/*
 * 	6502 Based Virtual Computer
 *	Copyright (C) 2011  Scott C. Duensing <scott@jaegertech.com>
 *	
 *	This program is free software; you can redistribute it and/or modify
 *	it under the terms of the GNU General Public License as published by
 *	the Free Software Foundation; either version 2 of the License, or
 *	(at your option) any later version.
 *	
 *	This program is distributed in the hope that it will be useful,
 *	but WITHOUT ANY WARRANTY; without even the implied warranty of
 *	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *	GNU General Public License for more details.
 *	
 *	You should have received a copy of the GNU General Public License along
 *	with this program; if not, write to the Free Software Foundation, Inc.,
 *	51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
 */

// Our Psuedo-Random Number Generator object.
function prng() {

	// --- Private variables.

	var MAX               = Math.pow(2, 32);
	var SEED              = Math.round(Math.random() * MAX);

	// Memory location for PRNG value.
	var MEMORY            = null;
	var MEMORY_START      = 0;
	var PRNG_ADDRESS      = 0;
	

	// --- Public methods.

	// Attach PRNG to DOM.
	this.attach = function(newMemory, newStartPosition) {
		// We create our own PRN to ensure recreatability between sessions and browsers.
		// http://weblog.bocoup.com/random-numbers/   https://gist.github.com/Protonk/5367430
		MEMORY       = newMemory;
		MEMORY_START = newStartPosition;
		PRNG_ADDRESS = MEMORY_START;
		LAST_RANDOM  = Math.random();
		MEMORY.addReadCallback(PRNG_ADDRESS, PRNG_ADDRESS, null, function(address, value, data){
			// On read, change the random value.  Based on WebKit2's invertible mapping generator.
			SEED += (SEED * SEED) | 5;
			var value = (SEED >>> 32) / MAX;
			MEMORY.writeByte(address, Math.floor(value * 256));
		});
	};
	
	// Deserialize PRNG state.
	this.deserialize = function(state) {
		SEED = state.S;
	};

	// Serialize PRNG state.
	this.serialize = function() {
		var state = {
			S: SEED
		};
		return state;
	};

}