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