.equals(fileLoc)) {
 					return data;
 				} else {
@@ -71,7 +71,7 @@ public abstract class RequestHandlerZip extends RequestHandlerHTTP {
 			}
 		}
 		
-		public Source getFileSource(URI loc) throws Exception {
+		Source getFileSource(SpaceSafeURI loc) throws Exception {
 			filesLock.lock();
 			// Assume files are only read once, allow GC by removing
 			Buffer file = readFiles.remove(loc);
@@ -82,15 +82,12 @@ public abstract class RequestHandlerZip extends RequestHandlerHTTP {
 			
 			file = findFile(loc);
 			filesLock.unlock();
-			if (file != null) {
-				return file;
-			}
-			return null;
+			return file;
 		}
-		
-		public URI findInZip(Predicate<URI> matches) throws Exception {
+
+		SpaceSafeURI findInZip(Predicate<SpaceSafeURI> matches) throws Exception {
 			filesLock.lock();
-			for (URI file : readFiles.keySet()) {
+			for (SpaceSafeURI file : readFiles.keySet()) {
 				if (matches.test(file)) {
 					filesLock.unlock();
 					return file;
@@ -104,7 +101,7 @@ public abstract class RequestHandlerZip extends RequestHandlerHTTP {
 					return null;
 				}
 				Buffer data = readCurrFile();
-				URI fileLoc = new URI(removeFolder(entry.getName()));
+				SpaceSafeURI fileLoc = new SpaceSafeURI(removeFolder(entry.getName()));
 				readFiles.put(fileLoc, data);
 				if (matches.test(fileLoc)) {
 					filesLock.unlock();
@@ -115,19 +112,19 @@ public abstract class RequestHandlerZip extends RequestHandlerHTTP {
 		
 	}
 	
-	private final Map<URI, ZipReader> cache = new HashMap<URI, ZipReader>();
-	final ReentrantReadWriteLock cacheLock = new ReentrantReadWriteLock();
+	private final Map<SpaceSafeURI, ZipReader> cache = new HashMap<>();
+	private final ReentrantReadWriteLock cacheLock = new ReentrantReadWriteLock();
 	
-	protected abstract URI getZipUri(URI loc) throws Exception;
+	protected abstract SpaceSafeURI getZipUri(SpaceSafeURI loc) throws Exception;
 	
-	protected abstract URI getLocationInZip(URI loc) throws Exception;
+	protected abstract SpaceSafeURI getLocationInZip(SpaceSafeURI loc) throws Exception;
 	
 	@Override
-	public abstract boolean matchesHandler(URI loc);
+	public abstract boolean matchesHandler(SpaceSafeURI loc);
 
 	@Override
-	public Source getFileSource(URI loc) throws Exception {
-		URI zipUri = getZipUri(loc);
+	public Source getFileSource(SpaceSafeURI loc) throws Exception {
+		SpaceSafeURI zipUri = getZipUri(loc);
 		cacheLock.readLock().lock();
 		ZipReader zr = cache.get(zipUri);
 		cacheLock.readLock().unlock();
@@ -150,8 +147,8 @@ public abstract class RequestHandlerZip extends RequestHandlerHTTP {
 		return zr.getFileSource(getLocationInZip(loc));
 	}
 	
-	protected URI findInZip(URI loc, Predicate<URI> matches) throws Exception {
-		URI zipUri = getZipUri(loc);
+	protected SpaceSafeURI findInZip(SpaceSafeURI loc, Predicate<SpaceSafeURI> matches) throws Exception {
+		SpaceSafeURI zipUri = getZipUri(loc);
 		cacheLock.readLock().lock();
 		ZipReader zr = cache.get(zipUri);
 		cacheLock.readLock().unlock();

src/main/java/link/infra/packwiz/installer/ui/CLIHandler.java

@@ -1,5 +1,9 @@
 package link.infra.packwiz.installer.ui;
 
+import java.util.List;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.Future;
+
 public class CLIHandler implements IUserInterface {
 
 	@Override
@@ -8,7 +12,7 @@ public class CLIHandler implements IUserInterface {
 	}
 
