ref-log: allow ref@{count} syntax.

Often I find myself wanting to say 'tip of "next" before I
merged the last three topics'.  Now I can say that with:

	git log next@{3}..next

Since small integers alone are invalid input strings to
approxidate, there is no fear of confusion.

Signed-off-by: Junio C Hamano <junkio@cox.net>

refs.c

@@ -795,7 +795,7 @@ int write_ref_sha1(struct ref_lock *lock,
 	return 0;
 }
 
-int read_ref_at(const char *ref, unsigned long at_time, unsigned char *sha1)
+int read_ref_at(const char *ref, unsigned long at_time, int cnt, unsigned char *sha1)
 {
 	const char *logfile, *logdata, *logend, *rec, *lastgt, *lastrec;
 	char *tz_c;
@@ -828,7 +828,7 @@ int read_ref_at(const char *ref, unsigned long at_time, unsigned char *sha1)
 		if (!lastgt)
 			die("Log %s is corrupt.", logfile);
 		date = strtoul(lastgt + 1, &tz_c, 10);
-		if (date <= at_time) {
+		if (date <= at_time || cnt == 0) {
 			if (lastrec) {
 				if (get_sha1_hex(lastrec, logged_sha1))
 					die("Log %s is corrupt.", logfile);
@@ -859,6 +859,8 @@ int read_ref_at(const char *ref, unsigned long at_time, unsigned char *sha1)
 			return 0;
 		}
 		lastrec = rec;
+		if (cnt > 0)
+			cnt--;
 	}
 
 	rec = logdata;

refs.h

@@ -39,7 +39,7 @@ extern void unlock_ref(struct ref_lock *lock);
 extern int write_ref_sha1(struct ref_lock *lock, const unsigned char *sha1, const char *msg);
 
 /** Reads log for the value of ref during at_time. **/
-extern int read_ref_at(const char *ref, unsigned long at_time, unsigned char *sha1);
+extern int read_ref_at(const char *ref, unsigned long at_time, int cnt, unsigned char *sha1);
 
 /** Returns 0 if target has the right format for a ref. **/
 extern int check_ref_format(const char *target);

sha1_name.c

@@ -249,24 +249,23 @@ static int get_sha1_basic(const char *str, int len, unsigned char *sha1)
 	static const char *warning = "warning: refname '%.*s' is ambiguous.\n";
 	const char **p, *ref;
 	char *real_ref = NULL;
-	int refs_found = 0, am;
-	unsigned long at_time = (unsigned long)-1;
+	int refs_found = 0;
+	int at, reflog_len;
 	unsigned char *this_result;
 	unsigned char sha1_from_ref[20];
 
 	if (len == 40 && !get_sha1_hex(str, sha1))
 		return 0;
 
-	/* At a given period of time? "@{2 hours ago}" */
-	for (am = 1; am < len - 1; am++) {
-		if (str[am] == '@' && str[am+1] == '{' && str[len-1] == '}') {
-			int date_len = len - am - 3;
-			char *date_spec = xmalloc(date_len + 1);
-			strlcpy(date_spec, str + am + 2, date_len + 1);
-			at_time = approxidate(date_spec);
-			free(date_spec);
-			len = am;
-			break;
+	/* basic@{time or number} format to query ref-log */
+	reflog_len = 0;
+	if (str[len-1] == '}') {
+		for (at = 1; at < len - 1; at++) {
+			if (str[at] == '@' && str[at+1] == '{') {
+				reflog_len = (len-1) - (at+2);
+				len = at;
+				break;
+			}
 		}
 	}
 
@@ -291,11 +290,22 @@ static int get_sha1_basic(const char *str, int len, unsigned char *sha1)
 	if (warn_ambiguous_refs && refs_found > 1)
 		fprintf(stderr, warning, len, str);
 
-	if (at_time != (unsigned long)-1) {
-		read_ref_at(
-			real_ref,
-			at_time,
-			sha1);
+	if (reflog_len) {
+		/* Is it asking for N-th entry, or approxidate? */
+		int nth, i;
+		unsigned long at_time;
+		for (i = nth = 0; 0 <= nth && i < reflog_len; i++) {
+			char ch = str[at+2+i];
+			if ('0' <= ch && ch <= '9')
+				nth = nth * 10 + ch - '0';
+			else
+				nth = -1;
+		}
+		if (0 <= nth)
+			at_time = 0;
+		else
+			at_time = approxidate(str + at + 2);
+		read_ref_at(real_ref, at_time, nth, sha1);
 	}
 
 	free(real_ref);