RFC v3, 6 partssha1_name: implement @{-N} syntax for N-th last checked out

diff --git a/refs.c b/refs.c
index 33ced65..4571fac 100644
--- a/refs.c
+++ b/refs.c
@@ -1337,24 +1337,68 @@ int create_symref(const char *ref_target, const char *refs_heads_master,
 	return 0;
 }
 
-static char *ref_msg(const char *line, const char *endp)
+/*
+ * Check and parse a single reflog line.  Assumes that there is only
+ * one newline in the range buf[0]..buf[len-1] (but does check that it
+ * is at buf[len-1]).
+ */
+static void parse_reflog_line(const char *buf, int len,
+			      unsigned char *osha1, unsigned char *nsha1,
+			      char **email,
+			      unsigned long *timestamp, int *tz,
+			      char **message,
+			      const char *logname)
 {
-	const char *ep;
-	line += 82;
-	ep = memchr(line, '\n', endp - line);
-	if (!ep)
-		ep = endp;
-	return xmemdupz(line, ep - line);
+	static char *retbuf = NULL;
+	static int retbufsz = 0;
+	char *tzstr, *email_end;
+
+	if (len < 83 || buf[len-1] != '\n')
+		die("Log %s is corrupt (entry too short or unterminated).", logname);
+
+	if (get_sha1_hex(buf, osha1) || buf[40] != ' ')
+		die("Log %s is corrupt (malformed old sha1).", logname);
+
+	if (get_sha1_hex(buf + 41, nsha1) || buf[81] != ' ')
+		die("Log %s is corrupt (malformed new sha1).", logname);
+
+	ALLOC_GROW(retbuf, len-82+1, retbufsz);
+	memcpy(retbuf, buf+82, len-82);
+	retbuf[len-82] = '\0';
+
+	email_end = strchr(retbuf, '>');
+	if (!email_end || email_end[1] != ' ')
+		die("Log %s is corrupt (malformed email field).", logname);
+
+	*email = retbuf;
+	email_end[1] = '\0';
+
+	*timestamp = strtoul(email_end + 2, &tzstr, 10);
+	if (!(*timestamp) || !tzstr || tzstr[0] != ' ' ||
+	    (tzstr[1] != '+' && tzstr[1] != '-') ||
+	    !isdigit(tzstr[2]) || !isdigit(tzstr[3]) ||
+	    !isdigit(tzstr[4]) || !isdigit(tzstr[5]))
+		die("Log %s is corrupt (malformed timezone).", logname);
+	if (!(tzstr[6] == '\t' || tzstr[6] == '\n'))
+		die("Log %s is corrupt (bad message field separator).", logname);
+	*tz = strtoul(tzstr, NULL, 10);
+
+	if (tzstr[6] == '\t')
+		*message = tzstr+7;
+	else
+		*message = tzstr+6;
 }
 
 int read_ref_at(const char *ref, unsigned long at_time, int cnt, unsigned char *sha1, char **msg, unsigned long *cutoff_time, int *cutoff_tz, int *cutoff_cnt)
 {
-	const char *logfile, *logdata, *logend, *rec, *lastgt, *lastrec;
-	char *tz_c;
+	const char *logfile, *logdata, *logend, *rec, *start;
+	char *email, *message;
 	int logfd, tz, reccnt = 0;
 	struct stat st;
 	unsigned long date;
-	unsigned char logged_sha1[20];
+	unsigned char new_sha1[20];
+	unsigned char old_sha1[20];
+	unsigned char next_sha1[20];
 	void *log_mapped;
 	size_t mapsz;
 
@@ -1370,86 +1414,55 @@ int read_ref_at(const char *ref, unsigned long at_time, int cnt, unsigned char *
 	logdata = log_mapped;
 	close(logfd);
 
-	lastrec = NULL;
 	rec = logend = logdata + st.st_size;
+	if (logdata < rec && *(rec-1) == '\n')
+		rec--;
 	while (logdata < rec) {
-		reccnt++;
-		if (logdata < rec && *(rec-1) == '\n')
-			rec--;
-		lastgt = NULL;
-		while (logdata < rec && *(rec-1) != '\n') {
-			rec--;
-			if (*rec == '>')
-				lastgt = rec;
-		}
-		if (!lastgt)
-			die("Log %s is corrupt.", logfile);
-		date = strtoul(lastgt + 1, &tz_c, 10);
+		start = memrchr(logdata, '\n', rec-logdata);
+		if (start)
+			start++;
+		else
+			start = logdata;
+		parse_reflog_line(start, rec-start+1,
+				  old_sha1, new_sha1,
+				  &email, &date, &tz, &message,
+				  logfile);
+
+		if (cutoff_time)
+			*cutoff_time = date;
+		if (cutoff_tz)
+			*cutoff_tz = tz;
+		if (cutoff_cnt)
+			*cutoff_cnt = reccnt;
+		if (msg)
+			*msg = message;
+
 		if (date <= at_time || cnt == 0) {
-			tz = strtoul(tz_c, NULL, 10);
-			if (msg)
-				*msg = ref_msg(rec, logend);
-			if (cutoff_time)
-				*cutoff_time = date;
-			if (cutoff_tz)
-				*cutoff_tz = tz;
-			if (cutoff_cnt)
-				*cutoff_cnt = reccnt - 1;
-			if (lastrec) {
-				if (get_sha1_hex(lastrec, logged_sha1))
-					die("Log %s is corrupt.", logfile);
-				if (get_sha1_hex(rec + 41, sha1))
-					die("Log %s is corrupt.", logfile);
-				if (hashcmp(logged_sha1, sha1)) {
-					fprintf(stderr,
-						"warning: Log %s has gap after %s.\n",
-						logfile, show_date(date, tz, DATE_RFC2822));
-				}
-			}
-			else if (date == at_time) {
-				if (get_sha1_hex(rec + 41, sha1))
-					die("Log %s is corrupt.", logfile);
-			}
-			else {
-				if (get_sha1_hex(rec + 41, logged_sha1))
-					die("Log %s is corrupt.", logfile);
-				if (hashcmp(logged_sha1, sha1)) {
-					fprintf(stderr,
-						"warning: Log %s unexpectedly ended on %s.\n",
-						logfile, show_date(date, tz, DATE_RFC2822));
-				}
-			}
+			if (reccnt && hashcmp(new_sha1, next_sha1))
+				fprintf(stderr,
+					"warning: Log %s has gap after %s.\n",
+					logfile, show_date(date, tz, DATE_RFC2822));
+			if (!reccnt && date < at_time && hashcmp(new_sha1, next_sha1))
+				fprintf(stderr,
+					"warning: Log %s unexpectedly ended on %s.\n",
+					logfile, show_date(date, tz, DATE_RFC2822));
+				/* leave caller's sha1 untouched */
+			else
+				hashcpy(sha1, new_sha1);
 			munmap(log_mapped, mapsz);
 			return 0;
 		}
-		lastrec = rec;
+
+		hashcpy(next_sha1, old_sha1);
+		rec = start-1;
 		if (cnt > 0)
 			cnt--;
+		reccnt++;
 	}
 
-	rec = logdata;
-	while (rec < logend && *rec != '>' && *rec != '\n')
-		rec++;
-	if (rec == logend || *rec == '\n')
-		die("Log %s is corrupt.", logfile);
-	date = strtoul(rec + 1, &tz_c, 10);
-	tz = strtoul(tz_c, NULL, 10);
-	if (get_sha1_hex(logdata, sha1))
-		die("Log %s is corrupt.", logfile);
-	if (is_null_sha1(sha1)) {
-		if (get_sha1_hex(logdata + 41, sha1))
-			die("Log %s is corrupt.", logfile);
-	}
-	if (msg)
-		*msg = ref_msg(logdata, logend);
-	munmap(log_mapped, mapsz);
+	hashcpy(sha1, new_sha1);
 
