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From: Junio C Hamano <gitster@pobox.com>
To: git@vger.kernel.org
Subject: [PATCH 04/12] xmerge.c: "diff3 -m" style clips merge reduction level to EAGER or less
Date: Fri, 29 Aug 2008 17:42:35 -0700	[thread overview]
Message-ID: <1220056963-2352-5-git-send-email-gitster@pobox.com> (raw)
In-Reply-To: <1220056963-2352-4-git-send-email-gitster@pobox.com>

When showing a conflicting merge result, and "--diff3 -m" style is asked
for, this patch makes sure that the merge reduction level does not exceed
XDL_MERGE_EAGER.  This is because "diff3 -m" style output would not make
sense for anything more aggressive than XDL_MERGE_EAGER, because of the
way how the merge reduction works.

"git merge-file" no longer has to force MERGE_EAGER when "--diff3" is
asked for because of this change.

Suppose a common ancestor (shared preimage) is modified to postimage #1
and #2 (each letter represents one line):

                     #####
    postimage#1: 1234ABCDE789
                    |    /
                    |   /
    preimage:    123456789
                    |   \
    postimage#2: 1234AXYE789
                     ####

XDL_MERGE_MINIMAL and XDL_MERGE_EAGER would:

 (1) find the s/56/ABCDE/ done on one side and s/56/AXYE/ done on the
     other side,

 (2) notice that they touch an overlapping area, and

 (3) mark it as a conflict, "ABCDE vs AXYE".

The difference between the two algorithms is that EAGER drops the hunk
altogether if the postimages match (i.e. both sides modified the same
way), while MINIMAL keeps it.  There is no other operation performed to
the hunk.  As the result, lines marked with "#" in the above picure will
be in the RCS merge style output like this (letters <, = and > represent
conflict marker lines):

    output:      1234<ABCDE=AXYE>789    ; with MINIMAL/EAGER

The part from the preimage that corresponds to these conflicting changes
is "56", which is what "diff3 -m" style output adds to it:

    output:      1234<ABCDE|56=AXYE>789 ; in "diff3 -m" style

Now, XDL_MERGE_ZEALOUS looks at the differences between the changes two
postimages made in order to reduce the number of lines in the conflicting
regions.  It notices that both sides start their new contents with "A",
and excludes it from the output (it also excludes "E" for the same
reason).  The conflict that used to be "ABCDE vs AXYE" is now "BCD vs XY":

    output:      1234A<BCD=XY>E789      ; with ZEALOUS

There could even be matching parts between two postimages in the middle.
Instead of one side rewriting the shared "56" to "ABCDE" and the other
side to "AXYE", imagine the case where the postimages are "ABCDE" and
"AXCYE", in which case instead of having one conflicted hunk "BCD vs XY",
you would have two conflicting hunks "B vs X" and "D vs Y".

In either case, once you reduce "ABCDE vs AXYE" to "BCD vs XY" (or "ABCDE
vs AXCYE" to "B vs X" and "D vs Y"), there is no part from the preimage
that corresponds to the conflicting change made in both postimages
anymore.  In other words, conflict reduced by ZEALOUS algorithm cannot be
expressed in "diff3 -m" style.  Representing the last illustration like
this is misleading to say the least:

    output:      1234A<BCD|56=XY>E789   ; broken "diff3 -m" style

because the preimage was not ...4A56E... to begin with.  "A" and "E" are
common only between the postimages.

Even worse, once a single conflicting hunk is split into multiple ones
(recall the example of breaking "ABCDE vs AXCYE" to "B vs X" and "D vs
Y"), there is no sane way to distribute the preimage text across split
conflicting hunks.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 builtin-merge-file.c |    4 +---
 xdiff/xmerge.c       |    9 +++++++++
 2 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/builtin-merge-file.c b/builtin-merge-file.c
index 5b4f020..1e92510 100644
--- a/builtin-merge-file.c
+++ b/builtin-merge-file.c
@@ -27,10 +27,8 @@ int cmd_merge_file(int argc, const char **argv, const char *prefix)
 		else if (!strcmp(argv[1], "-q") ||
 				!strcmp(argv[1], "--quiet"))
 			freopen("/dev/null", "w", stderr);
-		else if (!strcmp(argv[1], "--diff3")) {
+		else if (!strcmp(argv[1], "--diff3"))
 			merge_style = XDL_MERGE_DIFF3;
-			merge_level = XDL_MERGE_EAGER;
-		}
 		else
 			usage(merge_file_usage);
 		argc--;
diff --git a/xdiff/xmerge.c b/xdiff/xmerge.c
index 7dcd405..d9737f0 100644
--- a/xdiff/xmerge.c
+++ b/xdiff/xmerge.c
@@ -392,6 +392,15 @@ static int xdl_do_merge(xdfenv_t *xe1, xdchange_t *xscr1, const char *name1,
 	int level = flags & XDL_MERGE_LEVEL_MASK;
 	int style = flags & XDL_MERGE_STYLE_MASK;
 
