[ovs-dev] [multipath 1/7] hash: Make functions for hashing a few words easier to use.

[Ben Pfaff]

It's easier for the client to pass in data directly than it is for the
client to stuff it into an array.

These functions will have new users in upcoming commits.
---
 lib/hash.c |   23 +++++++++++++----------
 lib/hash.h |   13 +++++++++----
 2 files changed, 22 insertions(+), 14 deletions(-)

diff --git a/lib/hash.c b/lib/hash.c
index 63b4784..61e17b2 100644
--- a/lib/hash.c
+++ b/lib/hash.c
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2008, 2009 Nicira Networks.
+ * Copyright (c) 2008, 2009, 2010 Nicira Networks.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -52,21 +52,24 @@ hash_words(const uint32_t *p, size_t n, uint32_t basis)
     return c;
 }
 
-/* Returns the hash of the pair of aligned 32-bit words at 'p', starting from
- * 'basis'. */
+/* Returns the hash of 'a', 'b', and 'c'. */
 uint32_t
-hash_2words(const uint32_t *p, uint32_t basis)
+hash_3words(uint32_t a, uint32_t b, uint32_t c)
 {
-    uint32_t a, b, c;
-
-    a = b = c = 0xdeadbeef + (2 << 2) + basis;
-    b += p[1];
-    a += p[0];
+    a += 0xdeadbeef;
+    b += 0xdeadbeef;
+    c += 0xdeadbeef;
     HASH_FINAL(a, b, c);
-
     return c;
 }
 
+/* Returns the hash of 'a' and 'b'. */
+uint32_t
+hash_2words(uint32_t a, uint32_t b)
+{
+    return hash_3words(a, b, 0);
+}
+
 /* Returns the hash of the 'n' bytes at 'p', starting from 'basis'. */
 uint32_t
 hash_bytes(const void *p_, size_t n, uint32_t basis)
diff --git a/lib/hash.h b/lib/hash.h
index 5f6409c..109612f 100644
--- a/lib/hash.h
+++ b/lib/hash.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2008, 2009 Nicira Networks.
+ * Copyright (c) 2008, 2009, 2010 Nicira Networks.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -49,7 +49,8 @@
     } while (0)
 
 uint32_t hash_words(const uint32_t *, size_t n_word, uint32_t basis);
-uint32_t hash_2words(const uint32_t *, uint32_t basis);
+uint32_t hash_2words(uint32_t, uint32_t);
+uint32_t hash_3words(uint32_t, uint32_t, uint32_t);
 uint32_t hash_bytes(const void *, size_t n_bytes, uint32_t basis);
 
 static inline uint32_t hash_string(const char *s, uint32_t basis)
@@ -58,7 +59,11 @@ static inline uint32_t hash_string(const char *s, uint32_t basis)
 }
 
 /* This is Bob Jenkins' integer hash from
- * http://burtleburtle.net/bob/hash/integer.html, modified for style. */
+ * http://burtleburtle.net/bob/hash/integer.html, modified for style.
+ *
+ * This hash is faster than hash_2words(), but it isn't as good when 'basis' is
+ * important.  So use this function for speed or hash_2words() for hash
+ * quality. */
 static inline uint32_t hash_int(uint32_t x, uint32_t basis)
 {
     x -= x << 6;
@@ -87,7 +92,7 @@ static inline uint32_t hash_double(double x, uint32_t basis)
     BUILD_ASSERT_DECL(sizeof x == sizeof value);
 
     memcpy(value, &x, sizeof value);
-    return hash_2words(value, basis);
+    return hash_3words(value[0], value[1], basis);
 }
 
 static inline uint32_t hash_pointer(const void *p, uint32_t basis)
-- 
1.7.1