 	@Override
-	public void show() {}
+	public void show(InputStateHandler h) {}
 
 	@Override
 	public void submitProgress(InstallProgress progress) {
@@ -29,5 +33,23 @@ public class CLIHandler implements IUserInterface {
 		task.run();
 		System.out.println("Finished successfully!");
 	}
-	
+
+	@Override
+	public Future<Boolean> showOptions(List<IOptionDetails> options) {
+		for (IOptionDetails opt : options) {
+			opt.setOptionValue(true);
+			System.out.println("Warning: accepting option " + opt.getName() + " as option choosing is not implemented in the CLI");
+		}
+		CompletableFuture<Boolean> future = new CompletableFuture<>();
+		future.complete(false); // Can't be cancelled!
+		return future;
+	}
+
+	@Override
+	public Future<IExceptionDetails.ExceptionListResult> showExceptions(List<IExceptionDetails> opts, int numTotal, boolean allowsIgnore) {
+		CompletableFuture<IExceptionDetails.ExceptionListResult> future = new CompletableFuture<>();
+		future.complete(IExceptionDetails.ExceptionListResult.CANCEL);
+		return future;
+	}
+
 }

src/main/java/link/infra/packwiz/installer/ui/ExceptionListWindow.java

@@ -0,0 +1,183 @@
+package link.infra.packwiz.installer.ui;
+
+import link.infra.packwiz.installer.ui.IExceptionDetails.ExceptionListResult;
+
+import javax.swing.*;
+import javax.swing.border.EmptyBorder;
+import java.awt.*;
+import java.awt.event.WindowAdapter;
+import java.awt.event.WindowEvent;
+import java.io.IOException;
+import java.io.PrintWriter;
+import java.io.StringWriter;
+import java.net.URI;
+import java.net.URISyntaxException;
+import java.util.List;
+import java.util.concurrent.CompletableFuture;
+
+class ExceptionListWindow extends JDialog {
+
+	private static final long serialVersionUID = 1L;
+	private final JTextArea lblExceptionStacktrace;
+
+	/**
+	 * Create the dialog.
+	 */
+	ExceptionListWindow(List<IExceptionDetails> eList, CompletableFuture<ExceptionListResult> future, int numTotal, boolean allowsIgnore, JFrame parentWindow) {
+		super(parentWindow, "Failed file downloads", true);
+
+		setBounds(100, 100, 540, 340);
+		setLocationRelativeTo(parentWindow);
+		getContentPane().setLayout(new BorderLayout());
+		{
+			JPanel errorPanel = new JPanel();
+			getContentPane().add(errorPanel, BorderLayout.NORTH);
+			{
+				JLabel lblWarning = new JLabel("One or more errors were encountered while installing the modpack!");
+				lblWarning.setIcon(UIManager.getIcon("OptionPane.warningIcon"));
+				errorPanel.add(lblWarning);
+			}
+		}
+		JPanel contentPanel = new JPanel();
+		contentPanel.setBorder(new EmptyBorder(5, 5, 5, 5));
+		getContentPane().add(contentPanel, BorderLayout.CENTER);
+		contentPanel.setLayout(new BorderLayout(0, 0));
+		{
+			JSplitPane splitPane = new JSplitPane();
+			splitPane.setResizeWeight(0.3);
+			contentPanel.add(splitPane);
+			{
+				lblExceptionStacktrace = new JTextArea("Select a file");
+				lblExceptionStacktrace.setBackground(UIManager.getColor("List.background"));
+				lblExceptionStacktrace.setOpaque(true);
+				lblExceptionStacktrace.setWrapStyleWord(true);
+				lblExceptionStacktrace.setLineWrap(true);
+				lblExceptionStacktrace.setEditable(false);
+				lblExceptionStacktrace.setFocusable(true);
+				lblExceptionStacktrace.setFont(UIManager.getFont("Label.font"));
+				lblExceptionStacktrace.setBorder(new EmptyBorder(5, 5, 5, 5));
+				JScrollPane scrollPane = new JScrollPane(lblExceptionStacktrace);
+				scrollPane.setBorder(new EmptyBorder(0, 0, 0, 0));
+				splitPane.setRightComponent(scrollPane);
+			}
+			{
+				JList<String> list = new JList<>();
+				list.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);
+				list.setBorder(new EmptyBorder(5, 5, 5, 5));
+				ExceptionListModel listModel = new ExceptionListModel(eList);
+				list.setModel(listModel);
+				list.addListSelectionListener(e -> {
