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
   9 #include <limits>
  10
  11 #include "base/basictypes.h"
  12 #include "base/logging.h"
  13 #include "base/string_util.h"
  14
  15 namespace base {
  16
  17 int RandInt(int min, int max) {
  18   DCHECK_LE(min, max);
  19
  20   uint64 range = static_cast<uint64>(max) - min + 1;
  21   int result = min + static_cast<int>(base::RandGenerator(range));
  22   DCHECK_GE(result, min);
  23   DCHECK_LE(result, max);
  24   return result;
  25 }
  26
  27 double RandDouble() {
  28   return BitsToOpenEndedUnitInterval(base::RandUint64());
  29 }
  30
  31 double BitsToOpenEndedUnitInterval(uint64 bits) {
  32   // We try to get maximum precision by masking out as many bits as will fit
  33   // in the target type's mantissa, and raising it to an appropriate power to
  34   // produce output in the range [0, 1).  For IEEE 754 doubles, the mantissa
  35   // is expected to accommodate 53 bits.
  36
  37   COMPILE_ASSERT(std::numeric_limits<double>::radix == 2, otherwise_use_scalbn);
  38   static const int kBits = std::numeric_limits<double>::digits;
  39   uint64 random_bits = bits & ((GG_UINT64_C(1) << kBits) - 1);
  40   double result = ldexp(static_cast<double>(random_bits), -1 * kBits);
  41   DCHECK_GE(result, 0.0);
  42   DCHECK_LT(result, 1.0);
  43   return result;
  44 }
  45
  46 uint64 RandGenerator(uint64 range) {
  47   DCHECK_GT(range, 0u);
  48   // We must discard random results above this number, as they would
  49   // make the random generator non-uniform (consider e.g. if
  50   // MAX_UINT64 was 7 and |range| was 5, then a result of 1 would be twice
  51   // as likely as a result of 3 or 4).
  52   uint64 max_acceptable_value =
  53       (std::numeric_limits<uint64>::max() / range) * range - 1;
  54
  55   uint64 value;
  56   do {
  57     value = base::RandUint64();
  58   } while (value > max_acceptable_value);
  59
  60   return value % range;
  61 }
  62
  63 void RandBytes(void* output, size_t output_length) {
  64   uint64 random_int;
  65   size_t random_int_size = sizeof(random_int);
  66   for (size_t i = 0; i < output_length; i += random_int_size) {
  67     random_int = base::RandUint64();
  68     size_t copy_count = std::min(output_length - i, random_int_size);
  69     memcpy(((uint8*)output) + i, &random_int, copy_count);
  70   }
  71 }
  72
  73 std::string RandBytesAsString(size_t length) {
  74   DCHECK_GT(length, 0u);
  75   std::string result;
  76   RandBytes(WriteInto(&result, length + 1), length);
  77   return result;
  78 }
  79
  80 }  // namespace base