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Diffstat (limited to 'seedrandom.js')
| -rw-r--r-- | seedrandom.js | 273 |
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diff --git a/seedrandom.js b/seedrandom.js new file mode 100644 index 0000000..27751b2 --- /dev/null +++ b/seedrandom.js @@ -0,0 +1,273 @@ +// seedrandom.js version 2.0. +// Author: David Bau 4/2/2011 +// modified by Peter <i@peter.am> 2011 +// +// Defines a method Math.seedrandom() that, when called, substitutes +// an explicitly seeded RC4-based algorithm for Math.random(). Also +// supports automatic seeding from local or network sources of entropy. +// +// Usage: +// +// <script src=http://davidbau.com/encode/seedrandom-min.js></script> +// +// Math.seedrandom('yipee'); Sets Math.random to a function that is +// initialized using the given explicit seed. +// +// Math.seedrandom(); Sets Math.random to a function that is +// seeded using the current time, dom state, +// and other accumulated local entropy. +// The generated seed string is returned. +// +// Math.seedrandom('yowza', true); +// Seeds using the given explicit seed mixed +// together with accumulated entropy. +// +// <script src="http://bit.ly/srandom-512"></script> +// Seeds using physical random bits downloaded +// from random.org. +// +// <script src="https://jsonlib.appspot.com/urandom?callback=Math.seedrandom"> +// </script> Seeds using urandom bits from call.jsonlib.com, +// which is faster than random.org. +// +// Examples: +// +// Math.seedrandom("hello"); // Use "hello" as the seed. +// document.write(Math.random()); // Always 0.5463663768140734 +// document.write(Math.random()); // Always 0.43973793770592234 +// var rng1 = Math.random; // Remember the current prng. +// +// var autoseed = Math.seedrandom(); // New prng with an automatic seed. +// document.write(Math.random()); // Pretty much unpredictable. +// +// Math.random = rng1; // Continue "hello" prng sequence. +// document.write(Math.random()); // Always 0.554769432473455 +// +// Math.seedrandom(autoseed); // Restart at the previous seed. +// document.write(Math.random()); // Repeat the 'unpredictable' value. +// +// Notes: +// +// Each time seedrandom('arg') is called, entropy from the passed seed +// is accumulated in a pool to help generate future seeds for the +// zero-argument form of Math.seedrandom, so entropy can be injected over +// time by calling seedrandom with explicit data repeatedly. +// +// On speed - This javascript implementation of Math.random() is about +// 3-10x slower than the built-in Math.random() because it is not native +// code, but this is typically fast enough anyway. Seeding is more expensive, +// especially if you use auto-seeding. Some details (timings on Chrome 4): +// +// Our Math.random() - avg less than 0.002 milliseconds per call +// seedrandom('explicit') - avg less than 0.5 milliseconds per call +// seedrandom('explicit', true) - avg less than 2 milliseconds per call +// seedrandom() - avg about 38 milliseconds per call +// +// LICENSE (BSD): +// +// Copyright 2010 David Bau, all rights reserved. +// +// Redistribution and use in source and binary forms, with or without +// modification, are permitted provided that the following conditions are met: +// +// 1. Redistributions of source code must retain the above copyright +// notice, this list of conditions and the following disclaimer. +// +// 2. Redistributions in binary form must reproduce the above copyright +// notice, this list of conditions and the following disclaimer in the +// documentation and/or other materials provided with the distribution. +// +// 3. Neither the name of this module nor the names of its contributors may +// be used to endorse or promote products derived from this software +// without specific prior written permission. +// +// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS +// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT +// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR +// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT +// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, +// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT +// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, +// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY +// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT +// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE +// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. +// +/** + * All code is in an anonymous closure to keep the global namespace clean. + * + * @param {number=} overflow + * @param {number=} startdenom + */ +(function (pool, math, width, chunks, significance, overflow, startdenom) { + + +// +// seedrandom() +// This is the seedrandom function described above. +// +math['seedrandom'] = function