use a hashmap to make remotes faster

Remotes are stored as an array, so looking one up or adding one without
duplication is an O(n) operation.  Reading an entire config file full of
remotes is O(n^2) in the number of remotes.  For a repository with tens of
thousands of remotes, the running time can hit multiple minutes.

Hash tables are way faster.  So we add a hashmap from remote name to
struct remote and use it for all lookups.  The time to add a new remote to
a repo that already has 50,000 remotes drops from ~2 minutes to < 1
second.

We retain the old array of remotes so iterators proceed in config-file
order.
Signed-off-by: NPatrick Reynolds <patrick.reynolds@github.com>
Reviewed-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

remote.c

@@ -42,6 +42,7 @@ struct rewrites {
 static struct remote **remotes;
 static int remotes_alloc;
 static int remotes_nr;
+static struct hashmap remotes_hash;
 
 static struct branch **branches;
 static int branches_alloc;
@@ -136,26 +137,51 @@ static void add_url_alias(struct remote *remote, const char *url)
 	add_pushurl_alias(remote, url);
 }
 
+struct remotes_hash_key {
+	const char *str;
+	int len;
+};
+
+static int remotes_hash_cmp(const struct remote *a, const struct remote *b, const struct remotes_hash_key *key)
+{
+	if (key)
+		return strncmp(a->name, key->str, key->len) || a->name[key->len];
+	else
+		return strcmp(a->name, b->name);
+}
+
+static inline void init_remotes_hash(void)
+{
+	if (!remotes_hash.cmpfn)
+		hashmap_init(&remotes_hash, (hashmap_cmp_fn)remotes_hash_cmp, 0);
+}
+
 static struct remote *make_remote(const char *name, int len)
 {
-	struct remote *ret;
-	int i;
+	struct remote *ret, *replaced;
+	struct remotes_hash_key lookup;
+	struct hashmap_entry lookup_entry;
 
-	for (i = 0; i < remotes_nr; i++) {
-		if (len ? (!strncmp(name, remotes[i]->name, len) &&
-			   !remotes[i]->name[len]) :
-		    !strcmp(name, remotes[i]->name))
-			return remotes[i];
-	}
+	if (!len)
+		len = strlen(name);
+
+	init_remotes_hash();
+	lookup.str = name;
+	lookup.len = len;
+	hashmap_entry_init(&lookup_entry, memhash(name, len));
+
+	if ((ret = hashmap_get(&remotes_hash, &lookup_entry, &lookup)) != NULL)
+		return ret;
 
 	ret = xcalloc(1, sizeof(struct remote));
 	ret->prune = -1;  /* unspecified */
 	ALLOC_GROW(remotes, remotes_nr + 1, remotes_alloc);
 	remotes[remotes_nr++] = ret;
-	if (len)
-		ret->name = xstrndup(name, len);
-	else
-		ret->name = xstrdup(name);
+	ret->name = xstrndup(name, len);
+
+	hashmap_entry_init(ret, lookup_entry.hash);
+	replaced = hashmap_put(&remotes_hash, ret);
+	assert(replaced == NULL);  /* no previous entry overwritten */
 	return ret;
 }
 
@@ -717,13 +743,16 @@ struct remote *pushremote_get(const char *name)
 
 int remote_is_configured(const char *name)
 {
-	int i;
+	struct remotes_hash_key lookup;
+	struct hashmap_entry lookup_entry;
 	read_config();
 
-	for (i = 0; i < remotes_nr; i++)
-		if (!strcmp(name, remotes[i]->name))
-			return 1;
-	return 0;
+	init_remotes_hash();
+	lookup.str = name;
+	lookup.len = strlen(name);
+	hashmap_entry_init(&lookup_entry, memhash(name, lookup.len));
+
+	return hashmap_get(&remotes_hash, &lookup_entry, &lookup) != NULL;
 }
 
 int for_each_remote(each_remote_fn fn, void *priv)

remote.h

@@ -2,6 +2,7 @@
 #define REMOTE_H
 
 #include "parse-options.h"
+#include "hashmap.h"
 
 enum {
 	REMOTE_CONFIG,
@@ -10,6 +11,8 @@ enum {
 };
 
 struct remote {
+	struct hashmap_entry ent;  /* must be first */
+
 	const char *name;
 	int origin;