combine-diff: better hunk splitting.

It considered an otherwise unchanged line that had line removals
in front of it an interesting line, which caused hunks to have
one extra the trailing context line.

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

combine-diff.c

@@ -323,6 +323,85 @@ static unsigned long line_all_diff(struct sline *sline, unsigned long all_mask)
 	return different;
 }
 
+static unsigned long adjust_hunk_tail(struct sline *sline,
+				      unsigned long all_mask,
+				      unsigned long hunk_begin,
+				      unsigned long i)
+{
+	/* i points at the first uninteresting line.
+	 * If the last line of the hunk was interesting
+	 * only because it has some deletion, then
+	 * it is not all that interesting for the
+	 * purpose of giving trailing context lines.
+	 */
+	if ((hunk_begin + 1 <= i) &&
+	    ((sline[i-1].flag & all_mask) == all_mask))
+		i--;
+	return i;
+}
+
+static unsigned long next_interesting(struct sline *sline,
+				      unsigned long mark,
+				      unsigned long i,
+				      unsigned long cnt,
+				      int uninteresting)
+{
+	while (i < cnt)
+		if (uninteresting ?
+		    !(sline[i].flag & mark) :
+		    (sline[i].flag & mark))
+			return i;
+		else
+			i++;
+	return cnt;
+}
+
+static int give_context(struct sline *sline, unsigned long cnt, int num_parent)
+{
+	unsigned long all_mask = (1UL<<num_parent) - 1;
+	unsigned long mark = (1UL<<num_parent);
+	unsigned long i;
+
+	i = next_interesting(sline, mark, 0, cnt, 0);
+	if (cnt <= i)
+		return 0;
+
+	while (i < cnt) {
+		unsigned long j = (context < i) ? (i - context) : 0;
+		unsigned long k;
+		while (j < i)
+			sline[j++].flag |= mark;
+
+	again:
+		j = next_interesting(sline, mark, i, cnt, 1);
+		if (cnt <= j)
+			break; /* the rest are all interesting */
+
+		/* lookahead context lines */
+		k = next_interesting(sline, mark, j, cnt, 0);
+		j = adjust_hunk_tail(sline, all_mask, i, j);
+
+		if (k < j + context) {
+			/* k is interesting and [j,k) are not, but
+			 * paint them interesting because the gap is small.
+			 */
+			while (j < k)
+				sline[j++].flag |= mark;
+			i = k;
+			goto again;
+		}
+
+		/* j is the first uninteresting line and there is
+		 * no overlap beyond it within context lines.
+		 */
+		i = k;
+		k = (j + context < cnt) ? j + context : cnt;
+		while (j < k)
+			sline[j++].flag |= mark;
+	}
+	return 1;
+}
+
 static int make_hunks(struct sline *sline, unsigned long cnt,
 		       int num_parent, int dense)
 {
@@ -331,44 +410,54 @@ static int make_hunks(struct sline *sline, unsigned long cnt,
 	unsigned long i;
 	int has_interesting = 0;
 
-	i = 0;
-	while (i < cnt) {
-		if (interesting(&sline[i], all_mask)) {
-			unsigned long j = (context < i) ? i - context : 0;
-			while (j <= i)
-				sline[j++].flag |= mark;
-			while (++i < cnt) {
-				if (!interesting(&sline[i], all_mask))
-					break;
-				sline[i].flag |= mark;
-			}
-			j = (i + context < cnt) ? i + context : cnt;
-			while (i < j)
-				sline[i++].flag |= mark;
-			has_interesting = 1;
-			continue;
-		}
-		i++;
+	for (i = 0; i < cnt; i++) {
+		if (interesting(&sline[i], all_mask))
+			sline[i].flag |= mark;
+		else
+			sline[i].flag &= ~mark;
 	}
 	if (!dense)
-		return has_interesting;
+		return give_context(sline, cnt, num_parent);
 
 	/* Look at each hunk, and if we have changes from only one
 	 * parent, or the changes are the same from all but one
 	 * parent, mark that uninteresting.
 	 */
-	has_interesting = 0;
 	i = 0;
 	while (i < cnt) {
-		int j, hunk_end, same, diff;
+		unsigned long j, hunk_begin, hunk_end;
+		int same, diff;
 		unsigned long same_diff, all_diff;
 		while (i < cnt && !(sline[i].flag & mark))
 			i++;
 		if (cnt <= i)
 			break; /* No more interesting hunks */
-		for (hunk_end = i + 1; hunk_end < cnt; hunk_end++)
-			if (!(sline[hunk_end].flag & mark))
-				break;
+		hunk_begin = i;
+		for (j = i + 1; j < cnt; j++) {
+			if (!(sline[j].flag & mark)) {
+				/* Look beyond the end to see if there
+				 * is an interesting line after this
+				 * hunk within context span.
+				 */
+				unsigned long la; /* lookahead */
+				int contin = 0;
+				la = adjust_hunk_tail(sline, all_mask,
+						     hunk_begin, j);
+				la = (la + context < cnt) ?
+					(la + context) : cnt;
+				while (j <= --la) {
+					if (sline[la].flag & mark) {
+						contin = 1;
+						break;
+					}
+				}
+				if (!contin)
+					break;
+				j = la;
+			}
+		}
+		hunk_end = j;
+
 		/* [i..hunk_end) are interesting.  Now does it have
 		 * the same change with all but one parent?
 		 */
@@ -387,13 +476,13 @@ static int make_hunks(struct sline *sline, unsigned long cnt,
 		}
 		if ((num_parent - 1 <= same) || (diff == 1)) {
 			/* This hunk is not that interesting after all */
-			for (j = i; j < hunk_end; j++)
+			for (j = hunk_begin; j < hunk_end; j++)
 				sline[j].flag &= ~mark;
 		}
-		else
-			has_interesting = 1;
 		i = hunk_end;
 	}
+
+	has_interesting = give_context(sline, cnt, num_parent);
 	return has_interesting;
 }