additional useful hash function

2 files changed, 36 insertions, 0 deletions

diff --git a/src/include/gnunet_crypto_lib.h b/src/include/gnunet_crypto_lib.h
index 0b250311a..8a8e71a96 100644
--- a/src/include/gnunet_crypto_lib.h
+++ b/src/include/gnunet_crypto_lib.h
@@ -484,6 +484,20 @@ void GNUNET_CRYPTO_hash_to_aes_key (const GNUNET_HashCode * hc,
 int GNUNET_CRYPTO_hash_get_bit (const GNUNET_HashCode * code,
                                 unsigned int bit);
 
+/**
+ * Determine how many low order bits match in two
+ * GNUNET_HashCodes.  i.e. - 010011 and 011111 share
+ * the first two lowest order bits, and therefore the
+ * return value is two (NOT XOR distance, nor how many
+ * bits match absolutely!).
+ *
+ * @param first the first hashcode
+ * @param second the hashcode to compare first to
+ *
+ * @return the number of bits that match
+ */
+unsigned int GNUNET_CRYPTO_hash_matching_bits(const GNUNET_HashCode *first, const GNUNET_HashCode *second);
+
 
 /**
  * Compare function for HashCodes, producing a total ordering
diff --git a/src/util/crypto_hash.c b/src/util/crypto_hash.c
index af492e15e..0fb2451b2 100644
--- a/src/util/crypto_hash.c
+++ b/src/util/crypto_hash.c
@@ -757,6 +757,28 @@ GNUNET_CRYPTO_hash_get_bit (const GNUNET_HashCode * code, unsigned int bit)
 }
 
 /**
+ * Determine how many low order bits match in two
+ * GNUNET_HashCodes.  i.e. - 010011 and 011111 share
+ * the first two lowest order bits, and therefore the
+ * return value is two (NOT XOR distance, nor how many
+ * bits match absolutely!).
+ *
+ * @param first the first hashcode
+ * @param second the hashcode to compare first to
+ *
+ * @return the number of bits that match
+ */
+unsigned int GNUNET_CRYPTO_hash_matching_bits(const GNUNET_HashCode *first, const GNUNET_HashCode *second)
+{
+  unsigned int i;
+
+  for (i = 0; i < sizeof (GNUNET_HashCode) * 8; i++)
+    if (GNUNET_CRYPTO_hash_get_bit (first, i) != GNUNET_CRYPTO_hash_get_bit (second, i))
+      return i;
+  return sizeof (GNUNET_HashCode) * 8;
+}
+
+/**
  * Compare function for HashCodes, producing a total ordering
  * of all hashcodes.
  * @return 1 if h1 > h2, -1 if h1 < h2 and 0 if h1 == h2.