DiffProcessor: detect removed values which are added again

In this case, fuse the removal and addition operation into a move and remove the
removal operation.

Author: fge

src/main/java/com/github/fge/jsonpatch/diff2/DiffProcessor.java

@@ -60,6 +60,14 @@ void valueRemoved(final JsonPointer pointer, final JsonNode value)
 
     void valueAdded(final JsonPointer pointer, final JsonNode value)
     {
+        final int removalIndex = findPreviouslyRemoved(value);
+        if (removalIndex != -1) {
+            final DiffOperation removed = diffs.get(removalIndex);
+            diffs.remove(removalIndex);
+            diffs.add(DiffOperation.move(removed.getOldPointer(),
+                value, pointer, value));
+            return;
+        }
         final JsonPointer ptr = findUnchangedValue(value);
         final DiffOperation op = ptr != null
             ? DiffOperation.copy(ptr, pointer, value)
@@ -87,4 +95,19 @@ private JsonPointer findUnchangedValue(final JsonNode value)
                 return entry.getKey();
         return null;
     }
+
+    private int findPreviouslyRemoved(final JsonNode value)
+    {
+        final Predicate<JsonNode> predicate = EQUIVALENCE.equivalentTo(value);
+
+        DiffOperation op;
+
+        for (int i = 0; i < diffs.size(); i++) {
+            op = diffs.get(i);
+            if (op.getType() == DiffOperation.Type.REMOVE
+                && predicate.apply(op.getOldValue()))
+                return i;
+        }
+        return -1;
+    }
 }

src/test/resources/jsonpatch/diff2/diff.json

@@ -126,5 +126,13 @@
         "patch": [
             { "op": "copy", "path": "/d", "from": "/a" }
         ]
+    },
+    {
+        "message": "similar element removed then added is moved instead",
+        "first": { "a": "b" },
+        "second": { "c": "b" },
+        "patch": [
+            { "op": "move", "path": "/c", "from": "/a" }
+        ]
     }
 ]