[PATCH/RFC v4 5/5] interpret_nth_last_branch(): avoid traversing the reflogs twice

[Thomas Rast <trast@student.ethz.ch>]

From: Johannes Schindelin <Johannes.Schindelin@gmx.de>

Instead of traversing them twice, we just build a list of branch switches,
pick the one we're interested in, and free the list again.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 sha1_name.c |   61 ++++++++++++++++++++++++----------------------------------
 1 files changed, 25 insertions(+), 36 deletions(-)

diff --git a/sha1_name.c b/sha1_name.c
index 9e1538e..b21a1f0 100644
--- a/sha1_name.c
+++ b/sha1_name.c
@@ -5,6 +5,8 @@
 #include "blob.h"
 #include "tree-walk.h"
 #include "refs.h"
+#include "cache-tree.h"
+#include "string-list.h"
 
 static int find_short_object_filename(int len, const char *name, unsigned char *sha1)
 {
@@ -684,43 +686,31 @@ static int get_sha1_oneline(const char *prefix, unsigned char *sha1)
 	return retval;
 }
 
-struct grab_nth_branch_switch_cbdata {
-	int counting;
-	int nth;
-	struct strbuf *buf;
-};
-
-static int grab_nth_branch_switch(unsigned char *osha1, unsigned char *nsha1,
+static int add_one_branch_switch(unsigned char *osha1, unsigned char *nsha1,
 				  const char *email, unsigned long timestamp, int tz,
 				  const char *message, void *cb_data)
 {
-	struct grab_nth_branch_switch_cbdata *cb = cb_data;
+	struct string_list *list = cb_data;
 	const char *match = NULL, *target = NULL;
 	size_t len;
 
-	if (!prefixcmp(message, "checkout: moving from ")) {
-		match = message + strlen("checkout: moving from ");
-		if ((target = strstr(match, " to ")) != NULL)
-			target += 4;
-	}
-
-	if (!match)
+	if (prefixcmp(message, "checkout: moving from "))
 		return 0;
 
-	len = target - match - 4;
-	if (target[len] == '\n' && !strncmp(match, target, len))
-		return 0;
+	match = message + strlen("checkout: moving from ");
 
-	if (cb->counting) {
-		cb->nth++;
-		return 0;
+	/* Is it "moving" from a branch to itself?  Then ignore it. */
+	if ((target = strstr(match, " to ")) != NULL) {
+		target += 4;
+		len = target - match - 4;
+		if (target[len] == '\n' && !strncmp(match, target, len))
+			return 0;
 	}
+	else
+		len = strchrnul(match, ' ') - match;
+
+	string_list_append(xstrndup(match, len), list);
 
-	if (cb->nth-- <= 0) {
-		strbuf_reset(cb->buf);
-		strbuf_add(cb->buf, match, len);
-		return 1;
-	}
 	return 0;
 }
 
@@ -738,7 +728,7 @@ static int grab_nth_branch_switch(unsigned char *osha1, unsigned char *nsha1,
 int interpret_nth_last_branch(const char *name, struct strbuf *buf)
 {
 	int nth;
-	struct grab_nth_branch_switch_cbdata cb;
+	struct string_list branch_list = { NULL, 0, 0, 0 };
 	const char *brace;
 	char *num_end;
 
@@ -751,18 +741,17 @@ int interpret_nth_last_branch(const char *name, struct strbuf *buf)
 	if (num_end != brace)
 		return -1;
 
-	cb.counting = 1;
-	cb.nth = 0;
-	cb.buf = buf;
-	for_each_reflog_ent("HEAD", grab_nth_branch_switch, &cb);
+	for_each_reflog_ent("HEAD", add_one_branch_switch, &branch_list);
 
-	if (cb.nth < nth)
+	if (branch_list.nr < nth)
 		return 0;
 
-	cb.counting = 0;
-	cb.nth -= nth;
-	cb.buf = buf;
-	for_each_reflog_ent("HEAD", grab_nth_branch_switch, &cb);
+	strbuf_reset(buf);
+	strbuf_addstr(buf, branch_list.items[branch_list.nr - nth].string);
+
+	/* force free()ing the items */
+	branch_list.strdup_strings = 1;
+	string_list_clear(&branch_list, 0);
 
 	return brace-name+1;
 }
-- 
1.6.1.315.g92577