79 lines
1.3 KiB
C
79 lines
1.3 KiB
C
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// A simple Sieve of Eratosthenes
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#include <stdint.h>
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#include <stdbool.h>
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#define BITMAP_SIZE 64
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#define OUTPORT 0x10000000
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static uint32_t bitmap[BITMAP_SIZE/32];
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static void bitmap_set(int idx)
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{
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bitmap[idx/32] |= 1 << (idx % 32);
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}
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static bool bitmap_get(int idx)
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{
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return (bitmap[idx/32] & (1 << (idx % 32))) != 0;
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}
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static void print_str(const char *p)
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{
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while (*p != 0)
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*((volatile uint32_t*)OUTPORT) = *(p++);
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}
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static void print_dec(int val)
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{
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char buffer[10];
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char *p = buffer;
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while (val) {
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*(p++) = val % 10;
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val = val / 10;
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}
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while (p != buffer) {
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*((volatile uint32_t*)OUTPORT) = '0' + *(--p);
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}
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}
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static void print_prime(int idx, int val)
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{
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if (idx < 10)
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print_str(" ");
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print_dec(idx);
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if (idx / 10 == 1)
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goto force_th;
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switch (idx % 10) {
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case 1: print_str("st"); break;
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case 2: print_str("nd"); break;
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case 3: print_str("rd"); break;
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force_th:
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default: print_str("th"); break;
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}
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print_str(" prime is ");
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print_dec(val);
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print_str(".\n");
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}
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void sieve()
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{
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int i, j, k;
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int idx = 1;
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print_prime(idx++, 2);
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for (i = 0; i < BITMAP_SIZE; i++) {
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if (bitmap_get(i))
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continue;
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print_prime(idx++, 3+2*i);
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for (j = 2*(3+2*i);; j += 3+2*i) {
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if (j%2 == 0)
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continue;
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k = (j-3)/2;
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if (k >= BITMAP_SIZE)
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break;
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bitmap_set(k);
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}
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}
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}
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