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