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Annotation of 42BSD/games/primes.c, revision 1.1.1.1

1.1       root        1: #ifndef lint
                      2: static char sccsid[] = "@(#)primes.c   4.1 (Wollongong) 6/13/83";
                      3: #endif
                      4: 
                      5: /*
                      6:  *     primes [ number ]
                      7:  *
                      8:  *     Print all primes greater than argument (or number read from stdin).
                      9:  *
                     10:  *     A free translation of 'primes.s'
                     11:  *
                     12:  */
                     13: 
                     14: #include <stdio.h>
                     15: #include <math.h>
                     16: 
                     17: #define        TABSIZE 1000            /* size of sieve table */
                     18: #define        BIG     4294967296.     /* largest unsigned int */
                     19: 
                     20: char   table[TABSIZE];         /* table for sieve of Eratosthenes */
                     21: int    tabbits = 8*TABSIZE;    /* number of bits in table */
                     22: 
                     23: float  fstart;
                     24: unsigned       start;                  /* lowest number to test for prime */
                     25: char   bittab[] = {            /* bit positions (to save shifting) */
                     26:        01, 02, 04, 010, 020, 040, 0100, 0200
                     27: };
                     28: 
                     29: unsigned pt[] =        {               /* primes < 100 */
                     30:        2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43,
                     31:        47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97
                     32: };
                     33: 
                     34: unsigned factab[] = {          /* difference between succesive trial factors */
                     35:        10, 2, 4, 2, 4, 6, 2, 6, 4, 2, 4, 6, 6, 2, 6, 4,
                     36:        2, 6, 4, 6, 8, 4, 2, 4, 2, 4, 8, 6, 4, 6, 2, 4,
                     37:        6, 2, 6, 6, 4, 2, 4, 6, 2, 6, 4, 2, 4, 2, 10, 2
                     38: };
                     39: 
                     40: main(argc, argv)
                     41: int    argc;
                     42: char   **argv;
                     43: {
                     44:        register unsigned       *fp;
                     45:        register char   *p;
                     46:        register int    i;
                     47:        unsigned        quot;
                     48:        unsigned        factor, v;
                     49: 
                     50:        if (argc >= 2) {                /* get starting no. from arg */
                     51:                if (sscanf(argv[1], "%f", &fstart) != 1
                     52:                    || fstart < 0.0 || fstart >= BIG) {
                     53:                        ouch();
                     54:                        exit(1);
                     55:                }
                     56:        } else {                        /* get starting no. from stdin */
                     57:                while ((i = scanf("%f", &fstart)) != 1
                     58:                    || fstart < 0.0 || fstart >= BIG) {
                     59:                        if (i == EOF)
                     60:                                exit(1);
                     61:                        ouch();
                     62:                }
                     63:        }
                     64:        start = (unsigned)fstart;
                     65: 
                     66:        /*
                     67:         * Quick list of primes < 100
                     68:         */
                     69:        if (start <= 97) {
                     70:                for (fp = pt; *fp < start; fp++)
                     71:                        ;
                     72:                do
                     73:                        printf("%u\n", *fp);
                     74:                while (++fp < &pt[sizeof(pt) / sizeof(*pt)]);
                     75:                start = 100;
                     76:        }
                     77:        quot = start/2;
                     78:        start = quot * 2 + 1;
                     79: 
                     80: /*
                     81:  * Loop forever:
                     82:  */
                     83:     for (;;) {
                     84:        /*
                     85:         * Generate primes via sieve of Eratosthenes
                     86:         */
                     87:        for (p = table; p < &table[TABSIZE]; p++)       /* clear sieve */
                     88:                *p = '\0';
                     89:        v = (unsigned)sqrt((float)(start + tabbits)); /* highest useful factor */
                     90:        sieve(3);
                     91:        sieve(5);
                     92:        sieve(7);
                     93:        factor = 11;
                     94:        fp = &factab[1];
                     95:        do {
                     96:                sieve(factor);
                     97:                factor += *fp;
                     98:                if (++fp >= &factab[sizeof(factab) / sizeof(*factab)])
                     99:                        fp = factab;
                    100:        } while (factor <= v);
                    101:        /*
                    102:         * Print generated primes
                    103:         */
                    104:        for (i = 0; i < 8*TABSIZE; i += 2) {
                    105:                if ((table[i>>3] & bittab[i&07]) == 0)
                    106:                        printf("%u\n", start);
                    107:                start += 2;
                    108:        }
                    109:     }
                    110: }
                    111: 
                    112: /*
                    113:  * Insert all multiples of given factor into the sieve
                    114:  */
                    115: sieve(factor)
                    116: unsigned factor;
                    117: {
                    118:        register int    i;
                    119:        unsigned        off;
                    120:        unsigned        quot;
                    121: 
                    122:        quot = start / factor;
                    123:        off = (quot * factor) - start;
                    124:        if ((int)off < 0)
                    125:                off += factor;
                    126:        while (off < tabbits ) {
                    127:                i = (int)off;
                    128:                table[i>>3] |= bittab[i&07];
                    129:                off += factor;
                    130:        }
                    131: }
                    132: 
                    133: /*
                    134:  * Error message
                    135:  */
                    136: ouch()
                    137: {
                    138:        fprintf(stderr, "Ouch.\n");
                    139: }

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