This patch makes the processing of large really large files more efficient
by making sure that the sender's hash table is large enough to hold all the
checksum entries without being overloaded.

Updated to use the current hashtable method when possible, and the new
hashtable method (which requires a modulus calculation for up to every byte
of the source file) only on large files that need a larger hashtable size.
This avoids slowing down files that don't need the extra-large hashtable.

This was updated for the latest codebase from a patch written by Shachar
Shemesh.

To use this patch, run these commands for a successful build:

    patch -p1 <patches/dynamic_hash.diff
    ./configure                                 (optional if already run)
    make

--- old/match.c
+++ new/match.c
@@ -39,29 +39,50 @@ static int total_matches;
 
 extern struct stats stats;
 
-#define TABLESIZE (1<<16)
+#define TRADITIONAL_TABLESIZE (1<<16)
 
+static uint32 tablesize;
 static int32 *hash_table;
 
 #define SUM2HASH2(s1,s2) (((s1) + (s2)) & 0xFFFF)
 #define SUM2HASH(sum) SUM2HASH2((sum)&0xFFFF,(sum)>>16)
 
+#define BIG_SUM2HASH(sum) ((sum)%tablesize)
+
 static void build_hash_table(struct sum_struct *s)
 {
+	static uint32 alloc_size;
 	int32 i;
 
-	if (!hash_table) {
-		hash_table = new_array(int32, TABLESIZE);
+	/* Dynamically calculate the hash table size so that the hash load
+	 * for big files is about 80%.  This number must be odd or s2 will
+	 * not be able to span the entire set. */
+	tablesize = (uint32)(s->count/8) * 10 + 11;
+	if (tablesize < TRADITIONAL_TABLESIZE)
+		tablesize = TRADITIONAL_TABLESIZE;
+	if (tablesize > alloc_size || tablesize < alloc_size - 16*1024) {
+		if (hash_table)
+			free(hash_table);
+		hash_table = new_array(int32, tablesize);
 		if (!hash_table)
 			out_of_memory("build_hash_table");
+		alloc_size = tablesize;
 	}
 
-	memset(hash_table, 0xFF, TABLESIZE * sizeof hash_table[0]);
+	memset(hash_table, 0xFF, tablesize * sizeof hash_table[0]);
 
-	for (i = 0; i < s->count; i++) {
-		uint32 t = SUM2HASH(s->sums[i].sum1);
-		s->sums[i].chain = hash_table[t];
-		hash_table[t] = i;
+	if (tablesize == TRADITIONAL_TABLESIZE) {
+		for (i = 0; i < s->count; i++) {
+			uint32 t = SUM2HASH(s->sums[i].sum1);
+			s->sums[i].chain = hash_table[t];
+			hash_table[t] = i;
+		}
+	} else {
+		for (i = 0; i < s->count; i++) {
+			uint32 t = BIG_SUM2HASH(s->sums[i].sum1);
+			s->sums[i].chain = hash_table[t];
+			hash_table[t] = i;
+		}
 	}
 }
 
@@ -164,11 +185,16 @@ static void hash_search(int f,struct sum
 				(double)offset, s2 & 0xFFFF, s1 & 0xFFFF);
 		}
 
-		i = hash_table[SUM2HASH2(s1,s2)];
-		if (i < 0)
-			goto null_hash;
+		if (tablesize == TRADITIONAL_TABLESIZE) {
+			if ((i = hash_table[SUM2HASH2(s1,s2)]) < 0)
+				goto null_hash;
+			sum = (s1 & 0xffff) | (s2 << 16);
+		} else {
+			sum = (s1 & 0xffff) | (s2 << 16);
+			if ((i = hash_table[BIG_SUM2HASH(sum)]) < 0)
+				goto null_hash;
+		}
 
-		sum = (s1 & 0xffff) | (s2 << 16);
 		hash_hits++;
 		do {
 			int32 l;