rand_util.cc revision 868fa2fe829687343ffae624259930155e16dbd8 
1// Copyright (c) 2011 The Chromium 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#include "base/rand_util.h"
6
7#include <math.h>
8
9#include <limits>
10
11#include "base/basictypes.h"
12#include "base/logging.h"
13#include "base/strings/string_util.h"
14
15namespace base {
16
17int RandInt(int min, int max) {
18  DCHECK_LE(min, max);
19
20  uint64 range = static_cast<uint64>(max) - min + 1;
21  int result = min + static_cast<int>(base::RandGenerator(range));
22  DCHECK_GE(result, min);
23  DCHECK_LE(result, max);
24  return result;
25}
26
27double RandDouble() {
28  return BitsToOpenEndedUnitInterval(base::RandUint64());
29}
30
31double BitsToOpenEndedUnitInterval(uint64 bits) {
32  // We try to get maximum precision by masking out as many bits as will fit
33  // in the target type's mantissa, and raising it to an appropriate power to
34  // produce output in the range [0, 1).  For IEEE 754 doubles, the mantissa
35  // is expected to accommodate 53 bits.
36
37  COMPILE_ASSERT(std::numeric_limits<double>::radix == 2, otherwise_use_scalbn);
38  static const int kBits = std::numeric_limits<double>::digits;
39  uint64 random_bits = bits & ((GG_UINT64_C(1) << kBits) - 1);
40  double result = ldexp(static_cast<double>(random_bits), -1 * kBits);
41  DCHECK_GE(result, 0.0);
42  DCHECK_LT(result, 1.0);
43  return result;
44}
45
46uint64 RandGenerator(uint64 range) {
47  DCHECK_GT(range, 0u);
48  // We must discard random results above this number, as they would
49  // make the random generator non-uniform (consider e.g. if
50  // MAX_UINT64 was 7 and |range| was 5, then a result of 1 would be twice
51  // as likely as a result of 3 or 4).
52  uint64 max_acceptable_value =
53      (std::numeric_limits<uint64>::max() / range) * range - 1;
54
55  uint64 value;
56  do {
57    value = base::RandUint64();
58  } while (value > max_acceptable_value);
59
60  return value % range;
61}
62
63void RandBytes(void* output, size_t output_length) {
64  uint64 random_int;
65  size_t random_int_size = sizeof(random_int);
66  for (size_t i = 0; i < output_length; i += random_int_size) {
67    random_int = base::RandUint64();
68    size_t copy_count = std::min(output_length - i, random_int_size);
69    memcpy(((uint8*)output) + i, &random_int, copy_count);
70  }
71}
72
73std::string RandBytesAsString(size_t length) {
74  DCHECK_GT(length, 0u);
75  std::string result;
76  RandBytes(WriteInto(&result, length + 1), length);
77  return result;
78}
79
80}  // namespace base
81