[PATCH 4/8] xdl_change_compact(): do one final shift or the other, not both

[Michael Haggerty <mhagger@alum.mit.edu>]

There is no need to shift the group to match a diff in the other file if
we're just going to override that shift based on the compaction
heuristic. Note that this changes the behavior if the matching shift
would have shifted the group higher than the last blank line: the old
code would have ignored the compaction heuristic in that case, whereas
the new code always gives precedence to the compaction heuristic when it
is turned on.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
---
 xdiff/xdiffi.c | 44 ++++++++++++++++++++++----------------------
 1 file changed, 22 insertions(+), 22 deletions(-)

diff --git a/xdiff/xdiffi.c b/xdiff/xdiffi.c
index 0f235bc..c67cfe3 100644
--- a/xdiff/xdiffi.c
+++ b/xdiff/xdiffi.c
@@ -547,11 +547,28 @@ int xdl_change_compact(xdfile_t *xdf, xdfile_t *xdfo, long flags) {
 			}
 		} while (groupsize != end - start);
 
-		/*
-		 * Try to move back the possibly merged group of changes, to match
-		 * the recorded position in the other file.
-		 */
-		if (end_matching_other != -1) {
+		if ((flags & XDF_COMPACTION_HEURISTIC) && blank_lines) {
+			/*
+			 * Compaction heuristic: if a group can be moved back and
+			 * forth, then if possible shift the group to make its
+			 * bottom line a blank line.
+			 *
+			 * As we already shifted the group forward as far as
+			 * possible in the earlier loop, we only need to handle
+			 * backward shifts, not forward ones.
+			 */
+			while (start > 0 &&
+			       !is_blank_line(recs, end - 1, flags) &&
+			       recs_match(recs, start - 1, end - 1, flags)) {
+				rchg[--start] = 1;
+				rchg[--end] = 0;
+			}
+		} else if (end_matching_other != -1) {
+			/*
+			 * Move the possibly merged group of changes back to line
+			 * up with the last group of changes from the other file
+			 * that it can align with.
+			 */
 			while (end_matching_other < end) {
 				rchg[--start] = 1;
 				rchg[--end] = 0;
@@ -561,23 +578,6 @@ int xdl_change_compact(xdfile_t *xdf, xdfile_t *xdfo, long flags) {
 					io--;
 			}
 		}
-
-		/*
-		 * If a group can be moved back and forth, see if there is a
-		 * blank line in the moving space. If there is a blank line,
-		 * make sure the last blank line is the end of the group.
-		 *
-		 * As we already shifted the group forward as far as possible
-		 * in the earlier loop, we need to shift it back only if at all.
-		 */
-		if ((flags & XDF_COMPACTION_HEURISTIC) && blank_lines) {
-			while (start > 0 &&
-			       !is_blank_line(recs, end - 1, flags) &&
-			       recs_match(recs, start - 1, end - 1, flags)) {
-				rchg[--start] = 1;
-				rchg[--end] = 0;
-			}
-		}
 	}
 
 	return 0;
-- 
2.8.1