-	if (cutoff_time)
-		*cutoff_time = date;
-	if (cutoff_tz)
-		*cutoff_tz = tz;
-	if (cutoff_cnt)
-		*cutoff_cnt = reccnt;
+	munmap(log_mapped, mapsz);
 	return 1;
 }
 
@@ -1466,30 +1479,16 @@ int for_each_reflog_ent(const char *ref, each_reflog_ent_fn fn, void *cb_data)
 		return -1;
 	while (fgets(buf, sizeof(buf), logfp)) {
 		unsigned char osha1[20], nsha1[20];
-		char *email_end, *message;
+		char *email, *message;
 		unsigned long timestamp;
 		int len, tz;
 
-		/* old SP new SP name <email> SP time TAB msg LF */
 		len = strlen(buf);
-		if (len < 83 || buf[len-1] != '\n' ||
-		    get_sha1_hex(buf, osha1) || buf[40] != ' ' ||
-		    get_sha1_hex(buf + 41, nsha1) || buf[81] != ' ' ||
-		    !(email_end = strchr(buf + 82, '>')) ||
-		    email_end[1] != ' ' ||
-		    !(timestamp = strtoul(email_end + 2, &message, 10)) ||
-		    !message || message[0] != ' ' ||
-		    (message[1] != '+' && message[1] != '-') ||
-		    !isdigit(message[2]) || !isdigit(message[3]) ||
-		    !isdigit(message[4]) || !isdigit(message[5]))
-			continue; /* corrupt? */
-		email_end[1] = '\0';
-		tz = strtol(message + 1, NULL, 10);
-		if (message[6] != '\t')
-			message += 6;
-		else
-			message += 7;
-		ret = fn(osha1, nsha1, buf+82, timestamp, tz, message, cb_data);
+		parse_reflog_line(buf, len,
+				  osha1, nsha1,
+				  &email, &timestamp, &tz, &message,
+				  logfile);
+		ret = fn(osha1, nsha1, email, timestamp, tz, message, cb_data);
 		if (ret)
 			break;
 	}

diff --git a/refs.c b/refs.c
index 4571fac..0a57896 100644
--- a/refs.c
+++ b/refs.c
@@ -1389,12 +1389,31 @@ static void parse_reflog_line(const char *buf, int len,
 		*message = tzstr+6;
 }
 
+static char *open_reflog(const char *ref, size_t *mapsz, const char **logfile)
+{
+	struct stat st;
+	int logfd;
+	char *map;
+
+	*logfile = git_path("logs/%s", ref);
+	logfd = open(*logfile, O_RDONLY, 0);
+	if (logfd < 0)
+		return NULL;
+	if (fstat(logfd, &st))
+		return NULL;
+
+	*mapsz = xsize_t(st.st_size);
+	map = xmmap(NULL, *mapsz, PROT_READ, MAP_PRIVATE, logfd, 0);
+	close(logfd);
+	return map;
+}
+
+
 int read_ref_at(const char *ref, unsigned long at_time, int cnt, unsigned char *sha1, char **msg, unsigned long *cutoff_time, int *cutoff_tz, int *cutoff_cnt)
 {
-	const char *logfile, *logdata, *logend, *rec, *start;
+	const char *logfile, *logdata, *logend, *rec, *end;
 	char *email, *message;
-	int logfd, tz, reccnt = 0;
-	struct stat st;
+	int tz, reccnt = 0;
 	unsigned long date;
 	unsigned char new_sha1[20];
 	unsigned char old_sha1[20];
@@ -1402,28 +1421,20 @@ int read_ref_at(const char *ref, unsigned long at_time, int cnt, unsigned char *
 	void *log_mapped;
 	size_t mapsz;
 
-	logfile = git_path("logs/%s", ref);
-	logfd = open(logfile, O_RDONLY, 0);
-	if (logfd < 0)
+	logdata = log_mapped = open_reflog(ref, &mapsz, &logfile);
+	if (!logdata)
 		die("Unable to read log %s: %s", logfile, strerror(errno));
-	fstat(logfd, &st);
-	if (!st.st_size)
+	if (!mapsz)
 		die("Log %s is empty.", logfile);
-	mapsz = xsize_t(st.st_size);
-	log_mapped = xmmap(NULL, mapsz, PROT_READ, MAP_PRIVATE, logfd, 0);
-	logdata = log_mapped;
-	close(logfd);
 
-	rec = logend = logdata + st.st_size;
-	if (logdata < rec && *(rec-1) == '\n')
-		rec--;
+	rec = logend = logdata + mapsz;
 	while (logdata < rec) {
-		start = memrchr(logdata, '\n', rec-logdata);
-		if (start)
-			start++;
-		else
-			start = logdata;
-		parse_reflog_line(start, rec-start+1,
+		if (logdata < rec && rec[-1] == '\n')
+			rec--;
+		end = rec;
+		while (logdata < rec && rec[-1] != '\n')
+			rec--;
+		parse_reflog_line(rec, end-rec+1,
 				  old_sha1, new_sha1,
 				  &email, &date, &tz, &message,
 				  logfile);
@@ -1454,7 +1465,6 @@ int read_ref_at(const char *ref, unsigned long at_time, int cnt, unsigned char *
 		}
 
 		hashcpy(next_sha1, old_sha1);
-		rec = start-1;
 		if (cnt > 0)
 			cnt--;
 		reccnt++;

diff --git a/refs.c b/refs.c
index 0a57896..3848aa0 100644
--- a/refs.c
+++ b/refs.c
@@ -1478,22 +1478,28 @@ int read_ref_at(const char *ref, unsigned long at_time, int cnt, unsigned char *
 
 int for_each_reflog_ent(const char *ref, each_reflog_ent_fn fn, void *cb_data)
 {
-	const char *logfile;
-	FILE *logfp;
-	char buf[1024];
+	const char *log_mapped, *buf, *log_end, *logfile;
+	size_t mapsz;
 	int ret = 0;
 
-	logfile = git_path("logs/%s", ref);
-	logfp = fopen(logfile, "r");
-	if (!logfp)
+	buf = log_mapped = open_reflog(ref, &mapsz, &logfile);
+	if (!log_mapped)
 		return -1;
-	while (fgets(buf, sizeof(buf), logfp)) {
+	log_end = buf + mapsz;
+
+	while (buf < log_end) {
 		unsigned char osha1[20], nsha1[20];
 		char *email, *message;
+		const char *next;
 		unsigned long timestamp;
 		int len, tz;
 
-		len = strlen(buf);
+		next = memchr(buf, '\n', log_end - buf);
+		if (next)
+			next++;
+		else
+			next = log_end;
+		len = next - buf;
 		parse_reflog_line(buf, len,
 				  osha1, nsha1,
 				  &email, &timestamp, &tz, &message,
@@ -1501,8 +1507,9 @@ int for_each_reflog_ent(const char *ref, each_reflog_ent_fn fn, void *cb_data)
 		ret = fn(osha1, nsha1, email, timestamp, tz, message, cb_data);
 		if (ret)
 			break;
+		buf = next;
 	}
-	fclose(logfp);
+	munmap((void*) log_mapped, mapsz);
 	return ret;
 }
 

refs.c, refs.h

diff --git a/refs.c b/refs.c
index 3848aa0..cc78f63 100644
--- a/refs.c
+++ b/refs.c
@@ -1476,6 +1476,41 @@ int read_ref_at(const char *ref, unsigned long at_time, int cnt, unsigned char *
 	return 1;
 }
 
