From fe1009187c3e2b3bd1e3375154a15d6f6cf8777b Mon Sep 17 00:00:00 2001
From: DieGurke <maxi@kske.dev>
Date: Tue, 7 Dec 2021 19:13:51 +0100
Subject: [PATCH] New change interface and changeManager class

---
 src/main/java/dev/kske/undoredo/Change.java   |  33 ++++++
 .../java/dev/kske/undoredo/ChangeManager.java | 100 ++++++++++++++++++
 2 files changed, 133 insertions(+)
 create mode 100644 src/main/java/dev/kske/undoredo/Change.java
 create mode 100644 src/main/java/dev/kske/undoredo/ChangeManager.java

diff --git a/src/main/java/dev/kske/undoredo/Change.java b/src/main/java/dev/kske/undoredo/Change.java
new file mode 100644
index 0000000..6f88984
--- /dev/null
+++ b/src/main/java/dev/kske/undoredo/Change.java
@@ -0,0 +1,33 @@
+package dev.kske.undoredo;
+
+/**
+ * Base interface for changes to be registered in an undo manager.
+ * 
+ * @author Maximilian K&auml;fer
+ * @since 0.0.1
+ */
+public interface Change {
+
+	/**
+	 * Performs the action implemented by this change.
+	 *
+	 * @since 0.0.1
+	 */
+	void apply();
+
+	/**
+	 * Inverts this change.
+	 * 
+	 * @implSpec This method is not supposed to alter the state of this change, but rather to create
+	 *           a new complementary change.
+	 * @return the inverted change
+	 * @since 0.0.1
+	 */
+	Change invert();
+
+	/**
+	 * @return whether the application of this change would result in an identical state
+	 * @since 0.0.1
+	 */
+	boolean isIdentity();
+}
diff --git a/src/main/java/dev/kske/undoredo/ChangeManager.java b/src/main/java/dev/kske/undoredo/ChangeManager.java
new file mode 100644
index 0000000..8f2812e
--- /dev/null
+++ b/src/main/java/dev/kske/undoredo/ChangeManager.java
@@ -0,0 +1,100 @@
+package dev.kske.undoredo;
+
+import java.util.*;
+
+/**
+ * @param <C> the change types to store in this change manager
+ * @author Maximilian K&auml;fer
+ * @since 0.0.1
+ */
+public final class ChangeManager<C extends Change> {
+
+	private final List<C> changes = new LinkedList<>();
+
+	private int	index;
+	private int	markedIndex;
+
+	/**
+	 * Adds a change to the changes list.
+	 * 
+	 * @param change the change to add
+	 * @since 0.0.1
+	 */
+	public void addChange(C change) {
+		change.apply();
+		changes.add(change);
+	}
+
+	/**
+	 * Undoes the change at the current index position.
+	 * 
+	 * @return whether the operation could be executed due to one being currently available
+	 * @since 0.1.0
+	 */
+	public boolean undo() {
+		if (isUndoAvailable()) {
+			changes.get(index).invert().apply();
+			--index;
+			return true;
+		}
+		return false;
+	}
+
+	/**
+	 * Applies the change that was undone before.
+	 * 
+	 * @return whether the operation could be executed due to one being currently available
+	 * @since 0.0.1
+	 */
+	public boolean redo() {
+		if (isRedoAvailable()) {
+			changes.get(index + 1).apply();
+			++index;
+			return true;
+		}
+		return false;
+	}
+
+	/**
+	 * Marks the current index.
+	 * 
+	 * @since 0.0.1
+	 */
+	public void mark() {
+		markedIndex = index;
+	}
+
+	/**
+	 * @return whether the current index was marked
+	 * @since 0.0.1
+	 */
+	public boolean isAtMarkedIndex() {
+		return markedIndex == index;
+	}
+
+	/**
+	 * @return whether the undo operation is currently available
+	 * @since 0.0.1
+	 */
+	public boolean isUndoAvailable() {
+		return index > 0;
+	}
+
+	/**
+	 * @return whether the redo operation is currently available.
+	 * @since 0.0.1
+	 */
+	public boolean isRedoAvailable() {
+		return index < changes.size() - 1;
+	}
+
+	/**
+	 * Provides an unmodifiable view of the changes stored in this change manager.
+	 * 
+	 * @return all stored changes
+	 * @since 0.0.1
+	 */
+	public List<C> getChanges() {
+		return Collections.unmodifiableList(changes);
+	}
+}