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      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 void RandBytes(void* output, size_t output_length) {
     65   uint64 random_int;
     66   size_t random_int_size = sizeof(random_int);
     67   for (size_t i = 0; i < output_length; i += random_int_size) {
     68     random_int = base::RandUint64();
     69     size_t copy_count = std::min(output_length - i, random_int_size);
     70     memcpy(((uint8*)output) + i, &random_int, copy_count);
     71   }
     72 }
     73 
     74 std::string RandBytesAsString(size_t length) {
     75   DCHECK_GT(length, 0u);
     76   std::string result;
     77   RandBytes(WriteInto(&result, length + 1), length);
     78   return result;
     79 }
     80 
     81 }  // namespace base
     82