+int for_each_reflog_ent_backward(const char *ref, each_reflog_ent_fn fn, void *cb_data)
+{
+	const char *log_mapped, *buf, *log_end, *logfile;
+	size_t mapsz;
+	int ret = 0;
+
+	log_mapped = open_reflog(ref, &mapsz, &logfile);
+	if (!log_mapped)
+		return -1;
+	buf = log_end = log_mapped + mapsz;
+
+	while (buf > log_mapped) {
+		unsigned char osha1[20], nsha1[20];
+		char *email, *message;
+		const char *end;
+		unsigned long timestamp;
+		int tz;
+
+		while (buf > log_mapped && buf[-1] == '\n')
+			buf--;
+		end = buf;
+		while (buf > log_mapped && buf[-1] != '\n')
+			buf--;
+		parse_reflog_line(buf, end-buf+1,
+				  osha1, nsha1,
+				  &email, &timestamp, &tz, &message,
+				  logfile);
+		ret = fn(osha1, nsha1, email, timestamp, tz, message, cb_data);
+		if (ret)
+			break;
+	}
+	munmap((void*) log_mapped, mapsz);
+	return ret;
+}
+
 int for_each_reflog_ent(const char *ref, each_reflog_ent_fn fn, void *cb_data)
 {
 	const char *log_mapped, *buf, *log_end, *logfile;
diff --git a/refs.h b/refs.h
index 06ad260..723bddc 100644
--- a/refs.h
+++ b/refs.h
@@ -60,6 +60,7 @@ struct ref_lock {
 /* iterate over reflog entries */
 typedef int each_reflog_ent_fn(unsigned char *osha1, unsigned char *nsha1, const char *, unsigned long, int, const char *, void *);
 int for_each_reflog_ent(const char *ref, each_reflog_ent_fn fn, void *cb_data);
+int for_each_reflog_ent_backward(const char *ref, each_reflog_ent_fn fn, void *cb_data);
 
 /*
  * Calls the specified function for each reflog file until it returns nonzero,

Documentation/git-rev-parse.txt, cache.h, sha1_name.c, t/t1505-rev-parse-last.sh

diff --git a/Documentation/git-rev-parse.txt b/Documentation/git-rev-parse.txt
index 2921da3..3ccef2f 100644
--- a/Documentation/git-rev-parse.txt
+++ b/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}'
diff --git a/cache.h b/cache.h
index 8e1af26..0dd9168 100644
--- a/cache.h
+++ b/cache.h
@@ -663,6 +663,7 @@ static inline unsigned int hexval(unsigned char c)
 extern const char *resolve_ref(const char *path, unsigned char *sha1, int, int *);
 extern int dwim_ref(const char *str, int len, unsigned char *sha1, char **ref);
 extern int dwim_log(const char *str, int len, unsigned char *sha1, char **ref);
+extern int interpret_nth_last_branch(const char *str, struct strbuf *);
 
 extern int refname_match(const char *abbrev_name, const char *full_name, const char **rules);
 extern const char *ref_rev_parse_rules[];
diff --git a/sha1_name.c b/sha1_name.c
index 159c2ab..b2dd302 100644
--- a/sha1_name.c
+++ b/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 +326,14 @@ 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);
+		}
 		/* allow "@{...}" to mean the current branch reflog */
 		refs_found = dwim_ref("HEAD", 4, sha1, &real_ref);
 	} else if (reflog_len)
@@ -379,8 +389,6 @@ 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)
 {
@@ -674,6 +682,71 @@ static int get_sha1_oneline(const char *prefix, unsigned char *sha1)
 	return retval;
 }
 
+struct grab_nth_branch_switch_cbdata {
+	int nth;
+	struct strbuf *buf;
+};
+
+static int grab_nth_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;
+	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)
+		return 0;
+
+	len = target - match - 4;
+	if (target[len] == '\n' && !strncmp(match, target, len))
+		return 0;
+
+	if (--cb->nth <= 0) {
+		strbuf_reset(cb->buf);
+		strbuf_add(cb->buf, match, len);
+		return 1;
+	}
+	return 0;
+}
+
+/*
+ * This reads "@{-N}" syntax, finds the name of the Nth previous
+ * branch we were on, and places the name of the branch in the given
+ * buf and returns 0 if successful.
+ *
+ * If the input is not of the accepted format, it returns a negative
+ * number to signal an error.
+ */
+int interpret_nth_last_branch(const char *name, struct strbuf *buf)
+{
+	int nth;
+	struct grab_nth_branch_switch_cbdata cb;
+	const char *brace;
+	char *num_end;
+
+	if (name[0] != '@' || name[1] != '{' || name[2] != '-')
+		return -1;
+	brace = strchr(name, '}');
+	if (!brace)
+		return -1;
+	nth = strtol(name+3, &num_end, 10);
+	if (num_end != brace)
+		return -1;
+
+	cb.nth = nth;
+	cb.buf = buf;
+	for_each_reflog_ent_backward("HEAD", grab_nth_branch_switch, &cb);
+
+	return brace-name+1;
+}
+
 /*
  * This is like "get_sha1_basic()", except it allows "sha1 expressions",
  * notably "xyz^" for "parent of xyz"
diff --git a/t/t1505-rev-parse-last.sh b/t/t1505-rev-parse-last.sh
new file mode 100755
index 0000000..1e49dd2
--- /dev/null
+++ b/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
+
+

Documentation/git-checkout.txt, builtin-checkout.c, t/t2012-checkout-last.sh

diff --git a/Documentation/git-checkout.txt b/Documentation/git-checkout.txt
index 9cd5151..3bccffa 100644
--- a/Documentation/git-checkout.txt
+++ b/Documentation/git-checkout.txt
@@ -133,6 +133,10 @@ the conflicted merge in the specified paths.
 +
 When this parameter names a non-branch (but still a valid commit object),
 your HEAD becomes 'detached'.
++
+As a special case, the "`@\{-N\}`" syntax for the N-th last branch
+checks out the branch (instead of detaching).  You may also specify
+"`-`" which is synonymous with "`@\{-1\}`".
 
 
 Detached HEAD
diff --git a/builtin-checkout.c b/builtin-checkout.c
index b5dd9c0..b0a101b 100644
--- a/builtin-checkout.c
+++ b/builtin-checkout.c
@@ -361,8 +361,16 @@ struct branch_info {
 static void setup_branch_path(struct branch_info *branch)
 {
 	struct strbuf buf = STRBUF_INIT;
-	strbuf_addstr(&buf, "refs/heads/");
-	strbuf_addstr(&buf, branch->name);
+	int ret;
+
+	if ((ret = interpret_nth_last_branch(branch->name, &buf))
+	    && ret == strlen(branch->name)) {
+		branch->name = xstrdup(buf.buf);
+		strbuf_splice(&buf, 0, 0, "refs/heads/", 11);
+	} else {
+		strbuf_addstr(&buf, "refs/heads/");
+		strbuf_addstr(&buf, branch->name);
+	}
 	branch->path = strbuf_detach(&buf, NULL);
 }
 
@@ -671,6 +679,9 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 		arg = argv[0];
 		has_dash_dash = (argc > 1) && !strcmp(argv[1], "--");
 
+		if (!strcmp(arg, "-"))
+			arg = "@{-1}";
+
 		if (get_sha1(arg, rev)) {
 			if (has_dash_dash)          /* case (1) */
 				die("invalid reference: %s", arg);
diff --git a/t/t2012-checkout-last.sh b/t/t2012-checkout-last.sh
new file mode 100755
index 0000000..320f6eb
--- /dev/null
+++ b/t/t2012-checkout-last.sh
@@ -0,0 +1,50 @@
+#!/bin/sh
+
+test_description='checkout can switch to last branch'
+
+. ./test-lib.sh
+
+test_expect_success 'setup' '
+	echo hello >world &&
+	git add world &&
+	git commit -m initial &&
+	git branch other &&
+	echo "hello again" >>world &&
+	git add world &&
+	git commit -m second
+'
+
+test_expect_success '"checkout -" does not work initially' '
+	test_must_fail git checkout -
+'
+
+test_expect_success 'first branch switch' '
+	git checkout other
+'
+
+test_expect_success '"checkout -" switches back' '
+	git checkout - &&
+	test "z$(git symbolic-ref HEAD)" = "zrefs/heads/master"
+'
+
+test_expect_success '"checkout -" switches forth' '
+	git checkout - &&
+	test "z$(git symbolic-ref HEAD)" = "zrefs/heads/other"
+'
+
+test_expect_success 'detach HEAD' '
+	git checkout $(git rev-parse HEAD)
+'
+
+test_expect_success '"checkout -" attaches again' '
+	git checkout - &&
+	test "z$(git symbolic-ref HEAD)" = "zrefs/heads/other"
+'
+
+test_expect_success '"checkout -" detaches again' '
+	git checkout - &&
+	test "z$(git rev-parse HEAD)" = "z$(git rev-parse other)" &&
+	test_must_fail git symbolic-ref HEAD
+'
+
+test_done