+					int i = list.getSelectedIndex();
+					if (i > -1) {
+						StringWriter sw = new StringWriter();
+						listModel.getExceptionAt(i).printStackTrace(new PrintWriter(sw));
+						lblExceptionStacktrace.setText(sw.toString());
+						// Scroll to the top
+						lblExceptionStacktrace.setCaretPosition(0);
+					} else {
+						lblExceptionStacktrace.setText("Select a file");
+					}
+				});
+				JScrollPane scrollPane = new JScrollPane(list);
+				scrollPane.setBorder(new EmptyBorder(0, 0, 0, 0));
+				splitPane.setLeftComponent(scrollPane);
+			}
+		}
+		{
+			JPanel buttonPane = new JPanel();
+			getContentPane().add(buttonPane, BorderLayout.SOUTH);
+			buttonPane.setLayout(new BorderLayout(0, 0));
+			{
+				JPanel rightButtons = new JPanel();
+				buttonPane.add(rightButtons, BorderLayout.EAST);
+				{
+					JButton btnContinue = new JButton("Continue");
+					btnContinue.setToolTipText("Attempt to continue installing, excluding the failed downloads");
+					btnContinue.addActionListener(e -> {
+						future.complete(ExceptionListResult.CONTINUE);
+						ExceptionListWindow.this.dispose();
+					});
+					rightButtons.add(btnContinue);
+				}
+				{
+					JButton btnCancelLaunch = new JButton("Cancel launch");
+					btnCancelLaunch.setToolTipText("Stop launching the game");
+					btnCancelLaunch.addActionListener(e -> {
+						future.complete(ExceptionListResult.CANCEL);
+						ExceptionListWindow.this.dispose();
+					});
+					rightButtons.add(btnCancelLaunch);
+				}
+				{
+					JButton btnIgnoreUpdate = new JButton("Ignore update");
+					btnIgnoreUpdate.setEnabled(allowsIgnore);
+					btnIgnoreUpdate.setToolTipText("Start the game without attempting to update");
+					btnIgnoreUpdate.addActionListener(e -> {
+						future.complete(ExceptionListResult.IGNORE);
+						ExceptionListWindow.this.dispose();
+					});
+					rightButtons.add(btnIgnoreUpdate);
+					{
+						JLabel lblErrored = new JLabel(eList.size() + "/" + numTotal + " errored");
+						lblErrored.setHorizontalAlignment(SwingConstants.CENTER);
+						buttonPane.add(lblErrored, BorderLayout.CENTER);
+					}
+					{
+						JPanel leftButtons = new JPanel();
+						buttonPane.add(leftButtons, BorderLayout.WEST);
+						{
+							JButton btnReportIssue = new JButton("Report issue");
+							boolean supported = Desktop.isDesktopSupported() && Desktop.getDesktop().isSupported(Desktop.Action.BROWSE);
+							btnReportIssue.setEnabled(supported);
+							if (supported) {
+								btnReportIssue.addActionListener(e -> {
+									try {
+										Desktop.getDesktop().browse(new URI("https://github.com/comp500/packwiz-installer/issues/new"));
+									} catch (IOException | URISyntaxException e1) {
+										// lol the button just won't work i guess
+									}
+								});
+							}
+							leftButtons.add(btnReportIssue);
+						}
+					}
+				}
+			}
+		}
+		addWindowListener(new WindowAdapter() {
+			@Override
+			public void windowClosing(WindowEvent e) {
+				future.complete(ExceptionListResult.CANCEL);
+			}
+
+			@Override
+			public void windowClosed(WindowEvent e) {
+				// Just in case closing didn't get triggered - if something else called dispose() the
+				// future will have already completed
+				future.complete(ExceptionListResult.CANCEL);
+			}
+		});
+	}
+
+	private static class ExceptionListModel extends AbstractListModel<String> {
+		private static final long serialVersionUID = 1L;
+		private final List<IExceptionDetails> details;
+
+		ExceptionListModel(List<IExceptionDetails> details) {
+			this.details = details;
+		}
+
+		public int getSize() {
+			return details.size();
+		}
+
+		public String getElementAt(int index) {
+			return details.get(index).getName();
+		}
+
+		Exception getExceptionAt(int index) {
+			return details.get(index).getException();
+		}
+	}
+
+}

