for Mike, unfinished, untested, 20130923

cells.js

var Sheet = (function () {
    "use strict";
    var Integrity = []; // XXX TODO add integrity togglage -sh
    var CellStack = [];

    function BaseCell(formula) {
        this._cached_value = null;
        this._formula = formula;
        this._fresh = false;
        this._deps = [];
    }

    BaseCell.prototype.value = function () {
        if (this._fresh) {
            if (CellStack.length > 0) {
                CellStack[CellStack.length - 1].deps.push(this);
            }
            return this._cached_value;
        }
        else {
            var deps = {deps: []};
            CellStack.push(deps);
            var ret = this._formula();
            CellStack.pop();
            // XXX do some circularity checks to avoid stack overflow -sh
            this._cached_value = ret;
            this._fresh = true;
            this._deps = deps;
            if (this._filter_value(ret)) {
                this._pre_update(ret);
                for (var x in deps) {
                    //noinspection JSUnfilteredForInLoop
                    x._invalidate();
                }
                this._post_update();
            }
            return ret;
        }
    };
    BaseCell.prototype._invalidate = function() {
        self._fresh = false;
        for (var x in self._deps) {
            // NB all are derived from BaseCell -sh
            //noinspection JSUnfilteredForInLoop
            x._invalidate();
        }
    };
    BaseCell.prototype._filter_value = function(datum) {
        return datum;
    };
    BaseCell.prototype._pre_update = function() {
        // for events -sh
    };
    BaseCell.prototype._post_update = function() {
        // NB this is for events
        // typical usage, invalidate deps, force them to recompute,
        // revert back to null value -sh
    };
})();