## File: random.c

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tworld 1.3.0-5
 `123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100101102103104105106107108109110111112113114115116117118119120121122123` ``````/* random.c: The game's random-number generator. * * Copyright (C) 2001-2006 by Brian Raiter, under the GNU General Public * License. No warranty. See COPYING for details. */ /* * This module is not here because I don't trust the C library's * random-number generator. (In fact, this module uses the linear * congruential generator, which is hardly impressive. But this isn't * strong cryptography; it's a game.) It is here simply because it is * necessary for the game to use the same generator FOREVER. In order * for playback of solutions to work correctly, the game must use the * same sequence of random numbers as when it was recorded. This would * fail if the playback occurred on a version compiled with a * different C library's generator. Thus, this module. */ #include #include #include "gen.h" #include "random.h" /* The most recently generated random number is stashed here, so that * it can provide the initial seed of the next PRNG. */ static unsigned long lastvalue = 0x80000000UL; /* The standard linear congruential random-number generator needs no * introduction. */ static unsigned long nextvalue(unsigned long value) { return ((value * 1103515245UL) + 12345UL) & 0x7FFFFFFFUL; } /* Move to the next pseudorandom number in the generator's series. */ static void nextrandom(prng *gen) { if (gen->shared) gen->value = lastvalue = nextvalue(lastvalue); else gen->value = nextvalue(gen->value); } /* Create a new PRNG, reset to the shared sequence. */ prng createprng(void) { prng gen; resetprng(&gen); return gen; } /* We start off a fresh series by taking the current time. A few * numbers are generated and discarded to work out any biases in the * seed value. */ void resetprng(prng *gen) { if (lastvalue > 0x7FFFFFFFUL) lastvalue = nextvalue(nextvalue(nextvalue(nextvalue(time(NULL))))); gen->value = gen->initial = lastvalue; gen->shared = TRUE; } /* Reset a PRNG to an independent sequence. */ void restartprng(prng *gen, unsigned long seed) { gen->value = gen->initial = seed & 0x7FFFFFFFUL; gen->shared = FALSE; } /* Use the top two bits to get a random number between 0 and 3. */ int random4(prng *gen) { nextrandom(gen); return gen->value >> 29; } /* Randomly select an element from a list of three values. */ int randomof3(prng *gen, int a, int b, int c) { int n; nextrandom(gen); n = (int)((3.0 * (gen->value & 0x3FFFFFFFUL)) / (double)0x40000000UL); return n < 2 ? n < 1 ? a : b : c; } /* Randomly permute a list of three values. Two random numbers are * used, with the ranges [0,1] and [0,1,2]. */ void randomp3(prng *gen, int *array) { int n, t; nextrandom(gen); n = gen->value >> 30; t = array[n]; array[n] = array[1]; array[1] = t; n = (int)((3.0 * (gen->value & 0x3FFFFFFFUL)) / (double)0x40000000UL); t = array[n]; array[n] = array[2]; array[2] = t; } /* Randomly permute a list of four values. Three random numbers are * used, with the ranges [0,1], [0,1,2], and [0,1,2,3]. */ void randomp4(prng *gen, int *array) { int n, t; nextrandom(gen); n = gen->value >> 30; t = array[n]; array[n] = array[1]; array[1] = t; n = (int)((3.0 * (gen->value & 0x0FFFFFFFUL)) / (double)0x10000000UL); t = array[n]; array[n] = array[2]; array[2] = t; n = (gen->value >> 28) & 3; t = array[n]; array[n] = array[3]; array[3] = t; } ``````