sfc64.h (2546B)
1 /* 2 * Copyright (c) 2024 Michael Niedermayer <michael-ffmpeg@niedermayer.cc> 3 * 4 * This file is part of FFmpeg. 5 * 6 * FFmpeg is free software; you can redistribute it and/or 7 * modify it under the terms of the GNU Lesser General Public 8 * License as published by the Free Software Foundation; either 9 * version 2.1 of the License, or (at your option) any later version. 10 * 11 * FFmpeg is distributed in the hope that it will be useful, 12 * but WITHOUT ANY WARRANTY; without even the implied warranty of 13 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU 14 * Lesser General Public License for more details. 15 * 16 * You should have received a copy of the GNU Lesser General Public 17 * License along with FFmpeg; if not, write to the Free Software 18 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA 19 * 20 */ 21 22 /** 23 * @file 24 * simple Pseudo Random Number Generator 25 * 26 * This is a implementation of SFC64, a 64-bit PRNG by Chris Doty-Humphrey. 27 * 28 * This Generator is much faster (0m1.872s) than 64bit KISS (0m3.823s) and PCG-XSH-RR-64/32 (0m2.700s) 29 * And passes testu01 and practrand test suits. 30 */ 31 32 #ifndef AVUTIL_SFC64_H 33 #define AVUTIL_SFC64_H 34 35 #include <inttypes.h> 36 37 typedef struct FFSFC64 { 38 uint64_t a,b,c,counter; 39 } FFSFC64; 40 41 static inline uint64_t ff_sfc64_get(FFSFC64 *s) { 42 uint64_t tmp = s->a + s->b + s->counter++; 43 s->a = s->b ^ (s->b >> 11); 44 s->b = s->c + (s->c << 3); // This is a multiply by 9 45 s->c = (s->c << 24 | s->c >> 40) + tmp; 46 return tmp; 47 } 48 49 /** 50 * Return the previous random value, and step the generator backward. 51 * 52 * It is safe to take values before the first, but such values can be highly 53 * correlated to the seeds. 54 */ 55 static inline uint64_t ff_sfc64_reverse_get(FFSFC64 *s) { 56 uint64_t prev_c = s->b * 0x8E38E38E38E38E39; 57 uint64_t tmp = s->c - (prev_c << 24 | prev_c >> 40); 58 s->b = s->a ^ (s->a >> 11); 59 s->b ^= s->b >> 22; 60 s->b ^= s->b >> 44; 61 62 s->a = tmp - s->b - --s->counter; 63 s->c = prev_c; 64 65 return tmp; 66 } 67 68 /** 69 * Initialize sfc64 with up to 3 seeds. 70 * 71 * @param rounds number of rounds mixing up state during init. Generally 8-18, larger numbers will help with bad quality seeds. 72 * 12 is a good choice if all 3 seeds are equal 73 * 74 */ 75 static inline void ff_sfc64_init(FFSFC64 *s, uint64_t seeda, uint64_t seedb, uint64_t seedc, int rounds) { 76 s->a = seeda; 77 s->b = seedb; 78 s->c = seedc; 79 s->counter = 1; 80 while (rounds--) 81 ff_sfc64_get(s); 82 } 83 84 #endif // AVUTIL_SFC64_H