1 files changed, 45 insertions, 0 deletions
diff --git a/addon-sdk/source/lib/diffpatcher/diff.js b/addon-sdk/source/lib/diffpatcher/diff.js
new file mode 100644
index 000000000..967c137e6
--- /dev/null
+++ b/addon-sdk/source/lib/diffpatcher/diff.js
@@ -0,0 +1,45 @@
+"use strict";
+
+var method = require("../method/core")
+
+// Method is designed to work with data structures representing application
+// state. Calling it with a state should return object representing `delta`
+// that has being applied to a previous state to get to a current state.
+//
+// Example
+//
+// diff(state) // => { "item-id-1": { title: "some title" } "item-id-2": null }
+var diff = method("diff@diffpatcher")
+
+// diff between `null` / `undefined` to any hash is a hash itself.
+diff.define(null, function(from, to) { return to })
+diff.define(undefined, function(from, to) { return to })
+diff.define(Object, function(from, to) {
+  return calculate(from, to || {}) || {}
+})
+
+function calculate(from, to) {
+  var diff = {}
+  var changes = 0
+  Object.keys(from).forEach(function(key) {
+    changes = changes + 1
+    if (!(key in to) && from[key] != null) diff[key] = null
+    else changes = changes - 1
+  })
+  Object.keys(to).forEach(function(key) {
+    changes = changes + 1
+    var previous = from[key]
+    var current = to[key]
+    if (previous === current) return (changes = changes - 1)
+    if (typeof(current) !== "object") return diff[key] = current
+    if (typeof(previous) !== "object") return diff[key] = current
+    var delta = calculate(previous, current)
+    if (delta) diff[key] = delta
+    else changes = changes - 1
+  })
+  return changes ? diff : null
+}
+
+diff.calculate = calculate
+
+module.exports = diff