xcrcgen: tool to create a "generalized CRC" hash table

A so-called "generalized CRC" is a form of hash function based on a
table similar to the conventional bytewise software implementation of
CRC.  For each byte in each data set, it contains a random
byte permutation table.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

1 files changed, 80 insertions, 0 deletions

diff --git a/misc/xcrcgen.c b/misc/xcrcgen.c
new file mode 100644
index 0000000..acfd48e
--- /dev/null
+++ b/misc/xcrcgen.c
@@ -0,0 +1,80 @@
+/*
+ * Produce a "generalized CRC" table.  Assumes a platform with
+ * /dev/urandom -- otherwise reimplement get_random_byte().
+ */
+
+#include <errno.h>
+#include <fcntl.h>
+#include <inttypes.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+static uint8_t get_random_byte(void)
+{
+    static int fd = -1;
+    uint8_t buf;
+    int rv;
+
+    if (fd < 0)
+	fd = open("/dev/urandom", O_RDONLY);
+
+    do {
+	errno = 0;
+	rv = read(fd, &buf, 1);
+	if (rv < 1 && errno != EAGAIN)
+	    abort();
+    } while (rv < 1);
+
+    return buf;
+}
+
+static void random_permute(uint8_t *buf)
+{
+    int i, j, k;
+    int m;
+
+    for (i = 0; i < 256; i++)
+	buf[i] = i;
+
+    m = 255;
+    for (i = 255; i > 0; i--) {
+	if (i <= (m >> 1))
+	    m >>= 1;
+	do {
+	    j = get_random_byte() & m;
+	} while (j > i);
+	k = buf[i];
+	buf[i] = buf[j];
+	buf[j] = k;
+    }
+}
+
+static void xcrc_table(uint64_t *buf)
+{
+    uint8_t perm[256];
+    int i, j;
+
+    memset(buf, 0, 8*256);	/* Make static checkers happy */
+
+    for (i = 0; i < 8; i++) {
+	random_permute(perm);
+	for (j = 0; j < 256; j++)
+	    buf[j] = (buf[j] << 8) | perm[j];
+    }
+}
+
+int main(void)
+{
+    int i;
+    uint64_t buf[256];
+
+    xcrc_table(buf);
+
+    for (i = 0; i < 256; i++) {
+	printf("%016"PRIx64"\n", buf[i]);
+    }
+
+    return 0;
+}