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 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 32 double RandDouble() { 33 return BitsToOpenEndedUnitInterval(base::RandUint64()); 34 } 35 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 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 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
