ll-merge: make flag easier to populate

ll_merge() takes its options in a flag word, which has a few
advantages:

 - options flags can be cheaply passed around in registers, while
   an option struct passed by pointer cannot;

 - callers can easily pass 0 without trouble for no options,
   while an option struct passed by value would not allow that.

The downside is that code to populate and access the flag word can be
somewhat opaque.  Mitigate that with a few macros.

Cc: Avery Pennarun <apenwarr@gmail.com>
Cc: Bert Wesarg <bert.wesarg@googlemail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/api-merge.txt

@@ -49,12 +49,15 @@ supports this.
 
 The `flag` parameter is a bitfield:
 
- - The least significant bit indicates whether this is an internal
-   merge to consolidate ancestors for a recursive merge.
+ - The `LL_OPT_VIRTUAL_ANCESTOR` bit indicates whether this is an
+   internal merge to consolidate ancestors for a recursive merge.
 
- - The next two bits allow local conflicts to be automatically
+ - The `LL_OPT_FAVOR_MASK` bits allow local conflicts to be automatically
    resolved in favor of one side or the other (as in 'git merge-file'
-   `--ours`/`--theirs`/`--union` for 01, 10, and 11, respectively).
+   `--ours`/`--theirs`/`--union`).
+   They can be populated by `create_ll_flag`, whose argument can be
+   `XDL_MERGE_FAVOR_OURS`, `XDL_MERGE_FAVOR_THEIRS`, or
+   `XDL_MERGE_FAVOR_UNION`.
 
 Everything else
 ---------------

ll-merge.c

@@ -46,7 +46,7 @@ static int ll_binary_merge(const struct ll_merge_driver *drv_unused,
 	 * or common ancestor for an internal merge.  Still return
 	 * "conflicted merge" status.
 	 */
-	mmfile_t *stolen = (flag & 01) ? orig : src1;
+	mmfile_t *stolen = (flag & LL_OPT_VIRTUAL_ANCESTOR) ? orig : src1;
 
 	result->ptr = stolen->ptr;
 	result->size = stolen->size;
@@ -79,7 +79,7 @@ static int ll_xdl_merge(const struct ll_merge_driver *drv_unused,
 
 	memset(&xmp, 0, sizeof(xmp));
 	xmp.level = XDL_MERGE_ZEALOUS;
-	xmp.favor= (flag >> 1) & 03;
+	xmp.favor = ll_opt_favor(flag);
 	if (git_xmerge_style >= 0)
 		xmp.style = git_xmerge_style;
 	if (marker_size > 0)
@@ -99,7 +99,8 @@ static int ll_union_merge(const struct ll_merge_driver *drv_unused,
 			  int flag, int marker_size)
 {
 	/* Use union favor */
-	flag = (flag & 1) | (XDL_MERGE_FAVOR_UNION << 1);
+	flag = (flag & LL_OPT_VIRTUAL_ANCESTOR) |
+	       create_ll_flag(XDL_MERGE_FAVOR_UNION);
 	return ll_xdl_merge(drv_unused, result, path_unused,
 			    orig, NULL, src1, NULL, src2, NULL,
 			    flag, marker_size);
@@ -342,7 +343,7 @@ int ll_merge(mmbuffer_t *result_buf,
 	const char *ll_driver_name = NULL;
 	int marker_size = DEFAULT_CONFLICT_MARKER_SIZE;
 	const struct ll_merge_driver *driver;
-	int virtual_ancestor = flag & 01;
+	int virtual_ancestor = flag & LL_OPT_VIRTUAL_ANCESTOR;
 
 	if (merge_renormalize) {
 		normalize_file(ancestor, path);

ll-merge.h

@@ -5,6 +5,20 @@
 #ifndef LL_MERGE_H
 #define LL_MERGE_H
 
+#define LL_OPT_VIRTUAL_ANCESTOR	(1 << 0)
+#define LL_OPT_FAVOR_MASK	((1 << 1) | (1 << 2))
+#define LL_OPT_FAVOR_SHIFT 1
+
+static inline int ll_opt_favor(int flag)
+{
+	return (flag & LL_OPT_FAVOR_MASK) >> LL_OPT_FAVOR_SHIFT;
+}
+
+static inline int create_ll_flag(int favor)
+{
+	return ((favor << LL_OPT_FAVOR_SHIFT) & LL_OPT_FAVOR_MASK);
+}
+
 int ll_merge(mmbuffer_t *result_buf,
 	     const char *path,
 	     mmfile_t *ancestor, const char *ancestor_label,

merge-recursive.c

@@ -647,7 +647,8 @@ static int merge_3way(struct merge_options *o,
 
 	merge_status = ll_merge(result_buf, a->path, &orig, base_name,
 				&src1, name1, &src2, name2,
-				(!!o->call_depth) | (favor << 1));
+				((o->call_depth ? LL_OPT_VIRTUAL_ANCESTOR : 0) |
+				 create_ll_flag(favor)));
 
 	free(name1);
 	free(name2);