99 lines
2.7 KiB
C++
99 lines
2.7 KiB
C++
// Copyright (c) the JPEG XL Project Authors. All rights reserved.
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//
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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#ifndef LIB_JXL_BASE_RANDOM_
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#define LIB_JXL_BASE_RANDOM_
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// Random number generator + distributions.
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// We don't use <random> because the implementation (and thus results) differs
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// between libstdc++ and libc++.
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#include <stdint.h>
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#include <string.h>
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#include <algorithm>
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#include <cmath>
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#include "lib/jxl/base/status.h"
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namespace jxl {
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struct Rng {
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explicit Rng(uint64_t seed)
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: s{static_cast<uint64_t>(0x94D049BB133111EBull),
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static_cast<uint64_t>(0xBF58476D1CE4E5B9ull) + seed} {}
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// Xorshift128+ adapted from xorshift128+-inl.h
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uint64_t operator()() {
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uint64_t s1 = s[0];
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const uint64_t s0 = s[1];
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const uint64_t bits = s1 + s0; // b, c
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s[0] = s0;
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s1 ^= s1 << 23;
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s1 ^= s0 ^ (s1 >> 18) ^ (s0 >> 5);
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s[1] = s1;
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return bits;
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}
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// Uniformly distributed int64_t in [begin, end), under the assumption that
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// `end-begin` is significantly smaller than 1<<64, otherwise there is some
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// bias.
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int64_t UniformI(int64_t begin, int64_t end) {
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JXL_DASSERT(end > begin);
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return static_cast<int64_t>((*this)() %
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static_cast<uint64_t>(end - begin)) +
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begin;
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}
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// Same as UniformI, but for uint64_t.
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uint64_t UniformU(uint64_t begin, uint64_t end) {
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JXL_DASSERT(end > begin);
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return (*this)() % (end - begin) + begin;
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}
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// Uniformly distributed float in [begin, end) range. Note: only 23 bits of
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// randomness.
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float UniformF(float begin, float end) {
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float f;
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// Bits of a random [1, 2) float.
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uint32_t u = ((*this)() >> (64 - 23)) | 0x3F800000;
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static_assert(sizeof(f) == sizeof(u),
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"Float and U32 must have the same size");
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memcpy(&f, &u, sizeof(f));
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// Note: (end-begin) * f + (2*begin-end) may fail to return a number >=
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// begin.
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return (end - begin) * (f - 1.0f) + begin;
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}
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// Bernoulli trial
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bool Bernoulli(float p) { return UniformF(0, 1) < p; }
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// State for geometric distributions.
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// The stored value is inv_log_1mp
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using GeometricDistribution = float;
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static GeometricDistribution MakeGeometric(float p) {
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return 1.0 / std::log(1 - p);
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}
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uint32_t Geometric(const GeometricDistribution& dist) {
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float f = UniformF(0, 1);
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float inv_log_1mp = dist;
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float log = std::log(1 - f) * inv_log_1mp;
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return static_cast<uint32_t>(log);
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}
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template <typename T>
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void Shuffle(T* t, size_t n) {
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for (size_t i = 0; i + 1 < n; i++) {
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size_t a = UniformU(i, n);
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std::swap(t[a], t[i]);
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}
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}
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private:
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uint64_t s[2];
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};
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} // namespace jxl
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#endif // LIB_JXL_BASE_RANDOM_
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