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