From 6c043b44a1cec206f3b4f8063fb8383e66ae6d53 Mon Sep 17 00:00:00 2001
From: Andre Noll
Date: Mon, 26 May 2008 02:04:40 +0200
Subject: [PATCH] Add documentation for double_hash().

adu.c  17 ++++++++++++++++
1 file changed, 16 insertions(+), 1 deletion()
diff git a/adu.c b/adu.c
index f8096fb..8068d20 100644
 a/adu.c
+++ b/adu.c
@@ 299,7 +299,22 @@ static int create_tables(void)
return 1;
}

+/*
+ * We use a hash table of size s=2^uid_hash_bits to map the uids into the
+ * interval [0..s]. Hash collisions are treated by open addressing, i.e.
+ * unused slots in the table are used to store different uids that hash to the
+ * same slot.
+ *
+ * If a hash collision occurs, different slots are successively probed in order
+ * to find an unused slot for the new uid. Probing is implemented via a second
+ * hash function that maps the uid to h=(uid * PRIME2)  1, which is always an
+ * odd number.
+ *
+ * An odd number is sufficient to make sure each entry of the hash table gets
+ * probed for probe_num between 0 and s1 because s is a power of two, hence
+ * the second hash value never hash a common divisor with the hash table size.
+ * IOW: h is invertible in the ring [0..s].
+ */
static uint32_t double_hash(uint32_t uid, uint32_t probe_num)
{
return (uid * PRIME1 + ((uid * PRIME2)  1) * probe_num)

2.20.1