add a very simple prime generator that uses sieve of Eratosthenes

1 files changed, 69 insertions, 0 deletions

diff --git a/primegen.c b/primegen.c
new file mode 100644
index 0000000..045c2e5
--- /dev/null
+++ b/primegen.c
@@ -0,0 +1,69 @@
+#include <stdlib.h>
+#include <stdio.h>
+#include <stdint.h>
+#include <unistd.h>
+#include <string.h>
+
+int main(int argc, char **argv)
+{
+    int opt;
+    const char *optstr = "n:";
+    long count = -1;
+    int current, i, primecount = 0;
+
+    while ((opt = getopt(argc, argv, optstr)) != -1)
+    {
+        switch (opt)
+        {
+        case 'n':
+            count = strtol(optarg, NULL, 0);
+            break;
+
+        case '?':
+            exit(1);
+        }
+    }
+
+    if (count <= 0)
+    {
+        exit(1);
+    }
+
+    char *sieve = malloc(sizeof(char) * count);
+    memset(sieve, 1, sizeof(char) * count);
+
+    current = 2;
+
+    while (current < count)
+    {
+        if (sieve[current])
+        {
+            primecount++;
+
+            for (i = current*2; i < count; i+=current)
+            {
+                sieve[i] = 0;
+            }
+        }
+
+        current++;
+    }
+
+    printf("const long prime_table[%d] = { 2", primecount);
+
+    for (i = 3; i < count; ++i)
+    {
+        if (sieve[i])
+        {
+            printf(",%d", i);
+        }
+    }
+
+    printf("\n};\n");
+
+    printf("const long prime_table_size = %d;\n", primecount);
+
+    free(sieve);
+
+    return 0;
+}
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