lazy index hashing

This delays the hashing of index names until it becomes necessary for
the first time.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -191,6 +191,7 @@ struct index_state {
 	struct cache_tree *cache_tree;
 	time_t timestamp;
 	void *alloc;
+	unsigned name_hash_initialized : 1;
 	struct hash_table name_hash;
 };
 

read-cache.c

@@ -34,12 +34,11 @@ static unsigned int hash_name(const char *name, int namelen)
 	return hash;
 }
 
-static void set_index_entry(struct index_state *istate, int nr, struct cache_entry *ce)
+static void hash_index_entry(struct index_state *istate, struct cache_entry *ce)
 {
 	void **pos;
 	unsigned int hash = hash_name(ce->name, ce_namelen(ce));
 
-	istate->cache[nr] = ce;
 	pos = insert_hash(hash, ce, &istate->name_hash);
 	if (pos) {
 		ce->next = *pos;
@@ -47,6 +46,24 @@ static void set_index_entry(struct index_state *istate, int nr, struct cache_ent
 	}
 }
 
+static void lazy_init_name_hash(struct index_state *istate)
+{
+	int nr;
+
+	if (istate->name_hash_initialized)
+		return;
+	for (nr = 0; nr < istate->cache_nr; nr++)
+		hash_index_entry(istate, istate->cache[nr]);
+	istate->name_hash_initialized = 1;
+}
+
+static void set_index_entry(struct index_state *istate, int nr, struct cache_entry *ce)
+{
+	istate->cache[nr] = ce;
+	if (istate->name_hash_initialized)
+		hash_index_entry(istate, ce);
+}
+
 /*
  * We don't actually *remove* it, we can just mark it invalid so that
  * we won't find it in lookups.
@@ -75,7 +92,10 @@ static void replace_index_entry(struct index_state *istate, int nr, struct cache
 int index_name_exists(struct index_state *istate, const char *name, int namelen)
 {
 	unsigned int hash = hash_name(name, namelen);
-	struct cache_entry *ce = lookup_hash(hash, &istate->name_hash);
+	struct cache_entry *ce;
+
+	lazy_init_name_hash(istate);
+	ce = lookup_hash(hash, &istate->name_hash);
 
 	while (ce) {
 		if (!(ce->ce_flags & CE_UNHASHED)) {