83 lines
2.4 KiB
C++
83 lines
2.4 KiB
C++
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// Copyright (c) 2011 The Chromium Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file.
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#include "base/rand_util.h"
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#include <limits.h>
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#include <math.h>
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#include <stdint.h>
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#include <algorithm>
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#include <limits>
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#include "base/logging.h"
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#include "base/strings/string_util.h"
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namespace base {
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uint64_t RandUint64() {
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uint64_t number;
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RandBytes(&number, sizeof(number));
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return number;
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}
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int RandInt(int min, int max) {
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DCHECK_LE(min, max);
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uint64_t range = static_cast<uint64_t>(max) - min + 1;
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// |range| is at most UINT_MAX + 1, so the result of RandGenerator(range)
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// is at most UINT_MAX. Hence it's safe to cast it from uint64_t to int64_t.
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int result =
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static_cast<int>(min + static_cast<int64_t>(base::RandGenerator(range)));
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DCHECK_GE(result, min);
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DCHECK_LE(result, max);
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return result;
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}
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double RandDouble() {
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return BitsToOpenEndedUnitInterval(base::RandUint64());
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}
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double BitsToOpenEndedUnitInterval(uint64_t bits) {
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// We try to get maximum precision by masking out as many bits as will fit
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// in the target type's mantissa, and raising it to an appropriate power to
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// produce output in the range [0, 1). For IEEE 754 doubles, the mantissa
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// is expected to accommodate 53 bits.
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static_assert(std::numeric_limits<double>::radix == 2,
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"otherwise use scalbn");
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static const int kBits = std::numeric_limits<double>::digits;
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uint64_t random_bits = bits & ((UINT64_C(1) << kBits) - 1);
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double result = ldexp(static_cast<double>(random_bits), -1 * kBits);
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DCHECK_GE(result, 0.0);
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DCHECK_LT(result, 1.0);
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return result;
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}
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uint64_t RandGenerator(uint64_t range) {
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DCHECK_GT(range, 0u);
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// We must discard random results above this number, as they would
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// make the random generator non-uniform (consider e.g. if
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// MAX_UINT64 was 7 and |range| was 5, then a result of 1 would be twice
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// as likely as a result of 3 or 4).
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uint64_t max_acceptable_value =
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(std::numeric_limits<uint64_t>::max() / range) * range - 1;
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uint64_t value;
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do {
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value = base::RandUint64();
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} while (value > max_acceptable_value);
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return value % range;
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}
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std::string RandBytesAsString(size_t length) {
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DCHECK_GT(length, 0u);
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std::string result;
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RandBytes(WriteInto(&result, length + 1), length);
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return result;
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}
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} // namespace base
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