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

From: Elijah Newren via GitGitGadget <gitgitgadget@gmail.com>
Date: Jun 15, 2021, 05:16 UTC
Message-ID: <b7561a67c192d4bdede47fee5b7b1cb30c44b785.1623734171.git.gitgitgadget@gmail.com>
In-Reply-To: <pull.1036.git.git.1623734171.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.

Initial-patch-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Elijah Newren <newren@gmail.com>
---
 builtin/merge-file.c                   |  2 +
 contrib/completion/git-completion.bash |  2 +-
 xdiff-interface.c                      |  2 +
 xdiff/xdiff.h                          |  1 +
 xdiff/xmerge.c                         | 52 ++++++++++++++++++++++++--
 5 files changed, 55 insertions(+), 4 deletions(-)

diff --git a/builtin/merge-file.c b/builtin/merge-file.c
index 06a2f90c4875..e695867ee548 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 b50c5d0ea386..859455929814 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
diff --git a/xdiff-interface.c b/xdiff-interface.c
index 609615db2cd6..9977813a9d37 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 7a046051468f..8629ae287c79 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 1659edb45393..b1dc9df7ea0a 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;
@@ -482,6 +518,16 @@ 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.
+	 *
+	 * XDL_MERGE_ZEALOUS_DIFF3 will attempt to refine conflicts
+	 * looking for common areas of sides 1 & 2, despite the base
+	 * not matching and being shown, but will only look for common
+	 * areas at the beginning or ending of the conflict block.
+	 */
 	if (style == XDL_MERGE_DIFF3) {
 		/*
 		 * "diff3 -m" output does not make sense for anything
@@ -604,7 +650,7 @@ 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_refine_conflicts(xe1, xe2, changes, xpp, style) < 0 ||
 	     xdl_simplify_non_conflicts(xe1, changes,
 					XDL_MERGE_ZEALOUS < level) < 0)) {
 		xdl_cleanup_merge(changes);

-- 
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