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