Documentation/git-rev-parse.txt, cache.h, sha1_name.c, t/t1505-rev-parse-last.sh

diff --git a/Documentation/git-rev-parse.txt b/Documentation/git-rev-parse.txt
index 2921da3..3ccef2f 100644
--- a/Documentation/git-rev-parse.txt
+++ b/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}'
diff --git a/cache.h b/cache.h
index 8e1af26..0dd9168 100644
--- a/cache.h
+++ b/cache.h
@@ -663,6 +663,7 @@ static inline unsigned int hexval(unsigned char c)
 extern const char *resolve_ref(const char *path, unsigned char *sha1, int, int *);
 extern int dwim_ref(const char *str, int len, unsigned char *sha1, char **ref);
 extern int dwim_log(const char *str, int len, unsigned char *sha1, char **ref);
+extern int interpret_nth_last_branch(const char *str, struct strbuf *);
 
 extern int refname_match(const char *abbrev_name, const char *full_name, const char **rules);
 extern const char *ref_rev_parse_rules[];
diff --git a/sha1_name.c b/sha1_name.c
index 159c2ab..9e1538e 100644
--- a/sha1_name.c
+++ b/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)
 {
@@ -674,6 +684,89 @@ 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,
+				  const char *email, unsigned long timestamp, int tz,
+				  const char *message, void *cb_data)
+{
+	struct grab_nth_branch_switch_cbdata *cb = 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)
+		return 0;
+
+	len = target - match - 4;
+	if (target[len] == '\n' && !strncmp(match, target, len))
+		return 0;
+
+	if (cb->counting) {
+		cb->nth++;
+		return 0;
+	}
+
+	if (cb->nth-- <= 0) {
+		strbuf_reset(cb->buf);
+		strbuf_add(cb->buf, match, len);
+		return 1;
+	}
+	return 0;
+}
+
+/*
+ * This reads "@{-N}" syntax, finds the name of the Nth previous
+ * branch we were on, and places the name of the branch in the given
+ * buf and returns the number of characters parsed if successful.
+ *
+ * If the input is not of the accepted format, it returns a negative
+ * number to signal an error.
+ *
+ * If the input was ok but there are not N branch switches in the
+ * reflog, it returns 0.
+ */
+int interpret_nth_last_branch(const char *name, struct strbuf *buf)
+{
+	int nth;
+	struct grab_nth_branch_switch_cbdata cb;
+	const char *brace;
+	char *num_end;
+
+	if (name[0] != '@' || name[1] != '{' || name[2] != '-')
+		return -1;
+	brace = strchr(name, '}');
+	if (!brace)
+		return -1;
+	nth = strtol(name+3, &num_end, 10);
+	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);
+
+	if (cb.nth < nth)
+		return 0;
+
+	cb.counting = 0;
+	cb.nth -= nth;
+	cb.buf = buf;
+	for_each_reflog_ent("HEAD", grab_nth_branch_switch, &cb);
+
+	return brace-name+1;
+}
+
 /*
  * This is like "get_sha1_basic()", except it allows "sha1 expressions",
  * notably "xyz^" for "parent of xyz"
diff --git a/t/t1505-rev-parse-last.sh b/t/t1505-rev-parse-last.sh
new file mode 100755
index 0000000..1e49dd2
--- /dev/null
+++ b/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
+
+

Documentation/git-checkout.txt, builtin-checkout.c, t/t2012-checkout-last.sh

diff --git a/Documentation/git-checkout.txt b/Documentation/git-checkout.txt
index 9cd5151..3bccffa 100644
--- a/Documentation/git-checkout.txt
+++ b/Documentation/git-checkout.txt
@@ -133,6 +133,10 @@ the conflicted merge in the specified paths.
 +
 When this parameter names a non-branch (but still a valid commit object),
 your HEAD becomes 'detached'.
++
+As a special case, the "`@\{-N\}`" syntax for the N-th last branch
+checks out the branch (instead of detaching).  You may also specify
+"`-`" which is synonymous with "`@\{-1\}`".
 
 
 Detached HEAD
diff --git a/builtin-checkout.c b/builtin-checkout.c
index b5dd9c0..b0a101b 100644
--- a/builtin-checkout.c
+++ b/builtin-checkout.c
@@ -361,8 +361,16 @@ struct branch_info {
 static void setup_branch_path(struct branch_info *branch)
 {
 	struct strbuf buf = STRBUF_INIT;
-	strbuf_addstr(&buf, "refs/heads/");
-	strbuf_addstr(&buf, branch->name);
+	int ret;
+
+	if ((ret = interpret_nth_last_branch(branch->name, &buf))
+	    && ret == strlen(branch->name)) {
+		branch->name = xstrdup(buf.buf);
+		strbuf_splice(&buf, 0, 0, "refs/heads/", 11);
+	} else {
+		strbuf_addstr(&buf, "refs/heads/");
+		strbuf_addstr(&buf, branch->name);
+	}
 	branch->path = strbuf_detach(&buf, NULL);
 }
 
@@ -671,6 +679,9 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 		arg = argv[0];
 		has_dash_dash = (argc > 1) && !strcmp(argv[1], "--");
 
+		if (!strcmp(arg, "-"))
+			arg = "@{-1}";
+
 		if (get_sha1(arg, rev)) {
 			if (has_dash_dash)          /* case (1) */
 				die("invalid reference: %s", arg);
diff --git a/t/t2012-checkout-last.sh b/t/t2012-checkout-last.sh
new file mode 100755
index 0000000..320f6eb
--- /dev/null
+++ b/t/t2012-checkout-last.sh
@@ -0,0 +1,50 @@
+#!/bin/sh
+
+test_description='checkout can switch to last branch'
+
+. ./test-lib.sh
+
+test_expect_success 'setup' '
+	echo hello >world &&
+	git add world &&
+	git commit -m initial &&
+	git branch other &&
+	echo "hello again" >>world &&
+	git add world &&
+	git commit -m second
+'
+
+test_expect_success '"checkout -" does not work initially' '
+	test_must_fail git checkout -
+'
+
+test_expect_success 'first branch switch' '
+	git checkout other
+'
+
+test_expect_success '"checkout -" switches back' '
+	git checkout - &&
+	test "z$(git symbolic-ref HEAD)" = "zrefs/heads/master"
+'
+
+test_expect_success '"checkout -" switches forth' '
+	git checkout - &&
+	test "z$(git symbolic-ref HEAD)" = "zrefs/heads/other"
+'
+
+test_expect_success 'detach HEAD' '
+	git checkout $(git rev-parse HEAD)
+'
+
+test_expect_success '"checkout -" attaches again' '
+	git checkout - &&
+	test "z$(git symbolic-ref HEAD)" = "zrefs/heads/other"
+'
+
+test_expect_success '"checkout -" detaches again' '
+	git checkout - &&
+	test "z$(git rev-parse HEAD)" = "z$(git rev-parse other)" &&
+	test_must_fail git symbolic-ref HEAD
+'
+
+test_done

diff --git a/sha1_name.c b/sha1_name.c
index 2bbc5f1..306d04b 100644
--- a/sha1_name.c
+++ b/sha1_name.c
@@ -6,6 +6,7 @@
 #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)
 {
@@ -691,43 +692,31 @@ 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;
-
-	if (cb->counting) {
-		cb->nth++;
-		return 0;
-	}
-
-	if (cb->nth-- <= 0) {
-		strbuf_reset(cb->buf);
-		strbuf_add(cb->buf, match, len);
-		return 1;
-	}
+	match = message + strlen("checkout: moving from ");
+
+	/* 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);
+
 	return 0;
 }
 
