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 19 uint64_t RandUint64() { 20 uint64_t number; 21 RandBytes(&number, sizeof(number)); 22 return number; 23 } 24 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 38 double RandDouble() { 39 return BitsToOpenEndedUnitInterval(base::RandUint64()); 40 } 41 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 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 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
