xutils: Fix xdl_recmatch() on incomplete lines

Thell Fowler noticed that various "ignore whitespace" options to git diff
do not work well on an incomplete line.

The loop control of the function responsible for these bugs was extremely
difficult to follow.  This patch restructures the loops for three variants
of "ignore whitespace" logic.

The basic idea of the re-written logic is:

 - A loop runs while the characters from both strings we are looking at
   match.  We declare unmatch immediately when we find something that does
   not match and return false from the function.  We break out of the loop
   if we ran out of either side of the string.

   The way we skip spaces inside this loop varies depending on the style
   of ignoring whitespaces.

 - After the above loop breaks, we know that the parts of the strings we
   inspected so far match, ignoring the whitespaces.  The lines can match
   only if the remainder consists of nothing but whitespaces.  This part
   of the logic is shared across all three styles.

The new code is more obvious and should be much easier to follow.

Tested-by: Thell Fowler <git@tbfowler.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 49 insertions, 31 deletions

diff --git a/xdiff/xutils.c b/xdiff/xutils.c
index 9411fa96a..bc12f2989 100644
--- a/xdiff/xutils.c
+++ b/xdiff/xutils.c
@@ -190,48 +190,66 @@ int xdl_recmatch(const char *l1, long s1, const char *l2, long s2, long flags)
 {
 	int i1, i2;
 
+	if (!(flags & XDF_WHITESPACE_FLAGS))
+		return s1 == s2 && !memcmp(l1, l2, s1);
+
+	i1 = 0;
+	i2 = 0;
+
+	/*
+	 * -w matches everything that matches with -b, and -b in turn
+	 * matches everything that matches with --ignore-space-at-eol.
+	 *
+	 * Each flavor of ignoring needs different logic to skip whitespaces
+	 * while we have both sides to compare.
+	 */
 	if (flags & XDF_IGNORE_WHITESPACE) {
-		for (i1 = i2 = 0; i1 < s1 && i2 < s2; ) {
-			if (isspace(l1[i1]))
-				while (isspace(l1[i1]) && i1 < s1)
-					i1++;
-			if (isspace(l2[i2]))
-				while (isspace(l2[i2]) && i2 < s2)
-					i2++;
-			if (i1 < s1 && i2 < s2 && l1[i1++] != l2[i2++])
+		goto skip_ws;
+		while (i1 < s1 && i2 < s2) {
+			if (l1[i1++] != l2[i2++])
 				return 0;
+		skip_ws:
+			while (i1 < s1 && isspace(l1[i1]))
+				i1++;
+			while (i2 < s2 && isspace(l2[i2]))
+				i2++;
 		}
-		return (i1 >= s1 && i2 >= s2);
 	} else if (flags & XDF_IGNORE_WHITESPACE_CHANGE) {
-		for (i1 = i2 = 0; i1 < s1 && i2 < s2; ) {
-			if (isspace(l1[i1])) {
-				if (!isspace(l2[i2]))
-					return 0;
-				while (isspace(l1[i1]) && i1 < s1)
-					i1++;
-				while (isspace(l2[i2]) && i2 < s2)
-					i2++;
-			} else if (l1[i1++] != l2[i2++])
-				return 0;
-		}
-		return (i1 >= s1 && i2 >= s2);
-	} else if (flags & XDF_IGNORE_WHITESPACE_AT_EOL) {
-		for (i1 = i2 = 0; i1 < s1 && i2 < s2; ) {
-			if (l1[i1] != l2[i2]) {
+		while (i1 < s1 && i2 < s2) {
+			if (isspace(l1[i1]) && isspace(l2[i2])) {
+				/* Skip matching spaces and try again */
 				while (i1 < s1 && isspace(l1[i1]))
 					i1++;
 				while (i2 < s2 && isspace(l2[i2]))
 					i2++;
-				if (i1 < s1 || i2 < s2)
-					return 0;
-				return 1;
+				continue;
 			}
+			if (l1[i1++] != l2[i2++])
+				return 0;
+		}
+	} else if (flags & XDF_IGNORE_WHITESPACE_AT_EOL) {
+		while (i1 < s1 && i2 < s2 && l1[i1++] == l2[i2++])
+			; /* keep going */
+	}
+
+	/*
+	 * After running out of one side, the remaining side must have
+	 * nothing but whitespace for the lines to match.  Note that
+	 * ignore-whitespace-at-eol case may break out of the loop
+	 * while there still are characters remaining on both lines.
+	 */
+	if (i1 < s1) {
+		while (i1 < s1 && isspace(l1[i1]))
 			i1++;
+		if (s1 != i1)
+			return 0;
+	}
+	if (i2 < s2) {
+		while (i2 < s2 && isspace(l2[i2]))
 			i2++;
-		}
-		return i1 >= s1 && i2 >= s2;
-	} else
-		return s1 == s2 && !memcmp(l1, l2, s1);
+		return (s2 == i2);
+	}
+	return 1;
 }
 
 static unsigned long xdl_hash_record_with_whitespace(char const **data,