From a8be83fe00eded0bd6cb0001879ddf211a2cf2f2 Mon Sep 17 00:00:00 2001
From: Linus Torvalds <torvalds@ppc970.osdl.org>
Date: Mon, 20 Jun 2005 20:28:09 -0700
Subject: [PATCH] Make rev-parse understand "extended sha1" syntax

You can say "HEAD.p" for the "parent of HEAD". It nests, so

	HEAD.p2.p

means parent of second parent of HEAD (which obviously depends
on HEAD being a merge).
---
 rev-parse.c | 61 +++++++++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 57 insertions(+), 4 deletions(-)

diff --git a/rev-parse.c b/rev-parse.c
index 7419d18b3b..1b08b45581 100644
--- a/rev-parse.c
+++ b/rev-parse.c
@@ -4,6 +4,9 @@
  * Copyright (C) Linus Torvalds, 2005
  */
 #include "cache.h"
+#include "commit.h"
+
+static int get_extended_sha1(char *name, unsigned char *sha1);
 
 /*
  * Some arguments are relevant "revision" arguments,
@@ -32,6 +35,56 @@ static int is_rev_argument(const char *arg)
 	}
 }
 
+static int get_parent(char *name, unsigned char *result, int idx)
+{
+	unsigned char sha1[20];
+	int ret = get_extended_sha1(name, sha1);
+	struct commit *commit;
+	struct commit_list *p;
+
+	if (ret)
+		return ret;
+	commit = lookup_commit_reference(sha1);
+	if (!commit)
+		return -1;
+	if (parse_commit(commit))
+		return -1;
+	p = commit->parents;
+	while (p) {
+		if (!--idx) {
+			memcpy(result, p->item->object.sha1, 20);
+			return 0;
+		}
+		p = p->next;
+	}
+	return -1;
+}
+
+/*
+ * This is like "get_sha1()", except it allows "sha1 expressions",
+ * notably "xyz.p" for "parent of xyz"
+ */
+static int get_extended_sha1(char *name, unsigned char *sha1)
+{
+	int parent;
+	int len = strlen(name);
+
+	parent = 1;
+	if (len > 3 && name[len-1] >= '1' && name[len-1] <= '9') {
+		parent = name[len-1] - '0';
+		len--;
+	}
+	if (len > 2 && !memcmp(name + len - 2, ".p", 2)) {
+		int ret;
+		name[len-2] = 0;
+		ret = get_parent(name, sha1, parent);
+		name[len-2] = '.';
+		if (!ret)
+			return 0;
+	}
+	return get_sha1(name, sha1);
+}
+
 int main(int argc, char **argv)
 {
 	int i, as_is = 0, revs_only = 0, no_revs = 0;
@@ -83,10 +136,10 @@ int main(int argc, char **argv)
 			unsigned char end[20];
 			char *n = dotdot+2;
 			*dotdot = 0;
-			if (!get_sha1(arg, sha1)) {
+			if (!get_extended_sha1(arg, sha1)) {
 				if (!*n)
 					n = "HEAD";
-				if (!get_sha1(n, end)) {
+				if (!get_extended_sha1(n, end)) {
 					if (no_revs)
 						continue;
 					def = NULL;
@@ -97,14 +150,14 @@ int main(int argc, char **argv)
 			}
 			*dotdot = '.';
 		}
-		if (!get_sha1(arg, sha1)) {
+		if (!get_extended_sha1(arg, sha1)) {
 			if (no_revs)
 				continue;
 			def = NULL;
 			printf("%s\n", sha1_to_hex(sha1));
 			continue;
 		}
-		if (*arg == '^' && !get_sha1(arg+1, sha1)) {
+		if (*arg == '^' && !get_extended_sha1(arg+1, sha1)) {
 			if (no_revs)
 				continue;
 			def = NULL;