commit 654b8ce3f75d9038d71ec61c8bdb4d6f382e9bcf
Author: notwa <cloningdonor@gmail.com>
Date:   Fri May 31 21:06:00 2013 -0700

diff --git a/crc32.c b/crc32.c
new file mode 100644
index 0000000..15ddda0
--- /dev/null
+++ b/crc32.c
@@ -0,0 +1,60 @@
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+#ifndef BUFFER
+#define BUFFER 4096
+#endif
+uint8_t msg[BUFFER];
+
+/* CRC-32, eg. ethernet */
+const uint32_t crc32_tbl[] = {
+	0x00000000, 0x1db71064, 0x3b6e20c8, 0x26d930ac,
+	0x76dc4190, 0x6b6b51f4, 0x4db26158, 0x5005713c,
+	0xedb88320, 0xf00f9344, 0xd6d6a3e8, 0xcb61b38c,
+	0x9b64c2b0, 0x86d3d2d4, 0xa00ae278, 0xbdbdf21c
+};
+
+/* the idea is to act on nybbles instead of bytes to require less CPU cache */
+uint32_t crc32_calc(uint8_t *ptr, int cnt, uint32_t crc)
+{
+	while (cnt--) {
+		crc = (crc >> 4) ^ crc32_tbl[(crc & 0xf) ^ (*ptr & 0xf)];
+		crc = (crc >> 4) ^ crc32_tbl[(crc & 0xf) ^ (*(ptr++) >> 4)];
+	}
+	return crc;
+}
+
+int main(int argc, char **argv)
+{
+	int len;
+	uint32_t v32 = ~0;
+	FILE *f;
+
+	if (argc == 1) {
+		freopen(NULL, "rb", stdin);
+		f = stdin;
+	} else if (argc == 2) {
+		f = fopen(argv[1], "rb");
+		if (f == NULL) {
+			perror(argv[1]);
+			exit(1);
+		}
+	} else {
+		fputs("usage: crc32 [filename]", stderr);
+		exit(1);
+	}
+
+	do {
+		len = fread(msg, 1, BUFFER, f);
+		if (ferror(f)) {
+			perror(NULL);
+			exit(1);
+		}
+		v32 = crc32_calc(msg, len, v32);
+	} while (!feof(f));
+
+	fclose(f);
+
+	printf("%08lX\n", (unsigned long) ~v32);
+}