src/main/java/link/infra/packwiz/installer/ui/IExceptionDetails.java

@@ -0,0 +1,10 @@
+package link.infra.packwiz.installer.ui;
+
+public interface IExceptionDetails {
+	Exception getException();
+	String getName();
+
+	enum ExceptionListResult {
+		CONTINUE, CANCEL, IGNORE
+	}
+}

src/main/java/link/infra/packwiz/installer/ui/IOptionDetails.java

@@ -0,0 +1,8 @@
+package link.infra.packwiz.installer.ui;
+
+public interface IOptionDetails {
+	String getName();
+	boolean getOptionValue();
+	String getOptionDescription();
+	void setOptionValue(boolean value);
+}

src/main/java/link/infra/packwiz/installer/ui/IUserInterface.java

@@ -1,23 +1,38 @@
 package link.infra.packwiz.installer.ui;
 
+import java.util.List;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.Future;
+
 public interface IUserInterface {
 	
-	public void show();
+	void show(InputStateHandler handler);
 
-	public void handleException(Exception e);
-	
-	/**
-	 * This might not exit straight away, return after calling this!
-	 */
-	public default void handleExceptionAndExit(Exception e) {
+	void handleException(Exception e);
+
+	default void handleExceptionAndExit(Exception e) {
 		handleException(e);
 		System.exit(1);
-	};
-	
-	public default void setTitle(String title) {};
+	}
 
-	public void submitProgress(InstallProgress progress);
+	default void setTitle(String title) {}
 
-	public void executeManager(Runnable task);
+	void submitProgress(InstallProgress progress);
+
+	void executeManager(Runnable task);
+
+	// Return true if the installation was cancelled!
+	Future<Boolean> showOptions(List<IOptionDetails> option);
+
+	Future<IExceptionDetails.ExceptionListResult> showExceptions(List<IExceptionDetails> opts, int numTotal, boolean allowsIgnore);
+
+	default void disableOptionsButton() {}
+
+	// Should not return CONTINUE
+	default Future<IExceptionDetails.ExceptionListResult> showCancellationDialog() {
+		CompletableFuture<IExceptionDetails.ExceptionListResult> future = new CompletableFuture<>();
+		future.complete(IExceptionDetails.ExceptionListResult.CANCEL);
+		return future;
+	}
 	
 }

src/main/java/link/infra/packwiz/installer/ui/InputStateHandler.java

@@ -0,0 +1,22 @@
+package link.infra.packwiz.installer.ui;
+
+public class InputStateHandler {
+	private boolean optionsButtonPressed = false;
+	private boolean cancelButtonPressed = false;
+
+	synchronized void pressCancelButton() {
+		this.cancelButtonPressed = true;
+	}
+
+	synchronized void pressOptionsButton() {
+		this.optionsButtonPressed = true;
+	}
+
+	public synchronized boolean getCancelButton() {
+		return cancelButtonPressed;
+	}
+
+	public synchronized boolean getOptionsButton() {
+		return optionsButtonPressed;
+	}
+}

src/main/java/link/infra/packwiz/installer/ui/InstallWindow.java

@@ -1,51 +1,42 @@
 package link.infra.packwiz.installer.ui;
 
-import java.awt.BorderLayout;
-import java.awt.Component;
-import java.awt.EventQueue;
-import java.awt.GridBagConstraints;
-import java.awt.GridBagLayout;
-import java.awt.event.ActionEvent;
-import java.awt.event.ActionListener;
-import java.util.List;
-import java.util.concurrent.atomic.AtomicBoolean;
-
-import javax.swing.JButton;
-import javax.swing.JFrame;
-import javax.swing.JLabel;
-import javax.swing.JOptionPane;
-import javax.swing.JPanel;
-import javax.swing.JProgressBar;
-import javax.swing.UIManager;
+import javax.swing.*;
 import javax.swing.border.EmptyBorder;
+import java.awt.*;
+import java.util.List;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.Future;
+import java.util.concurrent.atomic.AtomicBoolean;
 
 public class InstallWindow implements IUserInterface {
 
 	private JFrame frmPackwizlauncher;
 	private JLabel lblProgresslabel;
 	private JProgressBar progressBar;
+	private InputStateHandler inputStateHandler;
 
 	private String title = "Updating modpack...";
 	private SwingWorkerButWithPublicPublish<Void, InstallProgress> worker;
 	private AtomicBoolean aboutToCrash = new AtomicBoolean();
+	private JButton btnOptions;
 
 	@Override
-	public void show() {
-		EventQueue.invokeLater(new Runnable() {
-			public void run() {
-				try {
-					UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
-					InstallWindow.this.initialize();
-					InstallWindow.this.frmPackwizlauncher.setVisible(true);
-				} catch (Exception e) {
-					e.printStackTrace();
-				}
+	public void show(InputStateHandler handler) {
+		this.inputStateHandler = handler;
+		EventQueue.invokeLater(() -> {
+			try {
+				UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
+				initialize();
+				frmPackwizlauncher.setVisible(true);
+			} catch (Exception e) {
+				e.printStackTrace();
 			}
 		});
 	}
 
