2 files changed, 218 insertions, 0 deletions
diff --git a/undo/09-test-undo.js b/undo/09-test-undo.js
new file mode 100644
index 0000000..2cceb55
--- /dev/null
+++ b/undo/09-test-undo.js
@@ -0,0 +1,165 @@
+var assert = require("assert")
+var UndoStack = require("./undo.js")
+UndoStack.debug = false
+
+describe('undo', function(){
+  
+  var state = "zero"
+  
+  describe('#register()', function(){
+  
+    UndoStack.register({
+      type: "demo",
+      undo: function(myState){
+        state = myState
+      },
+      redo: function(myState){
+        state = myState
+      },
+    })
+
+    it('registers undoable actions', function(){
+      assert( UndoStack.types.hasOwnProperty("demo") )
+    })
+  })
+  
+  describe('#push()', function(){
+    
+    it('starts empty', function(){
+      assert.equal(0, UndoStack.stack.length)
+      assert.equal(-1, UndoStack.pointer)
+    })
+    
+    it('pushes some actions', function(){
+
+      UndoStack.push({
+        type: "demo",
+        undo: state,
+        redo: "one"
+      })
+      state = "one"
+
+      UndoStack.push({
+        type: "demo",
+        undo: state,
+        redo: "two"
+      })
+      state = "two"
+
+      UndoStack.push({
+        type: "demo",
+        undo: state,
+        redo: "three"
+      })
+      state = "three"
+    
+      assert.equal(3, UndoStack.stack.length)
+      assert.equal(2, UndoStack.pointer)
+    })
+  })
+  
+  describe('#undo()', function(){
+  
+    it('retrieves old state', function(){
+      assert.equal("three", state)
+      UndoStack.undo()
+      assert.equal("two", state)
+      assert.equal(1, UndoStack.pointer)
+    })
+    
+    it('can only undo so far', function(){
+      var canUndo
+
+      canUndo = UndoStack.undo()
+      assert.equal("one", state)
+      assert.equal(0, UndoStack.pointer)
+      assert.equal(true, canUndo)
+
+      canUndo = UndoStack.undo()
+      assert.equal("zero", state)
+      assert.equal(-1, UndoStack.pointer)
+      assert.equal(false, canUndo)
+      
+      canUndo = UndoStack.undo()
+      assert.equal("zero", state)
+      assert.equal(-1, UndoStack.pointer)
+      assert.equal(false, canUndo)
+    })
+  
+  })
+
+  describe('#redo()', function(){
+
+    it('reassigns new state', function(){
+      UndoStack.redo()
+      assert.equal("one", state)
+    })
+    
+    it('can only redo so far', function(){
+      var canRedo
+      
+      canRedo = UndoStack.redo()
+      assert.equal("two", state)
+      assert.equal(true, canRedo)
+
+      canRedo = UndoStack.redo()
+      assert.equal("three", state)
+      assert.equal(false, canRedo)
+
+      canRedo = UndoStack.redo()
+      assert.equal("three", state)
+      assert.equal(false, canRedo)
+    })
+
+    it('clobbers old state if we undo then do something else', function(){
+      UndoStack.undo()
+      assert.equal("two", state)
+      assert.equal(1, UndoStack.pointer)
+
+      UndoStack.undo()
+      assert.equal("one", state)
+      assert.equal(0, UndoStack.pointer)
+      assert.equal(3, UndoStack.stack.length)
+
+      UndoStack.push({
+        type: "demo",
+        undo: state,
+        redo: "four"
+      })
+      state = "four"
+
+      assert.equal(1, UndoStack.pointer)
+      assert.equal(2, UndoStack.stack.length)
+
+      UndoStack.undo()
+      assert.equal("one", state)
+      assert.equal(0, UndoStack.pointer)
+      assert.equal(2, UndoStack.stack.length)
+
+      UndoStack.redo()
+      assert.equal("four", state)
+      assert.equal(1, UndoStack.pointer)
+      assert.equal(2, UndoStack.stack.length)
+    })
+  })
+  
+})
+
+// 1) push action
+// 2) push action
+// 3) push action
+// undo 3
+// undo 2
+// undo 1
+// undo * can't undo anymore
+// redo 1
+// redo 2
+// redo 3
+// redo * can't redo anymore
+// undo 3
+// 4) push action (clobbers action 3)
+// undo 4
+// undo 2
+// redo 2
+// redo 4
+
diff --git a/undo/undostack.js b/undo/undostack.js
new file mode 100644
index 0000000..5ba97e1
--- /dev/null
+++ b/undo/undostack.js
@@ -0,0 +1,53 @@
+(function(){
+
+  var UndoStack = function(){
+    this.debug = true
+    this.stack = []
+    this.types = {}
+    this.pointer = -1
+  }
+  UndoStack.prototype.push = function(action){
+    this.pointer++
+    this.stack[this.pointer] = action
+    this.purge()
+  }
+  UndoStack.prototype.purge = function(){
+    if (this.stack.length-1 == this.pointer) return
+    this.stack.length = this.pointer+1
+  }
+  UndoStack.prototype.undo = function(){
+    if (this.pointer == -1) return false
+    var action = this.stack[this.pointer]
+    this.debug && console.log("undo", action.type)
+    this.types[ action.type ].undo(action.undo)
+    this.pointer--
+    return this.pointer > -1
+  }
+  UndoStack.prototype.redo = function(){
+    if (this.pointer == this.stack.length-1) return false
+    this.pointer++
+    var action = this.stack[this.pointer]
+    this.debug && console.log("redo", action.type)
+    this.types[ action.type ].redo(action.redo)
+    return this.pointer < this.stack.length-1
+  }
+  UndoStack.prototype.register = function(actionType){
+  	if (actionType.length) {
+  		actionType.forEach(this.registerOne.bind(this))
+  	}
+  	else {
+  		this.registerOne(actionType)
+		}
+  }
+  UndoStack.prototype.registerOne = function(actionType){
+		this.types[ actionType.type ] = actionType
+	}  
+  if ('window' in this) {
+    window.UndoStack = new UndoStack
+  }
+  else {
+    module.exports = new UndoStack
+  }
+
+})()
+