140 lines
2.7 KiB
C
140 lines
2.7 KiB
C
#ifndef lint
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static char sccsid[] = "@(#)primes.c 1.1 92/07/30 SMI"; /* from UCB 4.1 06/13/83 */
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#endif
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/*
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* primes [ number ]
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*
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* Print all primes greater than argument (or number read from stdin).
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*
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* A free translation of 'primes.s'
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*
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*/
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#include <stdio.h>
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#include <math.h>
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#define TABSIZE 1000 /* size of sieve table */
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#define BIG 4294967296. /* largest unsigned int */
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char table[TABSIZE]; /* table for sieve of Eratosthenes */
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int tabbits = 8*TABSIZE; /* number of bits in table */
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float fstart;
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unsigned start; /* lowest number to test for prime */
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char bittab[] = { /* bit positions (to save shifting) */
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01, 02, 04, 010, 020, 040, 0100, 0200
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};
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unsigned pt[] = { /* primes < 100 */
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2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43,
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47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97
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};
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unsigned factab[] = { /* difference between succesive trial factors */
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10, 2, 4, 2, 4, 6, 2, 6, 4, 2, 4, 6, 6, 2, 6, 4,
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2, 6, 4, 6, 8, 4, 2, 4, 2, 4, 8, 6, 4, 6, 2, 4,
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6, 2, 6, 6, 4, 2, 4, 6, 2, 6, 4, 2, 4, 2, 10, 2
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};
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main(argc, argv)
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int argc;
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char **argv;
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{
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register unsigned *fp;
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register char *p;
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register int i;
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unsigned quot;
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unsigned factor, v;
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if (argc >= 2) { /* get starting no. from arg */
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if (sscanf(argv[1], "%f", &fstart) != 1
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|| fstart < 0.0 || fstart >= BIG) {
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ouch();
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exit(1);
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}
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} else { /* get starting no. from stdin */
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while ((i = scanf("%f", &fstart)) != 1
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|| fstart < 0.0 || fstart >= BIG) {
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if (i == EOF)
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exit(1);
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ouch();
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}
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}
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start = (unsigned)fstart;
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/*
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* Quick list of primes < 100
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*/
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if (start <= 97) {
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for (fp = pt; *fp < start; fp++)
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;
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do
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printf("%u\n", *fp);
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while (++fp < &pt[sizeof(pt) / sizeof(*pt)]);
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start = 100;
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}
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quot = start/2;
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start = quot * 2 + 1;
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/*
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* Loop forever:
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*/
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for (;;) {
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/*
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* Generate primes via sieve of Eratosthenes
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*/
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for (p = table; p < &table[TABSIZE]; p++) /* clear sieve */
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*p = '\0';
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v = (unsigned)sqrt((float)(start + tabbits)); /* highest useful factor */
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sieve(3);
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sieve(5);
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sieve(7);
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factor = 11;
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fp = &factab[1];
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do {
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sieve(factor);
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factor += *fp;
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if (++fp >= &factab[sizeof(factab) / sizeof(*factab)])
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fp = factab;
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} while (factor <= v);
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/*
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* Print generated primes
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*/
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for (i = 0; i < 8*TABSIZE; i += 2) {
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if ((table[i>>3] & bittab[i&07]) == 0)
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printf("%u\n", start);
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start += 2;
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}
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}
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}
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/*
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* Insert all multiples of given factor into the sieve
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*/
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sieve(factor)
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unsigned factor;
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{
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register int i;
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unsigned off;
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unsigned quot;
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quot = start / factor;
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off = (quot * factor) - start;
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if ((int)off < 0)
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off += factor;
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while (off < tabbits ) {
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i = (int)off;
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table[i>>3] |= bittab[i&07];
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off += factor;
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}
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}
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/*
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* Error message
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*/
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ouch()
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{
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fprintf(stderr, "Ouch.\n");
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}
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