 	/**
 	 * Initialize the contents of the frame.
+	 * @wbp.parser.entryPoint
 	 */
 	private void initialize() {
 		frmPackwizlauncher = new JFrame();
@@ -53,43 +44,41 @@ public class InstallWindow implements IUserInterface {
 		frmPackwizlauncher.setBounds(100, 100, 493, 95);
 		frmPackwizlauncher.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
 		frmPackwizlauncher.setLocationRelativeTo(null);
-		
+
 		JPanel panel = new JPanel();
 		panel.setBorder(new EmptyBorder(10, 10, 10, 10));
 		frmPackwizlauncher.getContentPane().add(panel, BorderLayout.CENTER);
 		panel.setLayout(new BorderLayout(0, 0));
-		
+
 		progressBar = new JProgressBar();
 		progressBar.setIndeterminate(true);
 		panel.add(progressBar, BorderLayout.CENTER);
-		
+
 		lblProgresslabel = new JLabel("Loading...");
 		panel.add(lblProgresslabel, BorderLayout.SOUTH);
-		
+
 		JPanel panel_1 = new JPanel();
 		panel_1.setBorder(new EmptyBorder(0, 5, 0, 5));
 		frmPackwizlauncher.getContentPane().add(panel_1, BorderLayout.EAST);
 		GridBagLayout gbl_panel_1 = new GridBagLayout();
 		panel_1.setLayout(gbl_panel_1);
-		
-		JButton btnOptions = new JButton("Options...");
+
+		btnOptions = new JButton("Optional mods...");
+		btnOptions.addActionListener(e -> {
+			btnOptions.setText("Loading...");
+			btnOptions.setEnabled(false);
+			inputStateHandler.pressOptionsButton();
+		});
 		btnOptions.setAlignmentX(Component.CENTER_ALIGNMENT);
 		GridBagConstraints gbc_btnOptions = new GridBagConstraints();
 		gbc_btnOptions.gridx = 0;
 		gbc_btnOptions.gridy = 0;
 		panel_1.add(btnOptions, gbc_btnOptions);
-		
+
 		JButton btnCancel = new JButton("Cancel");
-		btnCancel.addActionListener(new ActionListener() {
-			public void actionPerformed(ActionEvent arg0) {
-				if (worker != null) {
-					worker.cancel(true);
-				}
-				frmPackwizlauncher.dispose();
-				// TODO: show window to ask user what to do
-				System.out.println("Update process cancelled by user!");
-				System.exit(1);
-			}
+		btnCancel.addActionListener(e -> {
+			btnCancel.setEnabled(false);
+			inputStateHandler.pressCancelButton();
 		});
 		btnCancel.setAlignmentX(Component.CENTER_ALIGNMENT);
 		GridBagConstraints gbc_btnCancel = new GridBagConstraints();
@@ -101,10 +90,8 @@ public class InstallWindow implements IUserInterface {
 	@Override
 	public void handleException(Exception e) {
 		e.printStackTrace();
-		EventQueue.invokeLater(new Runnable() {
-			public void run() {
-				JOptionPane.showMessageDialog(null, "An error occurred: \n" + e.getClass().getCanonicalName() + ": " + e.getMessage(), title, JOptionPane.ERROR_MESSAGE);
-			}
+		EventQueue.invokeLater(() -> {
+			JOptionPane.showMessageDialog(null, "An error occurred: \n" + e.getClass().getCanonicalName() + ": " + e.getMessage(), title, JOptionPane.ERROR_MESSAGE);
 		});
 	}
 
@@ -113,28 +100,39 @@ public class InstallWindow implements IUserInterface {
 		e.printStackTrace();
 		// Used to prevent the done() handler of SwingWorker executing if the invokeLater hasn't happened yet
 		aboutToCrash.set(true);
-		EventQueue.invokeLater(new Runnable() {
-			public void run() {
-				JOptionPane.showMessageDialog(null, "A fatal error occurred: \n" + e.getClass().getCanonicalName() + ": " + e.getMessage(), title, JOptionPane.ERROR_MESSAGE);
-				System.exit(1);
-			}
+		EventQueue.invokeLater(() -> {
+			JOptionPane.showMessageDialog(null, "A fatal error occurred: \n" + e.getClass().getCanonicalName() + ": " + e.getMessage(), title, JOptionPane.ERROR_MESSAGE);
+			System.exit(1);
 		});
+		// Pause forever, so it blocks while we wait for System.exit to take effect
+		try {
+			Thread.currentThread().join();
+		} catch (InterruptedException ex) {
+			// no u
+		}
 	}
-	
+
 	@Override
 	public void setTitle(String title) {
 		this.title = title;
 		if (frmPackwizlauncher != null) {
-			EventQueue.invokeLater(new Runnable() {
-				public void run() {
-					InstallWindow.this.frmPackwizlauncher.setTitle(title);
-				}
-			});
+			EventQueue.invokeLater(() -> frmPackwizlauncher.setTitle(title));
 		}
 	}
 