@@ -745,7 +734,7 @@ 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;
 
@@ -758,18 +747,17 @@ 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);
-
-	if (cb.nth < nth)
+	for_each_reflog_ent("HEAD", add_one_branch_switch, &branch_list);
+
+	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;
 }

builtin-checkout.c, cache.h, sha1_name.c

diff --git a/builtin-checkout.c b/builtin-checkout.c
index b5dd9c0..a3b69d6 100644
--- a/builtin-checkout.c
+++ b/builtin-checkout.c
@@ -361,8 +361,14 @@ struct branch_info {
 static void setup_branch_path(struct branch_info *branch)
 {
 	struct strbuf buf = STRBUF_INIT;
-	strbuf_addstr(&buf, "refs/heads/");
-	strbuf_addstr(&buf, branch->name);
+
+	if (!interpret_nth_last_branch(branch->name, &buf)) {
+		branch->name = xstrdup(buf.buf);
+		strbuf_splice(&buf, 0, 0, "refs/heads/", 11);
+	} else {
+		strbuf_addstr(&buf, "refs/heads/");
+		strbuf_addstr(&buf, branch->name);
+	}
 	branch->path = strbuf_detach(&buf, NULL);
 }
 
diff --git a/cache.h b/cache.h
index 8e1af26..0dd9168 100644
--- a/cache.h
+++ b/cache.h
@@ -663,6 +663,7 @@ static inline unsigned int hexval(unsigned char c)
 extern const char *resolve_ref(const char *path, unsigned char *sha1, int, int *);
 extern int dwim_ref(const char *str, int len, unsigned char *sha1, char **ref);
 extern int dwim_log(const char *str, int len, unsigned char *sha1, char **ref);
+extern int interpret_nth_last_branch(const char *str, struct strbuf *);
 
 extern int refname_match(const char *abbrev_name, const char *full_name, const char **rules);
 extern const char *ref_rev_parse_rules[];
diff --git a/sha1_name.c b/sha1_name.c
index 159c2ab..6377264 100644
--- a/sha1_name.c
+++ b/sha1_name.c
@@ -674,6 +674,84 @@ 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,
+				  const char *email, unsigned long timestamp, int tz,
+				  const char *message, void *cb_data)
+{
+	struct grab_nth_branch_switch_cbdata *cb = cb_data;
+	const char *match = NULL;
+
+	if (!prefixcmp(message, "checkout: moving to "))
+		match = message + strlen("checkout: moving to ");
+	else if (!prefixcmp(message, "checkout: moving from ")) {
+		const char *cp = message + strlen("checkout: moving from ");
+		if ((cp = strstr(cp, " to ")) != NULL) {
+			match = cp + 4;
+		}
+	}
+
+	if (!match)
+		return 0;
+
+	if (cb->counting) {
+		cb->nth++;
+		return 0;
+	}
+
+	if (--cb->nth <= 0) {
+		size_t len = strlen(match);
+		while (match[len-1] == '\n')
+			len--;
+		strbuf_reset(cb->buf);
+		strbuf_add(cb->buf, match, len);
+		return 1;
+	}
+	return 0;
+}
+
+/*
+ * This reads "@{-N}" syntax, finds the name of the Nth previous
+ * branch we were on, and places the name of the branch in the given
+ * buf and returns 0 if successful.
+ *
+ * If the input is not of the accepted format, it returns a negative
+ * number to signal an error.
+ */
+int interpret_nth_last_branch(const char *name, struct strbuf *buf)
+{
+	int nth, i;
+	struct grab_nth_branch_switch_cbdata cb;
+
+	if (name[0] != '@' || name[1] != '{' || name[2] != '-')
+		return -1;
+	for (i = 3, nth = 0; name[i] && name[i] != '}'; i++) {
+		char ch = name[i];
+		if ('0' <= ch && ch <= '9')
+			nth = nth * 10 + ch - '0';
+		else
+			return -1;
+	}
+	if (nth < 0 || 10 <= nth)
+		return -1;
+
+	cb.counting = 1;
+	cb.nth = 0;
+	cb.buf = buf;
+	for_each_reflog_ent("HEAD", grab_nth_branch_switch, &cb);
+
+	cb.counting = 0;
+	cb.nth -= nth;
+	cb.buf = buf;
+	for_each_reflog_ent("HEAD", grab_nth_branch_switch, &cb);
+	return 0;
+}
+
 /*
  * This is like "get_sha1_basic()", except it allows "sha1 expressions",
  * notably "xyz^" for "parent of xyz"

builtin-checkout.c, sha1_name.c

diff --git a/builtin-checkout.c b/builtin-checkout.c
index a3b69d6..dc1de06 100644
--- a/builtin-checkout.c
+++ b/builtin-checkout.c
@@ -361,8 +361,10 @@ struct branch_info {
 static void setup_branch_path(struct branch_info *branch)
 {
 	struct strbuf buf = STRBUF_INIT;
+	int ret;
 
-	if (!interpret_nth_last_branch(branch->name, &buf)) {
+	if ((ret = interpret_nth_last_branch(branch->name, &buf))
+	    && ret == strlen(branch->name)) {
 		branch->name = xstrdup(buf.buf);
 		strbuf_splice(&buf, 0, 0, "refs/heads/", 11);
 	} else {
diff --git a/sha1_name.c b/sha1_name.c
index 6377264..34e39db 100644
--- a/sha1_name.c
+++ b/sha1_name.c
@@ -685,29 +685,28 @@ static int grab_nth_branch_switch(unsigned char *osha1, unsigned char *nsha1,
 				  const char *message, void *cb_data)
 {
 	struct grab_nth_branch_switch_cbdata *cb = cb_data;
-	const char *match = NULL;
-
-	if (!prefixcmp(message, "checkout: moving to "))
-		match = message + strlen("checkout: moving to ");
-	else if (!prefixcmp(message, "checkout: moving from ")) {
-		const char *cp = message + strlen("checkout: moving from ");
-		if ((cp = strstr(cp, " to ")) != NULL) {
-			match = cp + 4;
-		}
+	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)
 		return 0;
 
+	len = target - match - 4;
+	if (target[len] == '\n' && !strncmp(match, target, len))
+		return 0;
+
 	if (cb->counting) {
 		cb->nth++;
 		return 0;
 	}
 
-	if (--cb->nth <= 0) {
-		size_t len = strlen(match);
-		while (match[len-1] == '\n')
-			len--;
+	if (cb->nth-- <= 0) {
 		strbuf_reset(cb->buf);
 		strbuf_add(cb->buf, match, len);
 		return 1;
@@ -718,26 +717,28 @@ static int grab_nth_branch_switch(unsigned char *osha1, unsigned char *nsha1,
 /*
  * This reads "@{-N}" syntax, finds the name of the Nth previous
  * branch we were on, and places the name of the branch in the given
- * buf and returns 0 if successful.
+ * buf and returns the number of characters parsed if successful.
  *
  * If the input is not of the accepted format, it returns a negative
  * number to signal an error.
+ *
+ * If the input was ok but there are not N branch switches in the
+ * reflog, it returns 0.
  */
 int interpret_nth_last_branch(const char *name, struct strbuf *buf)
 {
-	int nth, i;
+	int nth;
 	struct grab_nth_branch_switch_cbdata cb;
+	const char *brace;
+	char *num_end;
 
 	if (name[0] != '@' || name[1] != '{' || name[2] != '-')
 		return -1;
-	for (i = 3, nth = 0; name[i] && name[i] != '}'; i++) {
-		char ch = name[i];
-		if ('0' <= ch && ch <= '9')
-			nth = nth * 10 + ch - '0';
-		else
-			return -1;
-	}
-	if (nth < 0 || 10 <= nth)
+	brace = strchr(name, '}');
+	if (!brace)
+		return -1;
+	nth = strtol(name+3, &num_end, 10);
+	if (num_end != brace)
 		return -1;
 
 	cb.counting = 1;
@@ -745,11 +746,15 @@ int interpret_nth_last_branch(const char *name, struct strbuf *buf)
 	cb.buf = buf;
 	for_each_reflog_ent("HEAD", grab_nth_branch_switch, &cb);
 
