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38 lines
934 B
C
38 lines
934 B
C
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#include <stdlib.h>
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/* Pseudo-random generator based on Minimal Standard by
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Lewis, Goodman, and Miller in 1969.
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I[j+1] = a*I[j] (mod m)
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where a = 16807
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m = 2147483647
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Using Schrage's algorithm, a*I[j] (mod m) can be rewritten as:
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a*(I[j] mod q) - r*{I[j]/q} if >= 0
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a*(I[j] mod q) - r*{I[j]/q} + m otherwise
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where: {} denotes integer division
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q = {m/a} = 127773
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r = m (mod a) = 2836
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note that the seed value of 0 cannot be used in the calculation as
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it results in 0 itself
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*/
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int
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_DEFUN (rand_r, (seed), unsigned int *seed)
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{
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long k;
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long s = (long)(*seed);
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if (s == 0)
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s = 0x12345987;
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k = s / 127773;
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s = 16807 * (s - k * 127773) - 2836 * k;
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if (s < 0)
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s += 2147483647;
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(*seed) = (unsigned int)s;
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return (int)(s & RAND_MAX);
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}
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