 	@Override
 	public void submitProgress(InstallProgress progress) {
+		StringBuilder sb = new StringBuilder();
+		if (progress.hasProgress) {
+			sb.append('(');
+			sb.append(progress.progress);
+			sb.append('/');
+			sb.append(progress.progressTotal);
+			sb.append(") ");
+		}
+		sb.append(progress.message);
+		// TODO: better logging library?
+		System.out.println(sb.toString());
 		if (worker != null) {
 			worker.publishPublic(progress);
 		}
@@ -142,48 +140,89 @@ public class InstallWindow implements IUserInterface {
 
 	@Override
 	public void executeManager(Runnable task) {
-		EventQueue.invokeLater(new Runnable() {
-			public void run() {
-				worker = new SwingWorkerButWithPublicPublish<Void, InstallProgress>() {
+		EventQueue.invokeLater(() -> {
+			worker = new SwingWorkerButWithPublicPublish<Void, InstallProgress>() {
 
-					@Override
-					protected Void doInBackground() throws Exception {
-						task.run();
-						return null;
-					}
+				@Override
+				protected Void doInBackground() {
+					task.run();
+					return null;
+				}
 
-					@Override
-					protected void process(List<InstallProgress> chunks) {
-						// Only process last chunk
-						if (chunks.size() > 0) {
-							InstallProgress prog = chunks.get(chunks.size() - 1);
-							if (prog.hasProgress) {
-								progressBar.setIndeterminate(false);
-								progressBar.setValue(prog.progress);
-								progressBar.setMaximum(prog.progressTotal);
-							} else {
-								progressBar.setIndeterminate(true);
-								progressBar.setValue(0);
-							}
-							lblProgresslabel.setText(prog.message);
+				@Override
+				protected void process(List<InstallProgress> chunks) {
+					// Only process last chunk
+					if (chunks.size() > 0) {
+						InstallProgress prog = chunks.get(chunks.size() - 1);
+						if (prog.hasProgress) {
+							progressBar.setIndeterminate(false);
+							progressBar.setValue(prog.progress);
+							progressBar.setMaximum(prog.progressTotal);
+						} else {
+							progressBar.setIndeterminate(true);
+							progressBar.setValue(0);
 						}
+						lblProgresslabel.setText(prog.message);
 					}
+				}
 
-					@Override
-					protected void done() {
-						if (aboutToCrash.get()) {
-							return;
-						}
-						// TODO: a better way to do this?
-						frmPackwizlauncher.dispose();
-						System.out.println("Finished successfully!");
-						System.exit(0);
+				@Override
+				protected void done() {
+					if (aboutToCrash.get()) {
+						return;
 					}
-			
-				};
-				worker.execute();
-			}
+					// TODO: a better way to do this?
+					frmPackwizlauncher.dispose();
+					System.out.println("Finished successfully!");
+					System.exit(0);
+				}
+
+			};
+			worker.execute();
 		});
 	}
 
+	@Override
+	public Future<Boolean> showOptions(List<IOptionDetails> opts) {
+		CompletableFuture<Boolean> future = new CompletableFuture<>();
+		EventQueue.invokeLater(() -> {
+			OptionsSelectWindow dialog = new OptionsSelectWindow(opts, future, frmPackwizlauncher);
+			dialog.setDefaultCloseOperation(JDialog.DISPOSE_ON_CLOSE);
+			dialog.setVisible(true);
+		});
+		return future;
+	}
+
+	@Override
+	public Future<IExceptionDetails.ExceptionListResult> showExceptions(List<IExceptionDetails> opts, int numTotal, boolean allowsIgnore) {
+		CompletableFuture<IExceptionDetails.ExceptionListResult> future = new CompletableFuture<>();
+		EventQueue.invokeLater(() -> {
+			ExceptionListWindow dialog = new ExceptionListWindow(opts, future, numTotal, allowsIgnore, frmPackwizlauncher);
+			dialog.setDefaultCloseOperation(JDialog.DISPOSE_ON_CLOSE);
+			dialog.setVisible(true);
+		});
+		return future;
+	}
+
+	@Override
+	public void disableOptionsButton() {
+		if (btnOptions != null) {
+			btnOptions.setText("Optional mods...");
+			btnOptions.setEnabled(false);
+		}
+	}
+
+	@Override
+	public Future<IExceptionDetails.ExceptionListResult> showCancellationDialog() {
+		CompletableFuture<IExceptionDetails.ExceptionListResult> future = new CompletableFuture<>();
+		EventQueue.invokeLater(() -> {
+			Object[] buttons = {"Quit", "Ignore"};
+			int result = JOptionPane.showOptionDialog(frmPackwizlauncher,
+					"The installation was cancelled. Would you like to quit the game, or ignore the update and start the game?",
+					"Cancelled installation",
+					JOptionPane.YES_NO_OPTION, JOptionPane.QUESTION_MESSAGE, null, buttons, buttons[0]);
+			future.complete(result == 0 ? IExceptionDetails.ExceptionListResult.CANCEL : IExceptionDetails.ExceptionListResult.IGNORE);
+		});
+		return future;
+	}
 }

