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80 lines
1.3 KiB
80 lines
1.3 KiB
2 months ago
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/*
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* UCW Library -- Prime Number Tests
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*
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* (c) 1997 Martin Mares <mj@ucw.cz>
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*
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* This software may be freely distributed and used according to the terms
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* of the GNU Lesser General Public License.
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*/
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#include <ucw/lib.h>
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#include <ucw/prime.h>
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static int /* Sequential search */
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__isprime(uint x) /* We know x != 2 && x != 3 */
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{
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uint test = 5;
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if (x == 5)
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return 1;
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for(;;)
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{
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if (!(x % test))
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return 0;
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if (x / test <= test)
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return 1;
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test += 2; /* 6k+1 */
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if (!(x % test))
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return 0;
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if (x / test <= test)
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return 1;
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test += 4; /* 6k-1 */
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}
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}
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int
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isprime(uint x)
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{
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if (x < 5)
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return (x == 2 || x == 3);
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switch (x % 6)
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{
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case 1:
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case 5:
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return __isprime(x);
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default:
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return 0;
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}
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}
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uint
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nextprime(uint x) /* Returns some prime greater than x */
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{
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x += 5 - (x % 6); /* x is 6k-1 */
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for(;;)
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{
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x += 2; /* 6k+1 */
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if (__isprime(x))
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return x;
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x += 4; /* 6k-1 */
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if (__isprime(x))
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return x;
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}
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}
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#ifdef TEST
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#include <stdio.h>
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#include <stdlib.h>
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int
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main(int argc, char **argv)
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{
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uint k = atol(argv[1]);
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printf("%d is%s prime\n", k, isprime(k) ? "" : "n't");
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printf("Next prime is %d\n", nextprime(k));
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return 0;
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}
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#endif
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