Minor bug fixing and commenting in lib/randomSupplies.js

Fixes #76

lib/randomSupplies.js

@@ -5,22 +5,30 @@
 (function(){
 	"use strict";
 	
-	const persistentRnd = {};
+	const persistentRnd = Object.create(null);
 	exports.persistent = {
 		getRng: function(length, window){
 			var domain = window.document.location.host;
 			if (!persistentRnd[domain]){
+				// create the (sub-)domains random numbers if not existing
 				persistentRnd[domain] = new Uint8Array(128);
-				for (var i = 0; i < persistentRnd[domain].length; i += 1){
-					persistentRnd[domain][i] = Math.floor(Math.random() * 0xFF);
-				}
+				window.crypto.getRandomValues(persistentRnd[domain]);
 			}
 			var bitSet = persistentRnd[domain];
 			
 			return function(value, i){
+				// use the last 7 bits from the value for the index of the
+				// random number
 				var index = value & 0x7F;
+				
+				// use the last 3 bits from the position and the first bit from
+				// from the value to get bit to use from the random number
 				var bitIndex = ((i & 0x03) << 1) | (value >>> 7);
+				
+				// extract the bit
 				var bit = (bitSet[index] >>> bitIndex) & 0x01;
+				
+				// XOR the bit the the value to alter the last bit of it... or not
 				return value ^ bit;
 			};
 		}
@@ -28,20 +36,24 @@
 	
 	exports.nonPersistent = {
 		getRng: function(length, window){
+			// Initialize the random number batch creation
 			var randomI = 65536;
-			// var randomOffset = 0;
 			var randomNumbers = new Uint8Array(Math.min(65536, length));
 			
 			return function(value, i){
 				if (randomI >= randomNumbers.length){
+					// refill the random number bucket if empty
 					randomI = 0;
-					// randomOffset += randomNumbers.length;
 					if (length - i < 65536){
 						randomNumbers = new Uint8Array(length - i);
 					}
 					window.crypto.getRandomValues(randomNumbers);
 				}
 				var rnd = randomNumbers[randomI];
+				randomI += 1;
+				
+				// do not alter the most significant bit that is set and
+				// the bit after it, to not disturb the image too much.
 				if (value >= 0x80){
 					value = value ^ (rnd & 0x1F);
 				}
@@ -60,7 +72,6 @@
 				// else if (value >= 0x04){
 					// value = value ^ (rnd * 0x00);
 				// }
-				randomI += 1;
 				return value;
 			};
 		}