reflog: add for_each_reflog_ent_reverse() API

"git checkout -" is a short-hand for "git checkout @{-1}" and the
"@{nth}" notation for a negative number is to find nth previous
checkout in the reflog of the HEAD to determine the name of the
branch the user was on.  We would want to find the nth most recent
reflog entry that matches "checkout: moving from X to Y" for this.

Unfortunately, reflog is implemented as an append-only file, and the
API to iterate over its entries, for_each_reflog_ent(), reads the
file in order, giving the entries from the oldest to newer.  For the
purpose of finding nth most recent one, this API forces us to record
the last n entries in a rotating buffer and give the result out only
after we read everything.  To optimize for a common case of finding
the nth most recent one for a small value of n, we also have a side
API for_each_recent_reflog_ent() that starts reading near the end of
the file, but it still has to read the entries in the "wrong" order.
The implementation of understanding @{-1} uses this interface.

This all becomes unnecessary if we add an API to let us iterate over
reflog entries in the reverse order, from the newest to older.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 86 insertions, 16 deletions

diff --git a/refs.c b/refs.c
index e302521..8e24060 100644
--- a/refs.c
+++ b/refs.c
@@ -2318,30 +2318,89 @@ static int show_one_reflog_ent(struct strbuf *sb, each_reflog_ent_fn fn, void *c
 	return fn(osha1, nsha1, sb->buf + 82, timestamp, tz, message, cb_data);
 }
 
-int for_each_recent_reflog_ent(const char *refname, each_reflog_ent_fn fn, long ofs, void *cb_data)
+static char *find_beginning_of_line(char *bob, char *scan)
+{
+	while (bob < scan && *(--scan) != '\n')
+		; /* keep scanning backwards */
+	/*
+	 * Return either beginning of the buffer, or LF at the end of
+	 * the previous line.
+	 */
+	return scan;
+}
+
+int for_each_reflog_ent_reverse(const char *refname, each_reflog_ent_fn fn, void *cb_data)
 {
-	FILE *logfp;
 	struct strbuf sb = STRBUF_INIT;
-	int ret = 0;
+	FILE *logfp;
+	long pos;
+	int ret = 0, at_tail = 1;
 
 	logfp = fopen(git_path("logs/%s", refname), "r");
 	if (!logfp)
 		return -1;
 
-	if (ofs) {
-		struct stat statbuf;
-		if (fstat(fileno(logfp), &statbuf) ||
-		    statbuf.st_size < ofs ||
-		    fseek(logfp, -ofs, SEEK_END) ||
-		    strbuf_getwholeline(&sb, logfp, '\n')) {
-			fclose(logfp);
-			strbuf_release(&sb);
-			return -1;
+	/* Jump to the end */
+	if (fseek(logfp, 0, SEEK_END) < 0)
+		return error("cannot seek back reflog for %s: %s",
+			     refname, strerror(errno));
+	pos = ftell(logfp);
+	while (!ret && 0 < pos) {
+		int cnt;
+		size_t nread;
+		char buf[BUFSIZ];
+		char *endp, *scanp;
+
+		/* Fill next block from the end */
+		cnt = (sizeof(buf) < pos) ? sizeof(buf) : pos;
+		if (fseek(logfp, pos - cnt, SEEK_SET))
+			return error("cannot seek back reflog for %s: %s",
+				     refname, strerror(errno));
+		nread = fread(buf, cnt, 1, logfp);
+		if (nread < 0)
+			return error("cannot read %d bytes from reflog for %s: %s",
+				     cnt, refname, strerror(errno));
+		pos -= cnt;
+
+		scanp = endp = buf + cnt;
+		if (at_tail && scanp[-1] == '\n')
+			/* Looking at the final LF at the end of the file */
+			scanp--;
+		at_tail = 0;
+
+		while (buf < scanp) {
+			/*
+			 * terminating LF of the previous line, or the beginning
+			 * of the buffer.
+			 */
+			char *bp;
+
+			bp = find_beginning_of_line(buf, scanp);
+
+			if (*bp != '\n') {
+				strbuf_splice(&sb, 0, 0, buf, endp - buf);
+				if (pos)
+					break; /* need to fill another block */
+				scanp = buf - 1; /* leave loop */
+			} else {
+				/*
+				 * (bp + 1) thru endp is the beginning of the
+				 * current line we have in sb
+				 */
+				strbuf_splice(&sb, 0, 0, bp + 1, endp - (bp + 1));
+				scanp = bp;
+				endp = bp + 1;
+			}
+			ret = show_one_reflog_ent(&sb, fn, cb_data);
+			strbuf_reset(&sb);
+			if (ret)
+				break;
 		}
-	}
 
-	while (!ret && !strbuf_getwholeline(&sb, logfp, '\n'))
+	}
+	if (!ret && sb.len)
 		ret = show_one_reflog_ent(&sb, fn, cb_data);
+
 	fclose(logfp);
 	strbuf_release(&sb);
 	return ret;
@@ -2349,9 +2408,20 @@ int for_each_recent_reflog_ent(const char *refname, each_reflog_ent_fn fn, long
 
 int for_each_reflog_ent(const char *refname, each_reflog_ent_fn fn, void *cb_data)
 {
-	return for_each_recent_reflog_ent(refname, fn, 0, cb_data);
-}
+	FILE *logfp;
+	struct strbuf sb = STRBUF_INIT;
+	int ret = 0;
+
+	logfp = fopen(git_path("logs/%s", refname), "r");
+	if (!logfp)
+		return -1;
 
+	while (!ret && !strbuf_getwholeline(&sb, logfp, '\n'))
+		ret = show_one_reflog_ent(&sb, fn, cb_data);
+	fclose(logfp);
+	strbuf_release(&sb);
+	return ret;
+}
 /*
  * Call fn for each reflog in the namespace indicated by name.  name
  * must be empty or end with '/'.  Name will be used as a scratch