sha1_name: support @{-N} syntax in get_sha1()

Let get_sha1() parse the @{-N} syntax, with docs and tests.

Note that while @{-1}^2, @{-2}~5 and such are supported, @{-1}@{1} is
currently not allowed.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-rev-parse.txt

@@ -212,6 +212,9 @@ when you run 'git-merge'.
   reflog of the current branch. For example, if you are on the
   branch 'blabla', then '@\{1\}' means the same as 'blabla@\{1\}'.
 
+* The special construct '@\{-<n>\}' means the <n>th branch checked out
+  before the current one.
+
 * A suffix '{caret}' to a revision parameter means the first parent of
   that commit object.  '{caret}<n>' means the <n>th parent (i.e.
   'rev{caret}'

sha1_name.c

@@ -297,6 +297,8 @@ int dwim_log(const char *str, int len, unsigned char *sha1, char **log)
 	return logs_found;
 }
 
+static int get_sha1_1(const char *name, int len, unsigned char *sha1);
+
 static int get_sha1_basic(const char *str, int len, unsigned char *sha1)
 {
 	static const char *warning = "warning: refname '%.*s' is ambiguous.\n";
@@ -307,7 +309,7 @@ static int get_sha1_basic(const char *str, int len, unsigned char *sha1)
 	if (len == 40 && !get_sha1_hex(str, sha1))
 		return 0;
 
-	/* basic@{time or number} format to query ref-log */
+	/* basic@{time or number or -number} format to query ref-log */
 	reflog_len = at = 0;
 	if (str[len-1] == '}') {
 		for (at = 0; at < len - 1; at++) {
@@ -324,6 +326,16 @@ static int get_sha1_basic(const char *str, int len, unsigned char *sha1)
 		return -1;
 
 	if (!len && reflog_len) {
+		struct strbuf buf = STRBUF_INIT;
+		int ret;
+		/* try the @{-N} syntax for n-th checkout */
+		ret = interpret_nth_last_branch(str+at, &buf);
+		if (ret > 0) {
+			/* substitute this branch name and restart */
+			return get_sha1_1(buf.buf, buf.len, sha1);
+		} else if (ret == 0) {
+			return -1;
+		}
 		/* allow "@{...}" to mean the current branch reflog */
 		refs_found = dwim_ref("HEAD", 4, sha1, &real_ref);
 	} else if (reflog_len)
@@ -379,8 +391,6 @@ static int get_sha1_basic(const char *str, int len, unsigned char *sha1)
 	return 0;
 }
 
-static int get_sha1_1(const char *name, int len, unsigned char *sha1);
-
 static int get_parent(const char *name, int len,
 		      unsigned char *result, int idx)
 {

t/t1505-rev-parse-last.sh

@@ -0,0 +1,71 @@
+#!/bin/sh
+
+test_description='test @{-N} syntax'
+
+. ./test-lib.sh
+
+
+make_commit () {
+	echo "$1" > "$1" &&
+	git add "$1" &&
+	git commit -m "$1"
+}
+
+
+test_expect_success 'setup' '
+
+	make_commit 1 &&
+	git branch side &&
+	make_commit 2 &&
+	make_commit 3 &&
+	git checkout side &&
+	make_commit 4 &&
+	git merge master &&
+	git checkout master
+
+'
+
+# 1 -- 2 -- 3 master
+#  \         \
+#   \         \
+#    --- 4 --- 5 side
+#
+# and 'side' should be the last branch
+
+git log --graph --all --pretty=oneline --decorate
+
+test_rev_equivalent () {
+
+	git rev-parse "$1" > expect &&
+	git rev-parse "$2" > output &&
+	test_cmp expect output
+
+}
+
+test_expect_success '@{-1} works' '
+	test_rev_equivalent side @{-1}
+'
+
+test_expect_success '@{-1}~2 works' '
+	test_rev_equivalent side~2 @{-1}~2
+'
+
+test_expect_success '@{-1}^2 works' '
+	test_rev_equivalent side^2 @{-1}^2
+'
+
+test_expect_failure '@{-1}@{1} works' '
+	test_rev_equivalent side@{1} @{-1}@{1}
+'
+
+test_expect_success '@{-2} works' '
+	test_rev_equivalent master @{-2}
+'
+
+test_expect_success '@{-3} fails' '
+	test_must_fail git rev-parse @{-3}
+'
+
+test_done
+
+