reverse diffs

index.js

@@ -1,7 +1,9 @@
 const diff = require('./diff');
 const patch = require('./patch');
+const reverse = require('./reverse');
 
 module.exports = {
   diff,
   patch,
+  reverse,
 };

reverse.js

@@ -0,0 +1,47 @@
+const { _clone, _objId } = require('./helpers');
+
+function reverse(changes) {
+
+  const reversed = _clone(changes).reverse();
+
+  for (const change of reversed) {
+
+    if (change.op == 'add') {
+      change.op = 'remove';
+      const id = _objId(change.value);
+      if (id) {
+        change._index = change.path.split('/').pop();
+        change.path = change.path.replace(/\d+$/, `[${id}]`);
+      }
+
+    } else if (change.op == 'remove') {
+      change.op = 'add';
+    }
+
+    if ('_prev' in change) {
+      var _prev = change._prev;
+    }
+
+    if ('value' in change) {
+      var _value = change.value;
+    }
+
+    if (_prev === undefined) {
+      delete change.value;
+    } else {
+      change.value = _prev;
+    }
+
+    if (_value === undefined) {
+      delete change._prev;
+    } else {
+      change._prev = _value;
+    }
+
+  }
+
+  return reversed;
+
+}
+
+module.exports = reverse;

test.js

@@ -2,6 +2,7 @@ const assert = require('assert');
 
 const diff = require('./diff');
 const patch = require('./patch');
+const reverse = require('./reverse');
 
 /* DIFF */
 
@@ -26,7 +27,7 @@ assert.deepEqual(
   [ { op: 'remove', path: '/title', _prev: 'hello' } ]
 );
 
-// object property remove
+// object property replace
 assert.deepEqual(
   diff({ id: 1, title: 'hello' }, { id: 1, title: 'salut' }),
   [ { op: 'replace', path: '/title', value: 'salut', _prev: 'hello' } ]
@@ -147,6 +148,19 @@ assert.deepEqual(
 );
 
 
+/* REVERSE */
 
+assert.deepEqual(
+  reverse(
+    [ { op: 'replace', path: '/name', value: 'henry', _prev: 'hank' } ]
+  ),
+  [ { op: 'replace', path: '/name', value: 'hank', _prev: 'henry' } ]
+);
 
+assert.deepEqual(
+  reverse(
+    [ { op: 'add', path: '/names/0', value: { id: 38, name: 'henry' } } ]
+  ),
+  [ { op: 'remove', path: '/names/[38]', _index: 0, _prev: { id: 38, name: 'henry' } } ]
+);