[PATCH v2 1/2] xdiff: implement a zealous diff3, or "zdiff3"

["Elijah Newren via GitGitGadget" <gitgitgadget@gmail.com>]

From: Elijah Newren <newren@gmail.com>

"zdiff3" is identical to ordinary diff3 except that it allows compaction
of common lines on the two sides of history at the beginning or end of
the conflict hunk.  For example, the following diff3 conflict:

    1
    2
    3
    4
    <<<<<<
    A
    B
    C
    D
    E
    ||||||
    5
    6
    ======
    A
    X
    C
    Y
    E
    >>>>>>
    7
    8
    9

has common lines 'A', 'C', and 'E' on the two sides.  With zdiff3, one
would instead get the following conflict:

    1
    2
    3
    4
    A
    <<<<<<
    B
    C
    D
    ||||||
    5
    6
    ======
    X
    C
    Y
    >>>>>>
    E
    7
    8
    9

Note that the common lines, 'A', and 'E' were moved outside the
conflict.  Unlike with the two-way conflicts from the 'merge'
conflictStyle, the zdiff3 conflict is NOT split into multiple conflict
regions to allow the common 'C' lines to be shown outside a conflict,
because zdiff3 shows the base version too and the base version cannot be
reasonably split.

Based-on-patch-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Helped-by: Phillip Wood <phillip.wood123@gmail.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
---
 builtin/merge-file.c                   |  2 +
 contrib/completion/git-completion.bash |  6 +--
 t/t6427-diff3-conflict-markers.sh      | 56 +++++++++++++++++++++
 xdiff-interface.c                      |  2 +
 xdiff/xdiff.h                          |  1 +
 xdiff/xmerge.c                         | 68 +++++++++++++++++++++++---
 6 files changed, 126 insertions(+), 9 deletions(-)

diff --git a/builtin/merge-file.c b/builtin/merge-file.c
index 06a2f90c487..e695867ee54 100644
--- a/builtin/merge-file.c
+++ b/builtin/merge-file.c
@@ -34,6 +34,8 @@ int cmd_merge_file(int argc, const char **argv, const char *prefix)
 	struct option options[] = {
 		OPT_BOOL('p', "stdout", &to_stdout, N_("send results to standard output")),
 		OPT_SET_INT(0, "diff3", &xmp.style, N_("use a diff3 based merge"), XDL_MERGE_DIFF3),
+		OPT_SET_INT(0, "zdiff3", &xmp.style, N_("use a zealous diff3 based merge"),
+				XDL_MERGE_ZEALOUS_DIFF3),
 		OPT_SET_INT(0, "ours", &xmp.favor, N_("for conflicts, use our version"),
 			    XDL_MERGE_FAVOR_OURS),
 		OPT_SET_INT(0, "theirs", &xmp.favor, N_("for conflicts, use their version"),
diff --git a/contrib/completion/git-completion.bash b/contrib/completion/git-completion.bash
index b50c5d0ea38..8489ca39497 100644
--- a/contrib/completion/git-completion.bash
+++ b/contrib/completion/git-completion.bash
@@ -1566,7 +1566,7 @@ _git_checkout ()
 
 	case "$cur" in
 	--conflict=*)
-		__gitcomp "diff3 merge" "" "${cur##--conflict=}"
+		__gitcomp "diff3 merge zdiff3" "" "${cur##--conflict=}"
 		;;
 	--*)
 		__gitcomp_builtin checkout
@@ -2445,7 +2445,7 @@ _git_switch ()
 
 	case "$cur" in
 	--conflict=*)
-		__gitcomp "diff3 merge" "" "${cur##--conflict=}"
+		__gitcomp "diff3 merge zdiff3" "" "${cur##--conflict=}"
 		;;
 	--*)
 		__gitcomp_builtin switch
@@ -2886,7 +2886,7 @@ _git_restore ()
 
 	case "$cur" in
 	--conflict=*)
-		__gitcomp "diff3 merge" "" "${cur##--conflict=}"
+		__gitcomp "diff3 merge zdiff3" "" "${cur##--conflict=}"
 		;;
 	--source=*)
 		__git_complete_refs --cur="${cur##--source=}"
diff --git a/t/t6427-diff3-conflict-markers.sh b/t/t6427-diff3-conflict-markers.sh
index 25c4b720e72..de9c6190b9c 100755
--- a/t/t6427-diff3-conflict-markers.sh
+++ b/t/t6427-diff3-conflict-markers.sh
@@ -211,4 +211,60 @@ test_expect_success 'rebase --apply describes fake ancestor base' '
 	)
 '
 
