xdiff: handle appended chunks better with -W

If lines are added at the end of a file, diff -W shows the whole file.
That's because get_func_line() only considers the pre-image and gives up
if it sees a record index beyond its end.

Consider the post-image as well to see if the added lines already make
up a full function.  If it doesn't then search for the previous function
line by starting from the bottom of the pre-image, thereby avoiding to
confuse get_func_line().

Reuse the existing label called "again", as it's exactly where we need
to jump to when we're done handling the pre-context, but rename it to
"post_context_calculation" in order to document its new purpose better.

Reported-by: Junio C Hamano <gitster@pobox.com>
Initial-patch-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: rscharfe

t/t4051-diff-function-context.sh

@@ -131,7 +131,7 @@ test_expect_success ' context includes end' '
 	grep "^[+].*End of second part" extended.diff
 '
 
-test_expect_failure ' context does not include other functions' '
+test_expect_success ' context does not include other functions' '
 	test $(grep -c "^[ +-].*Begin" extended.diff) -le 2
 '
 

xdiff/xemit.c

@@ -171,7 +171,28 @@ int xdl_emit_diff(xdfenv_t *xe, xdchange_t *xscr, xdemitcb_t *ecb,
 		s2 = XDL_MAX(xch->i2 - xecfg->ctxlen, 0);
 
 		if (xecfg->flags & XDL_EMIT_FUNCCONTEXT) {
-			long fs1 = get_func_line(xe, xecfg, NULL, xch->i1, -1);
+			long fs1, i1 = xch->i1;
+
+			/* Appended chunk? */
+			if (i1 >= xe->xdf1.nrec) {
+				char dummy[1];
+
+				/*
+				 * We don't need additional context if
+				 * a whole function was added.
+				 */
+				if (match_func_rec(&xe->xdf2, xecfg, xch->i2,
+						   dummy, sizeof(dummy)) >= 0)
+					goto post_context_calculation;
+
+				/*
+				 * Otherwise get more context from the
+				 * pre-image.
+				 */
+				i1 = xe->xdf1.nrec - 1;
+			}
+
+			fs1 = get_func_line(xe, xecfg, NULL, i1, -1);
 			if (fs1 < 0)
 				fs1 = 0;
 			if (fs1 < s1) {
@@ -180,7 +201,7 @@ int xdl_emit_diff(xdfenv_t *xe, xdchange_t *xscr, xdemitcb_t *ecb,
 			}
 		}
 
- again:
+ post_context_calculation:
 		lctx = xecfg->ctxlen;
 		lctx = XDL_MIN(lctx, xe->xdf1.nrec - (xche->i1 + xche->chg1));
 		lctx = XDL_MIN(lctx, xe->xdf2.nrec - (xche->i2 + xche->chg2));
@@ -209,7 +230,7 @@ int xdl_emit_diff(xdfenv_t *xe, xdchange_t *xscr, xdemitcb_t *ecb,
 				if (l <= e1 ||
 				    get_func_line(xe, xecfg, NULL, l, e1) < 0) {
 					xche = xche->next;
-					goto again;
+					goto post_context_calculation;
 				}
 			}
 		}