+	if (style == XDL_MERGE_DIFF3) {
+		/*
+		 * "diff3 -m" output does not make sense for anything
+		 * more aggressive than XDL_MERGE_EAGER.
+		 */
+		if (XDL_MERGE_EAGER < level)
+			level = XDL_MERGE_EAGER;
+	}
+
 	c = changes = NULL;
 
 	while (xscr1 && xscr2) {
-- 
1.6.0.1.149.ga4c44

  reply	other threads:[~2008-08-30  0:44 UTC|newest]

Thread overview: 26+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2008-08-30  0:42 [PATCH 00/12] Towards a better merge resolution support Junio C Hamano
2008-08-30  0:42 ` [PATCH 01/12] xdl_fill_merge_buffer(): separate out a too deeply nested function Junio C Hamano
2008-08-30  0:42   ` [PATCH 02/12] xdiff-merge: optionally show conflicts in "diff3 -m" style Junio C Hamano
2008-08-30  0:42     ` [PATCH 03/12] xmerge.c: minimum readability fixups Junio C Hamano
2008-08-30  0:42       ` Junio C Hamano [this message]
2008-08-30  0:42         ` [PATCH 05/12] rerere.c: use symbolic constants to keep track of parsing states Junio C Hamano
2008-08-30  0:42           ` [PATCH 06/12] rerere: understand "diff3 -m" style conflicts with the original Junio C Hamano
2008-08-30  0:42             ` [PATCH 07/12] merge.conflictstyle: choose between "merge" and "diff3 -m" styles Junio C Hamano
2008-08-30  0:42               ` [PATCH 08/12] git-merge-recursive: learn to honor merge.conflictstyle Junio C Hamano
2008-08-30  0:42                 ` [PATCH 09/12] checkout: do not check out unmerged higher stages randomly Junio C Hamano
2008-08-30  0:42                   ` [PATCH 10/12] checkout: allow ignoring unmerged paths when checking out of the index Junio C Hamano
2008-08-30  0:42                     ` [PATCH 11/12] checkout --ours/--theirs Junio C Hamano
2008-08-30  0:42                       ` [PATCH 12/12] checkout -m: recreate merge when checking out of unmerged index Junio C Hamano
2008-08-30  9:42               ` [PATCH 07/12] merge.conflictstyle: choose between "merge" and "diff3 -m" styles Johannes Schindelin
2008-08-30  9:34         ` [PATCH 04/12] xmerge.c: "diff3 -m" style clips merge reduction level to EAGER or less Johannes Schindelin
2008-08-30  9:31       ` [PATCH 03/12] xmerge.c: minimum readability fixups Johannes Schindelin
2008-08-30 15:42         ` Junio C Hamano
2008-08-30  9:29     ` [PATCH 02/12] xdiff-merge: optionally show conflicts in "diff3 -m" style Johannes Schindelin
2008-08-30  9:14   ` [PATCH 01/12] xdl_fill_merge_buffer(): separate out a too deeply nested function Johannes Schindelin
2008-09-01  9:39 ` [PATCH 00/12] Towards a better merge resolution support Alex Riesen
2008-09-01  9:44 ` Alex Riesen
2008-09-01  9:50   ` Abhijit Menon-Sen
2008-09-01 12:20     ` Thomas Rast
2008-09-01 10:38   ` Junio C Hamano
2008-09-01 11:34     ` Alex Riesen
2008-09-01 17:26       ` Junio C Hamano

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