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| 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 |
| 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 })(); |
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