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/*
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* Copyright (c) 2024 Michael Niedermayer <michael-ffmpeg@niedermayer.cc>
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*
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* This file is part of FFmpeg.
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*
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* FFmpeg is free software; you can redistribute it and/or
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* modify it under the terms of the GNU Lesser General Public
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* License as published by the Free Software Foundation; either
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* version 2.1 of the License, or (at your option) any later version.
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*
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* FFmpeg is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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* Lesser General Public License for more details.
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*
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* You should have received a copy of the GNU Lesser General Public
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* License along with FFmpeg; if not, write to the Free Software
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* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
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*
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*/
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/**
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* @file
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* simple Pseudo Random Number Generator
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*
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* This is a implementation of SFC64, a 64-bit PRNG by Chris Doty-Humphrey.
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*
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* This Generator is much faster (0m1.872s) than 64bit KISS (0m3.823s) and PCG-XSH-RR-64/32 (0m2.700s)
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* And passes testu01 and practrand test suits.
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*/
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#ifndef AVUTIL_SFC64_H
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#define AVUTIL_SFC64_H
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#include <inttypes.h>
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typedef struct FFSFC64 {
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uint64_t a,b,c,counter;
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} FFSFC64;
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static inline uint64_t ff_sfc64_get(FFSFC64 *s) {
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uint64_t tmp = s->a + s->b + s->counter++;
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s->a = s->b ^ (s->b >> 11);
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s->b = s->c + (s->c << 3); // This is a multiply by 9
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s->c = (s->c << 24 | s->c >> 40) + tmp;
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return tmp;
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}
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/**
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* Return the previous random value, and step the generator backward.
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*
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* It is safe to take values before the first, but such values can be highly
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* correlated to the seeds.
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*/
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static inline uint64_t ff_sfc64_reverse_get(FFSFC64 *s) {
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uint64_t prev_c = s->b * 0x8E38E38E38E38E39;
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uint64_t tmp = s->c - (prev_c << 24 | prev_c >> 40);
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s->b = s->a ^ (s->a >> 11);
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s->b ^= s->b >> 22;
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s->b ^= s->b >> 44;
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s->a = tmp - s->b - --s->counter;
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s->c = prev_c;
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return tmp;
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}
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/**
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* Initialize sfc64 with up to 3 seeds.
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*
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* @param rounds number of rounds mixing up state during init. Generally 8-18, larger numbers will help with bad quality seeds.
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* 12 is a good choice if all 3 seeds are equal
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*
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*/
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static inline void ff_sfc64_init(FFSFC64 *s, uint64_t seeda, uint64_t seedb, uint64_t seedc, int rounds) {
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s->a = seeda;
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s->b = seedb;
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s->c = seedc;
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s->counter = 1;
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while (rounds--)
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ff_sfc64_get(s);
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}
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#endif // AVUTIL_SFC64_H
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