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Task/Extensible-prime-generator/C/extensible-prime-generator.c

@@ -0,0 +1,98 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <math.h>
+
+#define CHUNK_BYTES (32 << 8)
+#define CHUNK_SIZE (CHUNK_BYTES << 6)
+
+int field[CHUNK_BYTES];
+#define GET(x) (field[(x)>>6] &  1<<((x)>>1&31))
+#define SET(x) (field[(x)>>6] |= 1<<((x)>>1&31))
+
+typedef unsigned uint;
+typedef struct {
+        uint *e;
+        uint cap, len;
+} uarray;
+uarray primes, offset;
+
+void push(uarray *a, uint n)
+{
+        if (a->len >= a->cap) {
+                if (!(a->cap *= 2)) a->cap = 16;
+                a->e = realloc(a->e, sizeof(uint) * a->cap);
+        }
+        a->e[a->len++] = n;
+}
+
+uint low;
+void init(void)
+{
+        uint p, q;
+
+        unsigned char f[1<<16];
+        memset(f, 0, sizeof(f));
+        push(&primes, 2);
+        push(&offset, 0);
+        for (p = 3; p < 1<<16; p += 2) {
+                if (f[p]) continue;
+                for (q = p*p; q < 1<<16; q += 2*p) f[q] = 1;
+                push(&primes, p);
+                push(&offset, q);
+        }
+        low = 1<<16;
+}
+
+void sieve(void)
+{
+        uint i, p, q, hi, ptop;
+        if (!low) init();
+
+        memset(field, 0, sizeof(field));
+
+        hi = low + CHUNK_SIZE;
+        ptop = sqrt(hi) * 2 + 1;
+
+        for (i = 1; (p = primes.e[i]*2) < ptop; i++) {
+                for (q = offset.e[i] - low; q < CHUNK_SIZE; q += p)
+                        SET(q);
+                offset.e[i] = q + low;
+        }
+
+        for (p = 1; p < CHUNK_SIZE; p += 2)
+                if (!GET(p)) push(&primes, low + p);
+
+        low = hi;
+}
+
+int main(void)
+{
+        uint i, p, c;
+
+        while (primes.len < 20) sieve();
+        printf("First 20:");
+        for (i = 0; i < 20; i++)
+                printf(" %u", primes.e[i]);
+        putchar('\n');
+
+        while (primes.e[primes.len-1] < 150) sieve();
+        printf("Between 100 and 150:");
+        for (i = 0; i < primes.len; i++) {
+                if ((p = primes.e[i]) >= 100 && p < 150)
+                        printf(" %u", primes.e[i]);
+        }
+        putchar('\n');
+
+        while (primes.e[primes.len-1] < 8000) sieve();
+        for (i = c = 0; i < primes.len; i++)
+                if ((p = primes.e[i]) >= 7700 && p < 8000) c++;
+        printf("%u primes between 7700 and 8000\n", c);
+
+        for (c = 10; c <= 100000000; c *= 10) {
+                while (primes.len < c) sieve();
+                printf("%uth prime: %u\n", c, primes.e[c-1]);
+        }
+
+        return 0;
+}