String hash function

string_hash.c

@@ -0,0 +1,40 @@
+/* http://www.cse.yorku.ca/~oz/hash.html */
+
+#include <string.h>
+#include <stdlib.h>
+#include <stdio.h>
+
+#define PER_LOOP (8)
+
+// macro HASH_SDBM is the equivalent of
+//   hash(i) = hash(i - 1) * 65599 + str[i];
+#define HASH_INIT (0)
+#define HASH_SDBM(h) (h << 6) + (h << 16) - h
+
+// macro HASH_DJB2M is the equivalent of
+//   hash(i) = hash(i - 1) * 33 + str[i];
+//#define HASH_INIT (5381)
+//#define HASH_DJB2(h) ((h << 5) + h)
+
+#define HASH_STEP(h, s) h = *s++ + HASH_SDBM(h);
+
+// loop unwound version using a duff device
+extern unsigned long string_hash(register unsigned char *s, const size_t l)
+{
+	register unsigned long h = HASH_INIT;                    // hash
+	register unsigned int n = (l + PER_LOOP - 1) / PER_LOOP; // n
+
+	switch (l % PER_LOOP) {
+		case 0: do { HASH_STEP(h, s)
+		case 7:      HASH_STEP(h, s)
+		case 6:      HASH_STEP(h, s)
+		case 5:      HASH_STEP(h, s)
+		case 4:      HASH_STEP(h, s)
+		case 3:      HASH_STEP(h, s)
+		case 2:      HASH_STEP(h, s)
+		case 1:      HASH_STEP(h, s)
+				} while (--n);
+	}
+
+	return h;
+}