+test_setup_zdiff3 () {
+	test_create_repo zdiff3 &&
+	(
+		cd zdiff3 &&
+
+		test_write_lines 1 2 3 4 5 6 7 8 9 >basic &&
+		test_write_lines 1 2 3 AA 4 5 BB 6 7 8 >middle-common &&
+		test_write_lines 1 2 3 4 5 6 7 8 9 >interesting &&
+
+		git add basic middle-common &&
+		git commit -m base &&
+
+		git branch left &&
+		git branch right &&
+
+		git checkout left &&
+		test_write_lines 1 2 3 4 A B C D E 7 8 9 >basic &&
+		test_write_lines 1 2 3 CC 4 5 DD 6 7 8 >middle-common &&
+		test_write_lines 1 2 3 4 A B C D E F G H I J 7 8 9 >interesting &&
+		git add -u &&
+		git commit -m letters &&
+
+		git checkout right &&
+		test_write_lines 1 2 3 4 A X C Y E 7 8 9 >basic &&
+		test_write_lines 1 2 3 EE 4 5 FF 6 7 8 >middle-common &&
+		test_write_lines 1 2 3 4 A B C 5 6 G H I J 7 8 9 >interesting &&
+		git add -u &&
+		git commit -m permuted
+	)
+}
+
+test_expect_failure 'check zdiff3 markers' '
+	test_setup_zdiff3 &&
+	(
+		cd zdiff3 &&
+
+		git checkout left^0 &&
+
+		test_must_fail git -c merge.conflictstyle=zdiff3 merge -s recursive right^0 &&
+
+		test_write_lines 1 2 3 4 A "<<<<<<< HEAD" B C D "||||||| $(git rev-parse --short HEAD^1)" 5 6 ======= X C Y ">>>>>>> right^0" E 7 8 9 >expect &&
+		test_cmp expect basic &&
+
+		test_write_lines 1 2 3 "<<<<<<< HEAD" CC "||||||| $(git rev-parse --short HEAD^1)" AA ======= EE ">>>>>>> right^0" 4 5 "<<<<<<< HEAD" DD "||||||| $(git rev-parse --short HEAD^1)" BB ======= FF ">>>>>>> right^0" 6 7 8 >expect &&
+		test_cmp expect middle-common &&
+
+		# Not passing this one yet.  For some reason, after extracting
+		# the common lines "A B C" and "G H I J", the remaining part
+		# is comparing "5 6" in the base to "5 6" on the left and
+		# "D E F" on the right.  And zdiff3 currently picks the side
+		# that matches the base as the merge result.  Weird.
+		test_write_lines 1 2 3 4 A B C D E F G H I J 7 8 9 >expect &&
+		test_cmp expect interesting
+	)
+'
+
 test_done
diff --git a/xdiff-interface.c b/xdiff-interface.c
index 609615db2cd..9977813a9d3 100644
--- a/xdiff-interface.c
+++ b/xdiff-interface.c
@@ -308,6 +308,8 @@ int git_xmerge_config(const char *var, const char *value, void *cb)
 			die("'%s' is not a boolean", var);
 		if (!strcmp(value, "diff3"))
 			git_xmerge_style = XDL_MERGE_DIFF3;
+		else if (!strcmp(value, "zdiff3"))
+			git_xmerge_style = XDL_MERGE_ZEALOUS_DIFF3;
 		else if (!strcmp(value, "merge"))
 			git_xmerge_style = 0;
 		/*
diff --git a/xdiff/xdiff.h b/xdiff/xdiff.h
index 7a046051468..8629ae287c7 100644
--- a/xdiff/xdiff.h
+++ b/xdiff/xdiff.h
@@ -65,6 +65,7 @@ extern "C" {
 
 /* merge output styles */
 #define XDL_MERGE_DIFF3 1
+#define XDL_MERGE_ZEALOUS_DIFF3 2
 
 typedef struct s_mmfile {
 	char *ptr;
diff --git a/xdiff/xmerge.c b/xdiff/xmerge.c
index 1659edb4539..df0c6041778 100644
--- a/xdiff/xmerge.c
+++ b/xdiff/xmerge.c
@@ -230,7 +230,7 @@ static int fill_conflict_hunk(xdfenv_t *xe1, const char *name1,
 	size += xdl_recs_copy(xe1, m->i1, m->chg1, needs_cr, 1,
 			      dest ? dest + size : NULL);
 
