1 // Copyright 2015 the V8 project authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style license that can be 3 // found in the LICENSE file. 4 5 // Flags: --random-seed=12 --nostress-opt --noalways-opt --predictable 6 7 (function() { 8 var kHistory = 2; 9 var kRepeats = 100; 10 var history = new Uint32Array(kHistory); 11 12 function random() { 13 return (Math.random() * Math.pow(2, 32)) >>> 0; 14 } 15 16 function ChiSquared(m, n) { 17 var ys_minus_np1 = (m - n / 2.0); 18 var chi_squared_1 = ys_minus_np1 * ys_minus_np1 * 2.0 / n; 19 var ys_minus_np2 = ((n - m) - n / 2.0); 20 var chi_squared_2 = ys_minus_np2 * ys_minus_np2 * 2.0 / n; 21 return chi_squared_1 + chi_squared_2; 22 } 23 for (var predictor_bit = -2; predictor_bit < 32; predictor_bit++) { 24 // The predicted bit is one of the bits from the PRNG. 25 for (var random_bit = 0; random_bit < 32; random_bit++) { 26 for (var ago = 0; ago < kHistory; ago++) { 27 // We don't want to check whether each bit predicts itself. 28 if (ago == 0 && predictor_bit == random_bit) continue; 29 // Enter the new random value into the history 30 for (var i = ago; i >= 0; i--) { 31 history[i] = random(); 32 } 33 // Find out how many of the bits are the same as the prediction bit. 34 var m = 0; 35 for (var i = 0; i < kRepeats; i++) { 36 for (var j = ago - 1; j >= 0; j--) history[j + 1] = history[j]; 37 history[0] = random(); 38 var predicted; 39 if (predictor_bit >= 0) { 40 predicted = (history[ago] >> predictor_bit) & 1; 41 } else { 42 predicted = predictor_bit == -2 ? 0 : 1; 43 } 44 var bit = (history[0] >> random_bit) & 1; 45 if (bit == predicted) m++; 46 } 47 // Chi squared analysis for k = 2 (2, states: same/not-same) and one 48 // degree of freedom (k - 1). 49 var chi_squared = ChiSquared(m, kRepeats); 50 if (chi_squared > 24) { 51 var percent = Math.floor(m * 100.0 / kRepeats); 52 if (predictor_bit < 0) { 53 var bit_value = predictor_bit == -2 ? 0 : 1; 54 print(`Bit ${random_bit} is ${bit_value} ${percent}% of the time`); 55 } else { 56 print(`Bit ${random_bit} is the same as bit ${predictor_bit} ` + 57 `${ago} ago ${percent}% of the time`); 58 } 59 } 60 // For 1 degree of freedom this corresponds to 1 in a million. We are 61 // running ~8000 tests, so that would be surprising. 62 assertTrue(chi_squared <= 24); 63 // If the predictor bit is a fixed 0 or 1 then it makes no sense to 64 // repeat the test with a different age. 65 if (predictor_bit < 0) break; 66 } 67 } 68 } 69 })(); 70
