xdiff: add new XDL_MERGE_FAVOR_BASE mode

Similar to XDL_MERGE_FAVOR_{OURS,THEIRS} add a new mode for favoring the
base version in the event of a conflict, and add a --base flag to
merge-file to allow testing this out.

This will be used in a subsequent commit to reduce the prevalence of
nested conflict markers in the virtual merge base when using diff3
conflict style, by having the virtual merge base (created by merging the
merge bases) favor the version from their merge base (i.e. from the merge
base of the merge bases).

Signed-off-by: Elijah Newren <[email protected]>

Author: newren

builtin/merge-file.c

@@ -73,6 +73,8 @@ int cmd_merge_file(int argc,
 		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, "base", &xmp.favor, N_("for conflicts, use base version"),
+			    XDL_MERGE_FAVOR_BASE),
 		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"),

xdiff/xdiff.h

@@ -63,6 +63,7 @@ extern "C" {
 #define XDL_MERGE_FAVOR_OURS 1
 #define XDL_MERGE_FAVOR_THEIRS 2
 #define XDL_MERGE_FAVOR_UNION 3
+#define XDL_MERGE_FAVOR_BASE 4
 
 /* merge output styles */
 #define XDL_MERGE_DIFF3 1

xdiff/xmerge.c

@@ -26,9 +26,10 @@ typedef struct s_xdmerge {
 	struct s_xdmerge *next;
 	/*
 	 * 0 = conflict,
-	 * 1 = no conflict, take first,
+	 * 1 = no conflict, take first.
 	 * 2 = no conflict, take second.
-	 * 3 = no conflict, take both.
+	 * 3 = no conflict, take both first & second.
+	 * 4 = no conflict, take base.
 	 */
 	int mode;
 	/*
@@ -313,6 +314,13 @@ static int xdl_fill_merge_buffer(xdfenv_t *xe1, const char *name1,
 			if (m->mode & 2)
 				size += xdl_recs_copy(xe2, m->i2, m->chg2, 0, 0,
 						      dest ? dest + size : NULL);
+		} else if (m->mode == XDL_MERGE_FAVOR_BASE) {
+			/* Before conflicting part */
+			size += xdl_recs_copy(xe1, i, m->i1 - i, 0, 0,
+					      dest ? dest + size : NULL);
+			/* Image from merge base */
+			size += xdl_orig_copy(xe1, m->i0, m->chg0, 0, 0,
+					      dest ? dest + size : NULL);
 		} else
 			continue;
 		i = m->i1 + m->chg1;