+	if (cb.nth < nth)
+		return 0;
+
 	cb.counting = 0;
 	cb.nth -= nth;
 	cb.buf = buf;
 	for_each_reflog_ent("HEAD", grab_nth_branch_switch, &cb);
-	return 0;
+
+	return brace-name+1;
 }
 
 /*

Documentation/git-rev-parse.txt, sha1_name.c, t/t1505-rev-parse-last.sh

diff --git a/Documentation/git-rev-parse.txt b/Documentation/git-rev-parse.txt
index 2921da3..3ccef2f 100644
--- a/Documentation/git-rev-parse.txt
+++ b/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}'
diff --git a/sha1_name.c b/sha1_name.c
index 34e39db..9e1538e 100644
--- a/sha1_name.c
+++ b/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)
 {
diff --git a/t/t1505-rev-parse-last.sh b/t/t1505-rev-parse-last.sh
new file mode 100755
index 0000000..1e49dd2
--- /dev/null
+++ b/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
+
+

Documentation/git-checkout.txt, builtin-checkout.c, t/t2012-checkout-last.sh

diff --git a/Documentation/git-checkout.txt b/Documentation/git-checkout.txt
index 9cd5151..3bccffa 100644
--- a/Documentation/git-checkout.txt
+++ b/Documentation/git-checkout.txt
@@ -133,6 +133,10 @@ the conflicted merge in the specified paths.
 +
 When this parameter names a non-branch (but still a valid commit object),
 your HEAD becomes 'detached'.
++
+As a special case, the "`@\{-N\}`" syntax for the N-th last branch
+checks out the branch (instead of detaching).  You may also specify
+"`-`" which is synonymous with "`@\{-1\}`".
 
 
 Detached HEAD
diff --git a/builtin-checkout.c b/builtin-checkout.c
index dc1de06..b0a101b 100644
--- a/builtin-checkout.c
+++ b/builtin-checkout.c
@@ -679,6 +679,9 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 		arg = argv[0];
 		has_dash_dash = (argc > 1) && !strcmp(argv[1], "--");
 
+		if (!strcmp(arg, "-"))
+			arg = "@{-1}";
+
 		if (get_sha1(arg, rev)) {
 			if (has_dash_dash)          /* case (1) */
 				die("invalid reference: %s", arg);
diff --git a/t/t2012-checkout-last.sh b/t/t2012-checkout-last.sh
new file mode 100755
index 0000000..320f6eb
--- /dev/null
+++ b/t/t2012-checkout-last.sh
@@ -0,0 +1,50 @@
+#!/bin/sh
+
+test_description='checkout can switch to last branch'
+
+. ./test-lib.sh
+
+test_expect_success 'setup' '
+	echo hello >world &&
+	git add world &&
+	git commit -m initial &&
+	git branch other &&
+	echo "hello again" >>world &&
+	git add world &&
+	git commit -m second
+'
+
+test_expect_success '"checkout -" does not work initially' '
+	test_must_fail git checkout -
+'
+
+test_expect_success 'first branch switch' '
+	git checkout other
+'
+
+test_expect_success '"checkout -" switches back' '
+	git checkout - &&
+	test "z$(git symbolic-ref HEAD)" = "zrefs/heads/master"
+'
+
+test_expect_success '"checkout -" switches forth' '
+	git checkout - &&
+	test "z$(git symbolic-ref HEAD)" = "zrefs/heads/other"
+'
+
+test_expect_success 'detach HEAD' '
+	git checkout $(git rev-parse HEAD)
+'
+
+test_expect_success '"checkout -" attaches again' '
+	git checkout - &&
+	test "z$(git symbolic-ref HEAD)" = "zrefs/heads/other"
+'
+
+test_expect_success '"checkout -" detaches again' '
+	git checkout - &&
+	test "z$(git rev-parse HEAD)" = "z$(git rev-parse other)" &&
+	test_must_fail git symbolic-ref HEAD
+'
+
+test_done

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;
 }

sha1_name.c, t/t1505-rev-parse-last.sh

diff --git a/sha1_name.c b/sha1_name.c
index 306d04b..ee0c456 100644
--- a/sha1_name.c
+++ b/sha1_name.c
@@ -240,8 +240,28 @@ static int ambiguous_path(const char *path, int len)
 	return slash;
 }
 
+/*
+ * *string and *len will only be substituted, and *string returned (for
+ * later free()ing) if the string passed in is of the form @{-<n>}.
+ */
+static char *substitute_nth_last_branch(const char **string, int *len)
+{
+	struct strbuf buf = STRBUF_INIT;
+	int ret = interpret_nth_last_branch(*string, &buf);
+
+	if (ret == *len) {
+		size_t size;
+		*string = strbuf_detach(&buf, &size);
+		*len = size;
+		return (char *)*string;
+	}
+
+	return NULL;
+}
+
 int dwim_ref(const char *str, int len, unsigned char *sha1, char **ref)
 {
+	char *last_branch = substitute_nth_last_branch(&str, &len);
 	const char **p, *r;
 	int refs_found = 0;
 
@@ -261,11 +281,13 @@ int dwim_ref(const char *str, int len, unsigned char *sha1, char **ref)
 				break;
 		}
 	}
+	free(last_branch);
 	return refs_found;
 }
 
 int dwim_log(const char *str, int len, unsigned char *sha1, char **log)
 {
+	char *last_branch = substitute_nth_last_branch(&str, &len);
 	const char **p;
 	int logs_found = 0;
 
@@ -296,6 +318,7 @@ int dwim_log(const char *str, int len, unsigned char *sha1, char **log)
 		if (!warn_ambiguous_refs)
 			break;
 	}
+	free(last_branch);
 	return logs_found;
 }
 
@@ -314,7 +337,7 @@ static int get_sha1_basic(const char *str, int len, unsigned char *sha1)
 	/* 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++) {
+		for (at = len-2; at >= 0; at--) {
 			if (str[at] == '@' && str[at+1] == '{') {
 				reflog_len = (len-1) - (at+2);
 				len = at;
diff --git a/t/t1505-rev-parse-last.sh b/t/t1505-rev-parse-last.sh
index 72e8322..2d6b31e 100755
--- a/t/t1505-rev-parse-last.sh
+++ b/t/t1505-rev-parse-last.sh
@@ -58,7 +58,7 @@ test_expect_success '@{-1}^2 works' '
 	test_rev_equivalent side^2 @{-1}^2
 '
 
-test_expect_failure '@{-1}@{1} works' '
+test_expect_success '@{-1}@{1} works' '
 	test_rev_equivalent side@{1} @{-1}@{1}
 '
 

diff --git a/t/t1505-rev-parse-last.sh b/t/t1505-rev-parse-last.sh
index 1e49dd2..72e8322 100755
--- a/t/t1505-rev-parse-last.sh
+++ b/t/t1505-rev-parse-last.sh
@@ -32,7 +32,11 @@ test_expect_success 'setup' '
 #
 # and 'side' should be the last branch
 
