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041f981960
* rand() use XOR to break up unwanted pair correlation This form of rand() cannot return the same value twice in a row. Two additonal EOR instructions produce a more even distribution of successive pairs. see comments on #951 * rand.s document purpose of XOR * suggested srand() optimization: zero fill unnecessary * test to validate implementation of rand() * srand() improving behaviour and adding startup test * srand() with a tail call to rand() for better initial shuffle * srand() can fall through to rand() instead of tail call
111 lines
2.5 KiB
C
111 lines
2.5 KiB
C
/* This test verifies that the assembly implementation of rand() matches its
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* theoretical high level equivalent.
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*
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* This does about 3000 tests from various starting srand() seeds.
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* A more thorough test might visit the entire sequence with 2^32 tests, but
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* that takes hours to simulate, and this should be a sufficient sampling.
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*
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* This will also fail if rand() is ever altered, which might be a warning to
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* tread carefully. Some past discussion of RNG here:
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* https://github.com/cc65/cc65/pull/951
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*/
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#include <stdlib.h>
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#include <stdio.h>
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#include <stdint.h>
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/* for faster execution */
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#pragma static-locals (on)
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/* values tested per seed */
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#define SUBTESTS 50
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/* increments used between tested seeds */
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/* 653 is prime and divides 32768 by ~50 */
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#define TESTINC 653
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static uint32_t seed;
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int ref_rand()
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{
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uint16_t output;
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/* seed follows the LCG sequence * 0x01010101 + 0xB3B3B3B3 */
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seed = seed * 0x01010101UL + 0xB3B3B3B3UL;
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/* output uses the top two bytes (reversed) XOR with bottom two bytes */
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{
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uint16_t s0 = (seed >> 0) & 0xFF;
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uint16_t s1 = (seed >> 8) & 0xFF;
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uint16_t s2 = (seed >> 16) & 0xFF;
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uint16_t s3 = (seed >> 24) & 0xFF;
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uint16_t o0 = s3 ^ s1;
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uint16_t o1 = s2 ^ s0;
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output = o0 | (o1 << 8);
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}
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return (int)(output & 0x7FFF);
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}
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void ref_srand(int ax)
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{
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uint32_t s = (unsigned int)ax;
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seed = s | (s << 16); /* low 16 bits is convenient filler for high 16 bits */
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ref_rand(); /* one pre-call "shuffles" the first rand() result so it isn't too predictable */
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}
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int main(void)
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{
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unsigned int i,j;
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int a,b;
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/* test that startup state is equivalent to srand(1) */
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{
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//srand(1); // implied
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ref_srand(1);
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for (j=0; j<SUBTESTS; ++j)
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{
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a = rand();
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b = ref_rand();
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if (a != b)
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{
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printf("failed startup seed at test %d. rand()=%d reference=%d\n",j,a,b);
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return EXIT_FAILURE;
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}
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}
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}
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/* test every power of 2 seed */
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for (i = 0; i < 16; ++i)
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{
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srand(1<<i);
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ref_srand(1<<i);
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for (j=0; j<SUBTESTS; ++j)
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{
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a = rand();
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b = ref_rand();
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if (a != b)
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{
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printf("failed seed %d at test %d. rand()=%d reference=%d\n",(1<<i),j,a,b);
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return EXIT_FAILURE;
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}
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}
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}
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/* test a sampling of seeds*/
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for (i = 0; i < 32768UL; i += TESTINC)
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{
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srand(i);
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ref_srand(i);
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for (j=0; j<SUBTESTS; ++j)
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{
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a = rand();
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b = ref_rand();
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if (a != b)
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{
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printf("failed seed %d at test %d. rand()=%d reference=%d\n",(1<<i),j,a,b);
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return EXIT_FAILURE;
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}
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}
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}
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return EXIT_SUCCESS;
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}
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