• Home
  • Line#
  • Scopes#
  • Navigate#
  • Raw
  • Download
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 <limits.h>
8 #include <math.h>
9 #include <stdint.h>
10 
11 #include <algorithm>
12 #include <limits>
13 
14 #include "base/logging.h"
15 #include "base/strings/string_util.h"
16 
17 namespace base {
18 
RandUint64()19 uint64_t RandUint64() {
20   uint64_t number;
21   RandBytes(&number, sizeof(number));
22   return number;
23 }
24 
RandInt(int min,int max)25 int RandInt(int min, int max) {
26   DCHECK_LE(min, max);
27 
28   uint64_t range = static_cast<uint64_t>(max) - min + 1;
29   // |range| is at most UINT_MAX + 1, so the result of RandGenerator(range)
30   // is at most UINT_MAX.  Hence it's safe to cast it from uint64_t to int64_t.
31   int result =
32       static_cast<int>(min + static_cast<int64_t>(base::RandGenerator(range)));
33   DCHECK_GE(result, min);
34   DCHECK_LE(result, max);
35   return result;
36 }
37 
RandDouble()38 double RandDouble() {
39   return BitsToOpenEndedUnitInterval(base::RandUint64());
40 }
41 
BitsToOpenEndedUnitInterval(uint64_t bits)42 double BitsToOpenEndedUnitInterval(uint64_t bits) {
43   // We try to get maximum precision by masking out as many bits as will fit
44   // in the target type's mantissa, and raising it to an appropriate power to
45   // produce output in the range [0, 1).  For IEEE 754 doubles, the mantissa
46   // is expected to accommodate 53 bits.
47 
48   static_assert(std::numeric_limits<double>::radix == 2,
49                 "otherwise use scalbn");
50   static const int kBits = std::numeric_limits<double>::digits;
51   uint64_t random_bits = bits & ((UINT64_C(1) << kBits) - 1);
52   double result = ldexp(static_cast<double>(random_bits), -1 * kBits);
53   DCHECK_GE(result, 0.0);
54   DCHECK_LT(result, 1.0);
55   return result;
56 }
57 
RandGenerator(uint64_t range)58 uint64_t RandGenerator(uint64_t range) {
59   DCHECK_GT(range, 0u);
60   // We must discard random results above this number, as they would
61   // make the random generator non-uniform (consider e.g. if
62   // MAX_UINT64 was 7 and |range| was 5, then a result of 1 would be twice
63   // as likely as a result of 3 or 4).
64   uint64_t max_acceptable_value =
65       (std::numeric_limits<uint64_t>::max() / range) * range - 1;
66 
67   uint64_t value;
68   do {
69     value = base::RandUint64();
70   } while (value > max_acceptable_value);
71 
72   return value % range;
73 }
74 
RandBytesAsString(size_t length)75 std::string RandBytesAsString(size_t length) {
76   DCHECK_GT(length, 0u);
77   std::string result;
78   RandBytes(WriteInto(&result, length + 1), length);
79   return result;
80 }
81 
82 }  // namespace base
83