Revert to previous merge method

richtextfx/src/main/java/org/fxmisc/richtext/model/TextChange.java

@@ -5,39 +5,14 @@
 
 public abstract class TextChange<S, Self extends TextChange<S, Self>> {
 
-    /**
-     * Indicates whether this change can be merged with another change if they are both {@link #INSERTION} or
-     * both {@link #DELETION}.
-     */
-    public static enum MergeType {
-        /** Indicates that the change will insert something but not remove anything */
-        INSERTION,
-        /** Indicates that the change will delete something but not insert anything */
-        DELETION,
-        /** Indicates that the change is a style change, a replacement or something other than the other two types */
-        NONE,
-    }
-
-
     protected final int position;
     protected final S removed;
     protected final S inserted;
-    protected final MergeType mergeType;
 
     public TextChange(int position, S removed, S inserted) {
         this.position = position;
         this.removed = removed;
         this.inserted = inserted;
-
-        if (insertedLength() == 0) {
-            mergeType = removedLength() != 0
-                    ? MergeType.DELETION
-                    : MergeType.NONE;
-        } else if (removedLength() == 0) {
-            mergeType = MergeType.INSERTION;
-        } else {
-            mergeType = MergeType.NONE;
-        }
     }
 
     public int getPosition() { return position; };
@@ -46,7 +21,6 @@ public TextChange(int position, S removed, S inserted) {
     public Self invert() { return create(position, inserted, removed); }
     public int getRemovalEnd() { return position + removedLength(); }
     public int getInsertionEnd() { return position + insertedLength(); }
-    public final MergeType getMergeType() { return mergeType; };
 
     protected abstract int removedLength();
     protected abstract int insertedLength();
@@ -63,20 +37,32 @@ public final boolean isIdentity() {
     }
 
     /**
-     * Merges this change with the given change only if the end of this change's inserted text equals the
-     * latter's position and both are either {@link MergeType#INSERTION} or {@link MergeType#DELETION} changes.
-     *
+     * Merges this change with the given change, if possible.
+     * This change is considered to be the former and the given
+     * change is considered to be the latter.
+     * Changes can be merged if either
+     * <ul>
+     *   <li>the latter's start matches the former's added text end; or</li>
+     *   <li>the latter's removed text end matches the former's added text end.</li>
+     * </ul>
      * @param latter change to merge with this change.
      * @return a new merged change if changes can be merged,
      * {@code null} otherwise.
      */
     public Optional<Self> mergeWith(Self latter) {
-        if((this.mergeType == MergeType.INSERTION || this.mergeType == MergeType.DELETION)
-                && this.mergeType == latter.mergeType
-                && this.getInsertionEnd() == latter.position) {
+        if(latter.position == this.position + this.insertedLength()) {
             S removedText = concat(this.removed, latter.removed);
             S addedText = concat(this.inserted, latter.inserted);
             return Optional.of(create(this.position, removedText, addedText));
+        } else if(latter.position + latter.removedLength() == this.position + this.insertedLength()) {
+            if(this.position <= latter.position) {
+                S addedText = concat(sub(this.inserted, 0, latter.position - this.position), latter.inserted);
+                return Optional.of(create(this.position, this.removed, addedText));
+            }
+            else {
+                S removedText = concat(sub(latter.removed, 0, this.position - latter.position), this.removed);
+                return Optional.of(create(latter.position, removedText, latter.inserted));
+            }
         } else {
             return Optional.empty();
         }
@@ -104,7 +90,6 @@ public final String toString() {
         return
                 this.getClass().getSimpleName() + "{\n" +
                 "\tposition: "  + position  + "\n" +
-                "\tmergeType: " + mergeType + "\n" +
                 "\tremoved: "   + removed   + "\n" +
                 "\tinserted: "  + inserted  + "\n" +
                 "}";