-	if (style == XDL_MERGE_DIFF3) {
+	if (style == XDL_MERGE_DIFF3 || style == XDL_MERGE_ZEALOUS_DIFF3) {
 		/* Shared preimage */
 		if (!dest) {
 			size += marker_size + 1 + needs_cr + marker3_size;
@@ -327,7 +327,7 @@ static int xdl_fill_merge_buffer(xdfenv_t *xe1, const char *name1,
  * lines. Try hard to show only these few lines as conflicting.
  */
 static int xdl_refine_conflicts(xdfenv_t *xe1, xdfenv_t *xe2, xdmerge_t *m,
-		xpparam_t const *xpp)
+				xpparam_t const *xpp, int style)
 {
 	for (; m; m = m->next) {
 		mmfile_t t1, t2;
@@ -368,6 +368,42 @@ static int xdl_refine_conflicts(xdfenv_t *xe1, xdfenv_t *xe2, xdmerge_t *m,
 			continue;
 		}
 		x = xscr;
+		if (style == XDL_MERGE_ZEALOUS_DIFF3) {
+			int advance1 = xscr->i1, advance2 = xscr->i2;
+
+			/*
+			 * Advance m->i1 and m->i2 so that conflict for sides
+			 * 1 and 2 start after common region.  Decrement
+			 * m->chg[12] since there are now fewer conflict lines
+			 * for those sides.
+			 */
+			m->i1 += advance1;
+			m->i2 += advance2;
+			m->chg1 -= advance1;
+			m->chg2 -= advance2;
+
+			/*
+			 * Splitting conflicts due to internal common regions
+			 * on the two sides would be inappropriate since we
+			 * are also showing the merge base and have no
+			 * reasonable way to split the merge base text.
+			 */
+			while (xscr->next)
+				xscr = xscr->next;
+
+			/*
+			 * Lower the number of conflict lines to not include
+			 * the final common lines, if any.  Do this by setting
+			 * number of conflict lines to
+			 *   (line offset for start of conflict in xscr) +
+			 *   (number of lines in the conflict in xscr)
+			 */
+			m->chg1 = (xscr->i1 - advance1) + (xscr->chg1);
+			m->chg2 = (xscr->i2 - advance2) + (xscr->chg2);
+			xdl_free_env(&xe);
+			xdl_free_script(x);
+			continue;
+		}
 		m->i1 = xscr->i1 + i1;
 		m->chg1 = xscr->chg1;
 		m->i2 = xscr->i2 + i2;
@@ -419,6 +455,7 @@ static int lines_contain_alnum(xdfenv_t *xe, int i, int chg)
 static void xdl_merge_two_conflicts(xdmerge_t *m)
 {
 	xdmerge_t *next_m = m->next;
+	m->chg0 = next_m->i0 + next_m->chg0 - m->i0;
 	m->chg1 = next_m->i1 + next_m->chg1 - m->i1;
 	m->chg2 = next_m->i2 + next_m->chg2 - m->i2;
 	m->next = next_m->next;
@@ -430,12 +467,12 @@ static void xdl_merge_two_conflicts(xdmerge_t *m)
  * it appears simpler -- because it takes up less (or as many) lines --
  * if the lines are moved into the conflicts.
  */
-static int xdl_simplify_non_conflicts(xdfenv_t *xe1, xdmerge_t *m,
+static int xdl_simplify_non_conflicts(xdfenv_t *xe1, xdmerge_t *m, int style,
 				      int simplify_if_no_alnum)
 {
 	int result = 0;
 
-	if (!m)
+	if (!m || style == XDL_MERGE_ZEALOUS_DIFF3)
 		return result;
 	for (;;) {
 		xdmerge_t *next_m = m->next;
@@ -482,6 +519,25 @@ static int xdl_do_merge(xdfenv_t *xe1, xdchange_t *xscr1,
 	int style = xmp->style;
 	int favor = xmp->favor;
 
+	/*
+	 * XDL_MERGE_DIFF3 does not attempt to refine conflicts by looking
+	 * at common areas of sides 1 & 2, because the base (side 0) does
+	 * not match and is being shown.  Similarly, simplification of
+	 * non-conflicts is also skipped due to the skipping of conflict
+	 * refinement.
+	 *
+	 * XDL_MERGE_ZEALOUS_DIFF3, on the other hand, will attempt to
+	 * refine conflicts looking for common areas of sides 1 & 2.
+	 * However, since the base is being shown and does not match,
+	 * it will only look for common areas at the beginning or end
+	 * of the conflict block.  Since XDL_MERGE_ZEALOUS_DIFF3's
+	 * conflict refinement is much more limited in this fashion, the
+	 * conflict simplification will be skipped.
+	 *
+	 * See xdl_refine_conflicts() and xdl_simplify_non_conflicts()
+	 * for more details, particularly looking for
+	 * XDL_MERGE_ZEALOUS_DIFF3.
+	 */
 	if (style == XDL_MERGE_DIFF3) {
 		/*
 		 * "diff3 -m" output does not make sense for anything
@@ -604,8 +660,8 @@ static int xdl_do_merge(xdfenv_t *xe1, xdchange_t *xscr1,
 		changes = c;
 	/* refine conflicts */
 	if (XDL_MERGE_ZEALOUS <= level &&
-	    (xdl_refine_conflicts(xe1, xe2, changes, xpp) < 0 ||
-	     xdl_simplify_non_conflicts(xe1, changes,
+	    (xdl_refine_conflicts(xe1, xe2, changes, xpp, style) < 0 ||
+	     xdl_simplify_non_conflicts(xe1, changes, style,
 					XDL_MERGE_ZEALOUS < level) < 0)) {
 		xdl_cleanup_merge(changes);
 		return -1;
-- 
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