michael@0: "use strict";
michael@0: 
michael@0: var method = require("../method/core")
michael@0: 
michael@0: // Method is designed to work with data structures representing application
michael@0: // state. Calling it with a state should return object representing `delta`
michael@0: // that has being applied to a previous state to get to a current state.
michael@0: //
michael@0: // Example
michael@0: //
michael@0: // diff(state) // => { "item-id-1": { title: "some title" } "item-id-2": null }
michael@0: var diff = method("diff@diffpatcher")
michael@0: 
michael@0: // diff between `null` / `undefined` to any hash is a hash itself.
michael@0: diff.define(null, function(from, to) { return to })
michael@0: diff.define(undefined, function(from, to) { return to })
michael@0: diff.define(Object, function(from, to) {
michael@0:   return calculate(from, to || {}) || {}
michael@0: })
michael@0: 
michael@0: function calculate(from, to) {
michael@0:   var diff = {}
michael@0:   var changes = 0
michael@0:   Object.keys(from).forEach(function(key) {
michael@0:     changes = changes + 1
michael@0:     if (!(key in to) && from[key] != null) diff[key] = null
michael@0:     else changes = changes - 1
michael@0:   })
michael@0:   Object.keys(to).forEach(function(key) {
michael@0:     changes = changes + 1
michael@0:     var previous = from[key]
michael@0:     var current = to[key]
michael@0:     if (previous === current) return (changes = changes - 1)
michael@0:     if (typeof(current) !== "object") return diff[key] = current
michael@0:     if (typeof(previous) !== "object") return diff[key] = current
michael@0:     var delta = calculate(previous, current)
michael@0:     if (delta) diff[key] = delta
michael@0:     else changes = changes - 1
michael@0:   })
michael@0:   return changes ? diff : null
michael@0: }
michael@0: 
michael@0: diff.calculate = calculate
michael@0: 
michael@0: module.exports = diff