-git log --graph --all --pretty=oneline --decorate
+test_expect_success 'show a log (for debugging)' '
+
+	git log --graph --all --pretty=oneline --decorate
+
+'
 
 test_rev_equivalent () {
 

sha1_name.c, t/t2012-checkout-last.sh

diff --git a/sha1_name.c b/sha1_name.c
index 9e1538e..d6622f2 100644
--- a/sha1_name.c
+++ b/sha1_name.c
@@ -684,10 +684,11 @@ static int get_sha1_oneline(const char *prefix, unsigned char *sha1)
 	return retval;
 }
 
+#define MAX_PREVIOUS_BRANCH 16
+
 struct grab_nth_branch_switch_cbdata {
-	int counting;
-	int nth;
-	struct strbuf *buf;
+	long cnt;
+	struct strbuf buf[MAX_PREVIOUS_BRANCH];
 };
 
 static int grab_nth_branch_switch(unsigned char *osha1, unsigned char *nsha1,
@@ -697,6 +698,7 @@ static int grab_nth_branch_switch(unsigned char *osha1, unsigned char *nsha1,
 	struct grab_nth_branch_switch_cbdata *cb = cb_data;
 	const char *match = NULL, *target = NULL;
 	size_t len;
+	int nth;
 
 	if (!prefixcmp(message, "checkout: moving from ")) {
 		match = message + strlen("checkout: moving from ");
@@ -711,16 +713,9 @@ static int grab_nth_branch_switch(unsigned char *osha1, unsigned char *nsha1,
 	if (target[len] == '\n' && !strncmp(match, target, len))
 		return 0;
 
-	if (cb->counting) {
-		cb->nth++;
-		return 0;
-	}
-
-	if (cb->nth-- <= 0) {
-		strbuf_reset(cb->buf);
-		strbuf_add(cb->buf, match, len);
-		return 1;
-	}
+	nth = cb->cnt++ % MAX_PREVIOUS_BRANCH;
+	strbuf_reset(&cb->buf[nth]);
+	strbuf_add(&cb->buf[nth], match, len);
 	return 0;
 }
 
@@ -737,7 +732,8 @@ 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;
+	long nth;
+	int i;
 	struct grab_nth_branch_switch_cbdata cb;
 	const char *brace;
 	char *num_end;
@@ -750,19 +746,19 @@ int interpret_nth_last_branch(const char *name, struct strbuf *buf)
 	nth = strtol(name+3, &num_end, 10);
 	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);
-
-	if (cb.nth < nth)
-		return 0;
-
-	cb.counting = 0;
-	cb.nth -= nth;
-	cb.buf = buf;
+	if (nth <= 0 || MAX_PREVIOUS_BRANCH < nth)
+		return -1;
+	for (i = 0; i < MAX_PREVIOUS_BRANCH; i++)
+		strbuf_init(&cb.buf[i], 20);
+	cb.cnt = 0;
 	for_each_reflog_ent("HEAD", grab_nth_branch_switch, &cb);
+	if (cb.cnt < nth)
+		return -1;
+	i = (cb.cnt + MAX_PREVIOUS_BRANCH - nth) % MAX_PREVIOUS_BRANCH;
+	strbuf_reset(buf);
+	strbuf_add(buf, cb.buf[i].buf, cb.buf[i].len);
+	for (i = 0; i < MAX_PREVIOUS_BRANCH; i++)
+		strbuf_release(&cb.buf[i]);
 
 	return brace-name+1;
 }
diff --git a/t/t2012-checkout-last.sh b/t/t2012-checkout-last.sh
index 320f6eb..87b30a2 100755
--- a/t/t2012-checkout-last.sh
+++ b/t/t2012-checkout-last.sh
@@ -47,4 +47,48 @@ test_expect_success '"checkout -" detaches again' '
 	test_must_fail git symbolic-ref HEAD
 '
 
+test_expect_success 'more switches' '
+	for i in 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1
+	do
+		git checkout -b branch$i
+	done
+'
+
+more_switches () {
+	for i in 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1
+	do
+		git checkout branch$i
+	done
+}
+
+test_expect_success 'switch to the last' '
+	more_switches &&
+	git checkout @{-1} &&
+	test "z$(git symbolic-ref HEAD)" = "zrefs/heads/branch2"
+'
+
+test_expect_success 'switch to second from the last' '
+	more_switches &&
+	git checkout @{-2} &&
+	test "z$(git symbolic-ref HEAD)" = "zrefs/heads/branch3"
+'
+
+test_expect_success 'switch to third from the last' '
+	more_switches &&
+	git checkout @{-3} &&
+	test "z$(git symbolic-ref HEAD)" = "zrefs/heads/branch4"
+'
+
+test_expect_success 'switch to fourth from the last' '
+	more_switches &&
+	git checkout @{-4} &&
+	test "z$(git symbolic-ref HEAD)" = "zrefs/heads/branch5"
+'
+
+test_expect_success 'switch to twelfth from the last' '
+	more_switches &&
+	git checkout @{-12} &&
+	test "z$(git symbolic-ref HEAD)" = "zrefs/heads/branch13"
+'
+
 test_done

diff --git a/sha1_name.c b/sha1_name.c
index 2c5461e..9e5f444 100644
--- a/sha1_name.c
+++ b/sha1_name.c
@@ -691,11 +691,9 @@ static int get_sha1_oneline(const char *prefix, unsigned char *sha1)
 	return retval;
 }
 
-#define MAX_PREVIOUS_BRANCH 16
-
 struct grab_nth_branch_switch_cbdata {
-	long cnt;
-	struct strbuf buf[MAX_PREVIOUS_BRANCH];
+	long cnt, alloc;
+	struct strbuf *buf;
 };
 
 static int grab_nth_branch_switch(unsigned char *osha1, unsigned char *nsha1,
@@ -720,7 +718,7 @@ static int grab_nth_branch_switch(unsigned char *osha1, unsigned char *nsha1,
 	if (target[len] == '\n' && !strncmp(match, target, len))
 		return 0;
 
-	nth = cb->cnt++ % MAX_PREVIOUS_BRANCH;
+	nth = cb->cnt++ % cb->alloc;
 	strbuf_reset(&cb->buf[nth]);
 	strbuf_add(&cb->buf[nth], match, len);
 	return 0;
@@ -753,19 +751,22 @@ int interpret_nth_last_branch(const char *name, struct strbuf *buf)
 	nth = strtol(name+3, &num_end, 10);
 	if (num_end != brace)
 		return -1;
-	if (nth <= 0 || MAX_PREVIOUS_BRANCH < nth)
+	if (nth <= 0)
 		return -1;
-	for (i = 0; i < MAX_PREVIOUS_BRANCH; i++)
+	cb.alloc = nth;
+	cb.buf = xmalloc(nth * sizeof(struct strbuf));
+	for (i = 0; i < nth; i++)
 		strbuf_init(&cb.buf[i], 20);
 	cb.cnt = 0;
 	for_each_reflog_ent("HEAD", grab_nth_branch_switch, &cb);
 	if (cb.cnt < nth)
-		return -1;
-	i = (cb.cnt + MAX_PREVIOUS_BRANCH - nth) % MAX_PREVIOUS_BRANCH;
+		return 0;
+	i = cb.cnt % nth;
 	strbuf_reset(buf);
 	strbuf_add(buf, cb.buf[i].buf, cb.buf[i].len);
-	for (i = 0; i < MAX_PREVIOUS_BRANCH; i++)
+	for (i = 0; i < nth; i++)
 		strbuf_release(&cb.buf[i]);
+	free(cb.buf);
 
 	return brace-name+1;
 }

diff --git a/sha1_name.c b/sha1_name.c
index f54b6cb..4c0370b 100644
--- a/sha1_name.c
+++ b/sha1_name.c
@@ -754,7 +754,7 @@ static int grab_nth_branch_switch(unsigned char *osha1, unsigned char *nsha1,
 int interpret_nth_last_branch(const char *name, struct strbuf *buf)
 {
 	long nth;
-	int i;
+	int i, retval;
 	struct grab_nth_branch_switch_cbdata cb;
 	const char *brace;
 	char *num_end;
@@ -774,17 +774,21 @@ int interpret_nth_last_branch(const char *name, struct strbuf *buf)
 	for (i = 0; i < nth; i++)
 		strbuf_init(&cb.buf[i], 20);
 	cb.cnt = 0;
+	retval = 0;
 	for_each_reflog_ent("HEAD", grab_nth_branch_switch, &cb);
 	if (cb.cnt < nth)
-		return 0;
+		goto release_return;
 	i = cb.cnt % nth;
 	strbuf_reset(buf);
 	strbuf_add(buf, cb.buf[i].buf, cb.buf[i].len);
+	retval = brace-name+1;
+
+release_return:
 	for (i = 0; i < nth; i++)
 		strbuf_release(&cb.buf[i]);
 	free(cb.buf);
 
