Commit | Line | Data |
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779464d0 SL |
1 | #ifndef lint |
2 | static char sccsid[] = "@(#)primes.c 4.1 (Wollongong) %G%"; | |
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 | } |