builtin-apply.c: push match-beginning/end logic down

This moves the logic to force match at the beginning and/or at
the end of the buffer to the actual function that finds the
match from its caller.  This is a necessary preparation for the
next step to allow matching disregarding certain differences,
such as whitespace changes.

We probably could optimize this even more by taking advantage of
the fact that match_beginning and match_end forces the match to
be at an exact location (anchored at the beginning and/or the
end), but that's for another commit.

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

builtin-apply.c

@@ -1439,18 +1439,36 @@ static int read_old_data(struct stat *st, const char *path, struct strbuf *buf)
 
 static int match_fragment(const char *buf, unsigned long size,
 			  unsigned long try,
-			  const char *fragment, unsigned long fragsize)
+			  const char *fragment, unsigned long fragsize,
+			  int match_beginning, int match_end)
 {
-	if (try + fragsize > size)
+	if (match_beginning && try)
 		return 0;
-	if (memcmp(buf + try, fragment, fragsize))
-		return 0;
-	return 1;
+
+	/*
+	 * Do we have an exact match?  If we were told to match
+	 * at the end, size must be exactly at try+fragsize,
+	 * otherwise try+fragsize must be still within the preimage,
+	 * and either case, the old piece should match the preimage
+	 * exactly.
+	 */
+	if ((match_end
+	     ? (try + fragsize == size)
+	     : (try + fragsize <= size)) &&
+	    !memcmp(buf + try, fragment, fragsize))
+		return 1;
+
+	/*
+	 * NEEDSWORK: We can optionally match fuzzily here, but
+	 * that is for a later round.
+	 */
+	return 0;
 }
 
 static int find_offset(const char *buf, unsigned long size,
 		       const char *fragment, unsigned long fragsize,
-		       int line, int *lines)
+		       int line, int *lines,
+		       int match_beginning, int match_end)
 {
 	int i, no_more_backwards, no_more_forwards;
 	unsigned long start, backwards, forwards, try;
@@ -1483,7 +1501,8 @@ static int find_offset(const char *buf, unsigned long size,
 		no_more_backwards = !backwards;
 		no_more_forwards = (forwards + fragsize > size);
 
-		if (match_fragment(buf, size, try, fragment, fragsize)) {
+		if (match_fragment(buf, size, try, fragment, fragsize,
+				   match_beginning, match_end)) {
 			int shift = ((i+1) >> 1);
 			if (i & 1)
 				shift = -shift;
@@ -1765,17 +1784,16 @@ static int apply_one_fragment(struct strbuf *buf, struct fragment *frag,
 	pos = frag->newpos;
 	for (;;) {
 		offset = find_offset(buf->buf, buf->len,
-				     oldlines, oldsize, pos, &lines);
-		if (match_end && offset + oldsize != buf->len)
-			offset = -1;
-		if (match_beginning && offset)
-			offset = -1;
+				     oldlines, oldsize, pos, &lines,
+				     match_beginning, match_end);
 		if (offset >= 0) {
 			if (ws_error_action == correct_ws_error &&
-			    (buf->len - oldsize - offset == 0)) /* end of file? */
+			    (buf->len - oldsize - offset == 0))
+				/* end of file? */
 				newsize -= new_blank_lines_at_end;
 
-			/* Warn if it was necessary to reduce the number
+			/*
+			 * Warn if it was necessary to reduce the number
 			 * of context lines.
 			 */
 			if ((leading != frag->leading) ||