-	return brace-name+1;
+	return retval;
 }
 
 /*

refs.c, refs.h, sha1_name.c

diff --git a/refs.c b/refs.c
index 33ced65..024211d 100644
--- a/refs.c
+++ b/refs.c
@@ -1453,7 +1453,7 @@ int read_ref_at(const char *ref, unsigned long at_time, int cnt, unsigned char *
 	return 1;
 }
 
-int for_each_reflog_ent(const char *ref, each_reflog_ent_fn fn, void *cb_data)
+int for_each_recent_reflog_ent(const char *ref, each_reflog_ent_fn fn, long ofs, void *cb_data)
 {
 	const char *logfile;
 	FILE *logfp;
@@ -1464,6 +1464,16 @@ int for_each_reflog_ent(const char *ref, each_reflog_ent_fn fn, void *cb_data)
 	logfp = fopen(logfile, "r");
 	if (!logfp)
 		return -1;
+
+	if (ofs) {
+		struct stat statbuf;
+		if (fstat(fileno(logfp), &statbuf) ||
+		    statbuf.st_size < ofs ||
+		    fseek(logfp, -ofs, SEEK_END) ||
+		    fgets(buf, sizeof(buf), logfp))
+			return -1;
+	}
+
 	while (fgets(buf, sizeof(buf), logfp)) {
 		unsigned char osha1[20], nsha1[20];
 		char *email_end, *message;
@@ -1497,6 +1507,11 @@ int for_each_reflog_ent(const char *ref, each_reflog_ent_fn fn, void *cb_data)
 	return ret;
 }
 
+int for_each_reflog_ent(const char *ref, each_reflog_ent_fn fn, void *cb_data)
+{
+	return for_each_recent_reflog_ent(ref, fn, 0, cb_data);
+}
+
 static int do_for_each_reflog(const char *base, each_ref_fn fn, void *cb_data)
 {
 	DIR *dir = opendir(git_path("logs/%s", base));
diff --git a/refs.h b/refs.h
index 06ad260..3bb529d 100644
--- a/refs.h
+++ b/refs.h
@@ -60,6 +60,7 @@ extern int read_ref_at(const char *ref, unsigned long at_time, int cnt, unsigned
 /* iterate over reflog entries */
 typedef int each_reflog_ent_fn(unsigned char *osha1, unsigned char *nsha1, const char *, unsigned long, int, const char *, void *);
 int for_each_reflog_ent(const char *ref, each_reflog_ent_fn fn, void *cb_data);
+int for_each_recent_reflog_ent(const char *ref, each_reflog_ent_fn fn, long, void *cb_data);
 
 /*
  * Calls the specified function for each reflog file until it returns nonzero,
diff --git a/sha1_name.c b/sha1_name.c
index 4c0370b..38c9f1b 100644
--- a/sha1_name.c
+++ b/sha1_name.c
@@ -775,7 +775,13 @@ int interpret_nth_last_branch(const char *name, struct strbuf *buf)
 		strbuf_init(&cb.buf[i], 20);
 	cb.cnt = 0;
 	retval = 0;
-	for_each_reflog_ent("HEAD", grab_nth_branch_switch, &cb);
+	for_each_recent_reflog_ent("HEAD", grab_nth_branch_switch, 40960, &cb);
+	if (cb.cnt < nth) {
+		cb.cnt = 0;
+		for (i = 0; i < nth; i++)
+			strbuf_release(&cb.buf[i]);
+		for_each_reflog_ent("HEAD", grab_nth_branch_switch, &cb);
+	}
 	if (cb.cnt < nth)
 		goto release_return;
 	i = cb.cnt % nth;

diff --git a/sha1_name.c b/sha1_name.c
index 4d10705..803f9d2 100644
--- a/sha1_name.c
+++ b/sha1_name.c
@@ -735,7 +735,7 @@ static int grab_nth_branch_switch(unsigned char *osha1, unsigned char *nsha1,
 	if ((target = strstr(match, " to ")) != NULL) {
 		len = target - match;
 		target += 4;
-		if (target[len] == '\n' && !strncmp(match, target, len))
+		if (len == strlen(target) && !strncmp(match, target, len))
 			return 0;
 	}
 	else

diff --git a/sha1_name.c b/sha1_name.c
index 38c9f1b..7d95bbb 100644
--- a/sha1_name.c
+++ b/sha1_name.c
@@ -723,17 +723,13 @@ static int grab_nth_branch_switch(unsigned char *osha1, unsigned char *nsha1,
 
 	if (!prefixcmp(message, "checkout: moving from ")) {
 		match = message + strlen("checkout: moving from ");
-		if ((target = strstr(match, " to ")) != NULL)
-			target += 4;
+		target = strstr(match, " to ");
 	}
 
 	if (!match || !target)
 		return 0;
 
-	len = target - match - 4;
-	if (target[len] == '\n' && !strncmp(match, target, len))
-		return 0;
-
+	len = target - match;
 	nth = cb->cnt++ % cb->alloc;
 	strbuf_reset(&cb->buf[nth]);
 	strbuf_add(&cb->buf[nth], match, len);

diff --git c/sha1_name.c w/sha1_name.c
index 38c9f1b..9aed8ae 100644
--- c/sha1_name.c
+++ w/sha1_name.c
@@ -731,7 +731,10 @@ static int grab_nth_branch_switch(unsigned char *osha1, unsigned char *nsha1,
 		return 0;
 
 	len = target - match - 4;
-	if (target[len] == '\n' && !strncmp(match, target, len))
+	if (len + 1 == strlen(target) &&
+	    target[len] == '\n' &&
+	    !memcmp(target, match, len))
+		/* switching same branch "from A to A\n" */
 		return 0;
 
 	nth = cb->cnt++ % cb->alloc;

diff --git a/t/t1505-rev-parse-last.sh b/t/t1505-rev-parse-last.sh
index c745ec4..d709ecf 100755
--- a/t/t1505-rev-parse-last.sh
+++ b/t/t1505-rev-parse-last.sh
@@ -32,8 +32,6 @@ test_expect_success 'setup' '
 #
 # and 'side' should be the last branch
 
-git log --graph --all --pretty=oneline --decorate
-
 test_rev_equivalent () {
 
 	git rev-parse "$1" > expect &&

diff --git a/sha1_name.c b/sha1_name.c
index 9e5f444..853bac6 100644
--- a/sha1_name.c
+++ b/sha1_name.c
@@ -705,18 +705,18 @@ static int grab_nth_branch_switch(unsigned char *osha1, unsigned char *nsha1,
 	size_t len;
 	int nth;
 
-	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 ((target = strstr(match, " to ")) != NULL) {
+		len = target - match - 4;
+		target += 4;
+		if (target[len] == '\n' && !strncmp(match, target, len))
+			return 0;
+	}
+	else
+		len = strchrnul(match, ' ') - match;
 
 	nth = cb->cnt++ % cb->alloc;
 	strbuf_reset(&cb->buf[nth]);