src/main/java/link/infra/packwiz/installer/ui/OptionTempHandler.java

@@ -0,0 +1,33 @@
+package link.infra.packwiz.installer.ui;
+
+// Serves as a proxy for IOptionDetails, so that setOptionValue isn't called until OK is clicked
+class OptionTempHandler implements IOptionDetails {
+	private final IOptionDetails opt;
+	private boolean tempValue;
+
+	OptionTempHandler(IOptionDetails opt) {
+		this.opt = opt;
+		tempValue = opt.getOptionValue();
+	}
+
+	public String getName() {
+		return opt.getName();
+	}
+
+	public String getOptionDescription() {
+		return opt.getOptionDescription();
+	}
+
+	public boolean getOptionValue() {
+		return tempValue;
+	}
+
+	public void setOptionValue(boolean value) {
+		tempValue = value;
+	}
+
+	void finalise() {
+		opt.setOptionValue(tempValue);
+	}
+
+}

src/main/java/link/infra/packwiz/installer/ui/OptionsSelectWindow.java

@@ -0,0 +1,205 @@
+package link.infra.packwiz.installer.ui;
+
+import javax.swing.*;
+import javax.swing.border.EmptyBorder;
+import javax.swing.event.ListSelectionEvent;
+import javax.swing.event.ListSelectionListener;
+import javax.swing.event.TableModelListener;
+import javax.swing.table.TableModel;
+import java.awt.*;
+import java.awt.event.ActionEvent;
+import java.awt.event.ActionListener;
+import java.awt.event.WindowAdapter;
+import java.awt.event.WindowEvent;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.concurrent.CompletableFuture;
+
+public class OptionsSelectWindow extends JDialog implements ActionListener {
+
+	private static final long serialVersionUID = 1L;
+	private final JTextArea lblOptionDescription;
+	private final OptionTableModel tableModel;
+	private final CompletableFuture<Boolean> future;
+
+	/**
+	 * Create the dialog.
+	 */
+	OptionsSelectWindow(List<IOptionDetails> optList, CompletableFuture<Boolean> future, JFrame parentWindow) {
+		super(parentWindow, "Select optional mods...", true);
+
+		tableModel = new OptionTableModel(optList);
+		this.future = future;
+
+		setBounds(100, 100, 450, 300);
+		setLocationRelativeTo(parentWindow);
+		getContentPane().setLayout(new BorderLayout());
+		JPanel contentPanel = new JPanel();
+		contentPanel.setBorder(new EmptyBorder(5, 5, 5, 5));
+		getContentPane().add(contentPanel, BorderLayout.CENTER);
+		contentPanel.setLayout(new BorderLayout(0, 0));
+		{
+			JSplitPane splitPane = new JSplitPane();
+			splitPane.setResizeWeight(0.5);
+			contentPanel.add(splitPane);
+			{
+				JTable table = new JTable();
+				table.setShowVerticalLines(false);
+				table.setShowHorizontalLines(false);
+				table.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);
+				table.setShowGrid(false);
+				table.setModel(tableModel);
+				table.getColumnModel().getColumn(0).setResizable(false);
+				table.getColumnModel().getColumn(0).setPreferredWidth(15);
+				table.getColumnModel().getColumn(0).setMaxWidth(15);
+				table.getColumnModel().getColumn(1).setResizable(false);
+				table.getSelectionModel().addListSelectionListener(new ListSelectionListener() {
+					@Override
+					public void valueChanged(ListSelectionEvent e) {
+						int i = table.getSelectedRow();
+						if (i > -1) {
+							lblOptionDescription.setText(tableModel.getDescription(i));
+						} else {
+							lblOptionDescription.setText("Select an option...");
+						}
+					}
+				});
+				table.setTableHeader(null);
+				JScrollPane scrollPane = new JScrollPane(table);
+				scrollPane.getViewport().setBackground(UIManager.getColor("List.background"));
+				scrollPane.setHorizontalScrollBarPolicy(ScrollPaneConstants.HORIZONTAL_SCROLLBAR_NEVER);
+				scrollPane.setBorder(new EmptyBorder(0, 0, 0, 0));
+				splitPane.setLeftComponent(scrollPane);
+			}
+			{
