base/rand_util.cc

   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 #include <stdint.h>
   9
  10 #include <algorithm>
  11 #include <limits>
  12
  13 #include "base/basictypes.h"
  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 range = static_cast<uint64>(max) - min + 1;
  23   int result = min + static_cast<int>(base::RandGenerator(range));
  24   DCHECK_GE(result, min);
  25   DCHECK_LE(result, max);
  26   return result;
  27 }
  28
  29 double RandDouble() {
  30   return BitsToOpenEndedUnitInterval(base::RandUint64());
  31 }
  32
  33 double BitsToOpenEndedUnitInterval(uint64 bits) {
  34   // We try to get maximum precision by masking out as many bits as will fit
  35   // in the target type's mantissa, and raising it to an appropriate power to
  36   // produce output in the range [0, 1).  For IEEE 754 doubles, the mantissa
  37   // is expected to accommodate 53 bits.
  38
  39   COMPILE_ASSERT(std::numeric_limits<double>::radix == 2, otherwise_use_scalbn);
  40   static const int kBits = std::numeric_limits<double>::digits;
  41   uint64 random_bits = bits & ((UINT64_C(1) << kBits) - 1);
  42   double result = ldexp(static_cast<double>(random_bits), -1 * kBits);
  43   DCHECK_GE(result, 0.0);
  44   DCHECK_LT(result, 1.0);
  45   return result;
  46 }
  47
  48 uint64 RandGenerator(uint64 range) {
  49   DCHECK_GT(range, 0u);
  50   // We must discard random results above this number, as they would
  51   // make the random generator non-uniform (consider e.g. if
  52   // MAX_UINT64 was 7 and |range| was 5, then a result of 1 would be twice
  53   // as likely as a result of 3 or 4).
  54   uint64 max_acceptable_value =
  55       (std::numeric_limits<uint64>::max() / range) * range - 1;
  56
  57   uint64 value;
  58   do {
  59     value = base::RandUint64();
  60   } while (value > max_acceptable_value);
  61
  62   return value % range;
  63 }
  64
  65 std::string RandBytesAsString(size_t length) {
  66   DCHECK_GT(length, 0u);
  67   std::string result;
  68   RandBytes(WriteInto(&result, length + 1), length);
  69   return result;
  70 }
  71
  72 }  // namespace base