context_tree.js

var __contexts = 0;  // Just for debugging.

class Context {
    constructor(parent, transaction=false) {
        this._data = {};
        this._children = [];
        this._parent = parent;
        this._transaction = transaction;
        this._revision = __contexts++;
    }
    get(k) {
        if (this._data[k] !== undefined) return this._data[k];
        if (this._parent) return this._parent.get(k);
    }
    set(k, v) {
        var fork = (this._transaction) ? this : this.fork();
        if (v == undefined && k instanceof Object) {
            for (let j in k) fork._data[j] = k[j];
        } else fork._data[k] = v;
        return fork;
    }
    sub(k, n=1) {
        if (this.get(k) < n) this.raise("Insufficient " + k);
        return this.add(k, n*-1);
    }
    add(k, n=1) {
        return this.set(k, this.get(k)+n);
    }
    transfer(a, b, n=1) {
        if (n < 0) return this.transfer(b, a, n);
        if (this.get(a) < n) this.raise("Insufficient " + a);
        var fork = (this._transaction) ? this : this.fork();
        fork._data[a] = this.get(a) - n;
        fork._data[b] = this.get(b) + n;
        return fork;
    }
    transaction() {
        return this.fork(true);
    }
    commit() {
        if (!this._transaction) throw "Not a transaction.";
        this._transaction = false;
        return this;
    }
    fork(transaction=false) {
        var child = new this.constructor(this, transaction);
        this._children.push(child);
        return child;
    }
    tree(depth=0) {
        var d = JSON.parse(JSON.stringify(this._data));
        if (this._transaction) d['___TRANSACTION__'] = true;
        var str = '-'+this._revision+":"+JSON.stringify(d)+"\n";
        for (let c of this._children) {
            for (let i=0;i<=depth;i++) str += "-";
            str += c.tree(depth+1);
        }
        return str;
    }
    dump() {
        var d = {};
        for (let k in this._data) d[k] = this._data[k];
        if (this._parent) {
            let p = this._parent.dump();
            for (let k in p) if (d[k] === undefined) d[k] = p[k];
        }
        return JSON.parse(JSON.stringify(d));
    }
}

var a = new Context(),
    b = a.set('foo', 1),
      c = b.set('foo', 2),
    d = a.set('foo', 3),
    e = a.set('bar', 4),
      f = e.set('foo', 5),
        g = f.set({'bizz':3, 'foo':6});

var h = a.transaction();
h.set('foo', 7);
h.set('bar', 1);
h.transfer('foo', 'bar', 3);

console.log(a.tree());

/*
-0:{}
--1:{"foo":1}
---2:{"foo":2}
--3:{"foo":3}
--4:{"bar":4}
---5:{"foo":5}
----6:{"bizz":3,"foo":6}
--7:{"foo":4,"bar":4,"___TRANSACTION__":true}
*/

h.commit();

var i = h.set('foo', 1),
    j = i.transfer('foo', 'bar', 1);

console.log(a.tree());

/*
-0:{}
--1:{"foo":1}
---2:{"foo":2}
--3:{"foo":3}
--4:{"bar":4}
---5:{"foo":5}
----6:{"bizz":3,"foo":6}
--7:{"foo":4,"bar":4}
---8:{"foo":1,"bar"}
----9:{"foo":0,"bar":5}
*/

console.log('b', b.dump())  // {foo: 1}
console.log('f', f.dump())  // {foo: 3, bar: 5}
console.log('g', g.dump()); // {bizz: 3, foo: 6, bar: 5}
console.log('h', h.dump()); // {foo: 4, bar: 4}
console.log('i', i.dump()); // {foo: 1, bar: 4}
console.log('j', j.dump()); // {foo: 0, bar: 5}