+				lblOptionDescription = new JTextArea("Select an option...");
+				lblOptionDescription.setBackground(UIManager.getColor("List.background"));
+				lblOptionDescription.setOpaque(true);
+				lblOptionDescription.setWrapStyleWord(true);
+				lblOptionDescription.setLineWrap(true);
+				lblOptionDescription.setEditable(false);
+				lblOptionDescription.setFocusable(false);
+				lblOptionDescription.setFont(UIManager.getFont("Label.font"));
+				lblOptionDescription.setBorder(new EmptyBorder(10, 10, 10, 10));
+				JScrollPane scrollPane = new JScrollPane(lblOptionDescription);
+				scrollPane.setBorder(new EmptyBorder(0, 0, 0, 0));
+				splitPane.setRightComponent(scrollPane);
+			}
+		}
+		{
+			JPanel buttonPane = new JPanel();
+			buttonPane.setLayout(new FlowLayout(FlowLayout.RIGHT));
+			getContentPane().add(buttonPane, BorderLayout.SOUTH);
+			{
+				JButton okButton = new JButton("OK");
+				okButton.setActionCommand("OK");
+				okButton.addActionListener(this);
+				buttonPane.add(okButton);
+				getRootPane().setDefaultButton(okButton);
+			}
+			{
+				JButton cancelButton = new JButton("Cancel");
+				cancelButton.setActionCommand("Cancel");
+				cancelButton.addActionListener(this);
+				buttonPane.add(cancelButton);
+			}
+		}
+		addWindowListener(new WindowAdapter() {
+			@Override
+			public void windowClosing(WindowEvent e) {
+				future.complete(true);
+			}
+
+			@Override
+			public void windowClosed(WindowEvent e) {
+				// Just in case closing didn't get triggered - if something else called dispose() the
+				// future will have already completed
+				future.complete(true);
+			}
+		});
+	}
+
+	private static class OptionTableModel implements TableModel {
+		private List<OptionTempHandler> opts = new ArrayList<>();
+
+		OptionTableModel(List<IOptionDetails> givenOpts) {
+			for (IOptionDetails opt : givenOpts) {
+				opts.add(new OptionTempHandler(opt));
+			}
+		}
+
+		@Override
+		public int getRowCount() {
+			return opts.size();
+		}
+
+		@Override
+		public int getColumnCount() {
+			return 2;
+		}
+
+		private final String[] columnNames = {"Enabled", "Mod name"};
+		private final Class<?>[] columnTypes = {Boolean.class, String.class};
+		private final boolean[] columnEditables = {true, false};
+
+		@Override
+		public String getColumnName(int columnIndex) {
+			return columnNames[columnIndex];
+		}
+
+		@Override
+		public Class<?> getColumnClass(int columnIndex) {
+			return columnTypes[columnIndex];
+		}
+
+		@Override
+		public boolean isCellEditable(int rowIndex, int columnIndex) {
+			return columnEditables[columnIndex];
+		}
+
+		@Override
+		public Object getValueAt(int rowIndex, int columnIndex) {
+			OptionTempHandler opt = opts.get(rowIndex);
+			return columnIndex == 0 ? opt.getOptionValue() : opt.getName();
+		}
+
+		@Override
+		public void setValueAt(Object aValue, int rowIndex, int columnIndex) {
+			if (columnIndex == 0) {
+				OptionTempHandler opt = opts.get(rowIndex);
+				opt.setOptionValue((boolean) aValue);
+			}
+		}
+
+		// Noop, the table model doesn't change!
+		@Override
+		public void addTableModelListener(TableModelListener l) {}
+
+		@Override
+		public void removeTableModelListener(TableModelListener l) {}
+
+		String getDescription(int rowIndex) {
+			return opts.get(rowIndex).getOptionDescription();
+		}
+
+		void finalise() {
+			for (OptionTempHandler opt : opts) {
+				opt.finalise();
+			}
+		}
+
+	}
+
+	@Override
+	public void actionPerformed(ActionEvent e) {
+		if (e.getActionCommand().equals("OK")) {
+			tableModel.finalise();
+			future.complete(false);
+			dispose();
+		} else if (e.getActionCommand().equals("Cancel")) {
+			future.complete(true);
+			dispose();
+		}
+	}
+
+}