seedrandom(seed, use_entropy) { + var key = []; + var arc4; + + // Flatten the seed string or build one from local entropy if needed. + seed = mixkey(flatten( + use_entropy ? [seed, pool] : + arguments.length ? seed : + [new Date().getTime(), pool, window], 3), key); + + // Use the seed to initialize an ARC4 generator. + arc4 = new ARC4(key); + + // Mix the randomness into accumulated entropy. + mixkey(arc4.S, pool); + + // Override Math.random + + // This function returns a random double in [0, 1) that contains + // randomness in every bit of the mantissa of the IEEE 754 value. + + math['random2'] = function random2() { // Closure to return a random double: + var n = arc4.g(chunks); // Start with a numerator n < 2 ^ 48 + var d = startdenom; // and denominator d = 2 ^ 48. + var x = 0; // and no 'extra last byte'. + while (n < significance) { // Fill up all significant digits by + n = (n + x) * width; // shifting numerator and + d *= width; // denominator and generating a + x = arc4.g(1); // new least-significant-byte. + } + while (n >= overflow) { // To avoid rounding up, before adding + n /= 2; // last byte, shift everything + d /= 2; // right using integer math until + x >>>= 1; // we have exactly the desired bits. + } + return (n + x) / d; // Form the number within [0, 1). + }; + + // Return the seed that was used + return seed; +}; + +// +// ARC4 +// +// An ARC4 implementation. The constructor takes a key in the form of +// an array of at most (width) integers that should be 0 <= x < (width). +// +// The g(count) method returns a pseudorandom integer that concatenates +// the next (count) outputs from ARC4. Its return value is a number x +// that is in the range 0 <= x < (width ^ count). +// +/** @constructor */ +function ARC4(key) { + var t, u, me = this, keylen = key.length; + var i = 0, j = me.i = me.j = me.m = 0; + me.S = []; + me.c = []; + + // The empty key [] is treated as [0]. + if (!keylen) { key = [keylen++]; } + + // Set up S using the standard key scheduling algorithm. + while (i < width) { me.S[i] = i++; } + for (i = 0; i < width; i++) { + t = me.S[i]; + j = lowbits(j + t + key[i % keylen]); + u = me.S[j]; + me.S[i] = u; + me.S[j] = t; + } + + // The "g" method returns the next (count) outputs as one number. + me.g = function getnext(count) { + var s = me.S; + var i = lowbits(me.i + 1); var t = s[i]; + var j = lowbits(me.j + t); var u = s[j]; + s[i] = u; + s[j] = t; + var r = s[lowbits(t + u)]; + while (--count) { + i = lowbits(i + 1); t = s[i]; + j = lowbits(j + t); u = s[j]; + s[i] = u; + s[j] = t; + r = r * width + s[lowbits(t + u)]; + } + me.i = i; + me.j = j; + return r; + }; + // For robust unpredictability discard an initial batch of values. + // See http://www.rsa.com/rsalabs/node.asp?id=2009 + me.g(width); +} + +// +// flatten() +// Converts an object tree to nested arrays of strings. +// +/** @param {Object=} result + * @param {string=} prop + * @param {string=} typ */ +function flatten(obj, depth, result, prop, typ) { + result = []; + typ = typeof(obj); + if (depth && typ == 'object') { + for (prop in obj) { + if (prop.indexOf('S') < 5) { // Avoid FF3 bug (local/sessionStorage) + try { result.push(flatten(obj[prop], depth - 1)); } catch (e) {} + } + } + } + return (result.length ? result : obj + (typ != 'string' ? '\0' : '')); +} + +// +// mixkey() +// Mixes a string seed into a key that is an array of integers, and +// returns a shortened string seed that is equivalent to the result key. +// +/** @param {number=} smear + * @param {number=} j */ +function mixkey(seed, key, smear, j) { + seed += ''; // Ensure the seed is a string + smear = 0; + for (j = 0; j < seed.length; j++) { + key[lowbits(j)] = + lowbits((smear ^= key[lowbits(j)] * 19) + seed.charCodeAt(j)); + } + seed = ''; + for (j in key) { seed += String.fromCharCode(key[j]); } + return seed; +} + +// +// lowbits() +// A quick "n mod width" for width a power of 2. +// +function lowbits(n) { return n & (width - 1); } + +// +// The following constants are related to IEEE 754 limits. +// +startdenom = math.pow(width, chunks); +significance = math.pow(2, significance); +overflow = significance * 2; + +// +// When seedrandom.js is loaded, we immediately mix a few bits +// from the built-in RNG into the entropy pool. Because we do +// not want to intefere with determinstic PRNG state later, +// seedrandom will not call math.random on its own again after +// initialization. +// +mixkey(math.random(), pool); + +// End anonymous scope, and pass initial values. +})( + [], // pool: entropy pool starts empty + Math, // math: package containing random, pow, and seedrandom + 256, // width: each RC4 output is 0 <= x < 256 + 6, // chunks: at least six RC4 outputs for each double + 52 // significance: there are 52 significant digits in a double +); |