iD/js/id/graph/history.js

iD.History = function() {
    if (!(this instanceof iD.History)) return new iD.History();
    this.stack = [iD.Graph()];
    this.index = 0;
};

iD.History.prototype = {
    graph: function() {
        return this.stack[this.index];
    },

    merge: function(graph) {
        for (var i = 0; i < this.stack.length; i++) {
            this.stack[i] = this.stack[i].merge(graph);
        }
    },

    perform: function(action) {
        this.stack = this.stack.slice(0, this.index + 1);
        this.stack.push(action(this.graph()));
        this.index++;
    },

    replace: function(action) {
        // assert(this.index == this.stack.length - 1)
        this.stack[this.index] = action(this.graph());
    },

    undo: function() {
        while (this.index > 0) {
            this.index--;
            if (this.stack[this.index].annotation) break;
        }
    },

    redo: function() {
        while (this.index < this.stack.length - 1) {
            this.index++;
            if (this.stack[this.index].annotation) break;
        }
    },

    undoAnnotation: function() {
        var index = this.index;
        while (index >= 0) {
            if (this.stack[index].annotation) return this.stack[index].annotation;
            index--;
        }
    },

    redoAnnotation: function() {
        var index = this.index + 1;
        while (index <= this.stack.length - 1) {
            if (this.stack[index].annotation) return this.stack[index].annotation;
            index++;
        }
    },

    // generate reports of changes for changesets to use
    modify: function() {
        return this.stack[this.index].modifications();
    },

    create: function() {
        return this.stack[this.index].creations();
    },

    'delete': function() {
        return _.difference(
            _.pluck(this.stack[0].entities, 'id'),
            _.pluck(this.stack[this.index].entities, 'id')
            ).map(function(id) {
                return this.stack[0].entity(id);
            }.bind(this));
    },

    changes: function() {
        return {
            modify: this.modify(),
            create: this.create(),
            'delete': this['delete']()
        };
    }
};