+static int threeway_merge(struct cache_entry *stages[4],
+ struct cache_entry **dst,
+ struct cache_entry **next, int tail)
+{
+ struct cache_entry *old = stages[0];
+ struct cache_entry *a = stages[1], *b = stages[2], *c = stages[3];
+ struct cache_entry *merge;
+ int count;
+
+ /* #5ALT */
+ if (!a && b && c && same(b, c)) {
+ if (old && !same(b, old))
+ return -1;
+ return merged_entry_allow_dirty(b, old, dst);
+ }
+ /* #2ALT and #3ALT */
+ if (!a && (!!b != !!c)) {
+ /*
+ * The reason we need to worry about directory/file
+ * conflicts only in #2ALT and #3ALT case is this:
+ *
+ * (1) For all other cases that read-tree internally
+ * resolves a path, we always have such a path in
+ * *both* stage2 and stage3 when we begin.
+ * Traditionally, the behaviour has been even
+ * stricter and we did not resolve a path without
+ * initially being in all of stage1, 2, and 3.
+ *
+ * (2) When read-tree finishes, all resolved paths (i.e.
+ * the paths that are in stage0) must have come from
+ * either stage2 or stage3. It is not possible to
+ * have a stage0 path as a result of a merge if
+ * neither stage2 nor stage3 had that path.
+ *
+ * (3) It is guaranteed that just after reading the
+ * stages, each stage cannot have directory/file
+ * conflicts on its own, because they are populated
+ * by reading hierarchy of a tree. Combined with
+ * (1) and (2) above, this means that no matter what
+ * combination of paths we take from stage2 and
+ * stage3 as a result of a merge, they cannot cause
+ * a directory/file conflict situation (otherwise
+ * the "guilty" path would have already had such a
+ * conflict in the original stage, either stage2
+ * or stage3). Although its stage2 is synthesized
+ * by overlaying the current index on top of "our
+ * head" tree, --emu23 case also has this guarantee,
+ * by calling add_cache_entry() to create such stage2
+ * entries.
+ *
+ * (4) Only #2ALT and #3ALT lack the guarantee (1).
+ * They resolve paths that exist only in stage2
+ * or stage3. The stage2 tree may have a file DF
+ * while stage3 tree may have a file DF/DF. If
+ * #2ALT and #3ALT rules happen to apply to both
+ * of them, we would end up having DF (coming from
+ * stage2) and DF/DF (from stage3) in the result.
+ * When we attempt to resolve a path that exists
+ * only in stage2, we need to make sure there is
+ * no path that would conflict with it in stage3
+ * and vice versa.
+ */
+ if (c) { /* #2ALT */
+ if (!causes_df_conflict(c, 2, dst, next, tail) &&
+ (!old || same(c, old)))
+ return merged_entry_allow_dirty(c, old, dst);
+ }
+ else { /* #3ALT */
+ if (!causes_df_conflict(b, 3, dst, next, tail) &&
+ (!old || same(b, old)))
+ return merged_entry_allow_dirty(b, old, dst);
+ }
+ /* otherwise we will apply the original rule */
+ }
+ /* #14ALT */
+ if (a && b && c && same(a, b) && !same(a, c)) {
+ if (old && same(old, c))
+ return merged_entry_allow_dirty(c, old, dst);
+ /* otherwise the regular rule applies */
+ }
+ /*
+ * If we have an entry in the index cache ("old"), then we want
+ * to make sure that it matches any entries in stage 2 ("first
+ * branch", aka "b").
+ */
+ if (old) {
+ if (!b || !same(old, b))
+ return -1;
+ }
+ merge = merge_entries(a, b, c);
+ if (merge)
+ return merged_entry(merge, old, dst);
+ if (old)
+ verify_uptodate(old);
+ count = 0;
+ if (a) { *dst++ = a; count++; }
+ if (b) { *dst++ = b; count++; }
+ if (c) { *dst++ = c; count++; }
+ return count;
+}
+
+/*
+ * Two-way merge.
+ *
+ * The rule is to "carry forward" what is in the index without losing
+ * information across a "fast forward", favoring a successful merge
+ * over a merge failure when it makes sense. For details of the
+ * "carry forward" rule, please see <Documentation/git-read-tree.txt>.
+ *
+ */
+static int twoway_merge(struct cache_entry **src, struct cache_entry **dst,
+ struct cache_entry **next, int tail)
+{
+ struct cache_entry *current = src[0];
+ struct cache_entry *oldtree = src[1], *newtree = src[2];
+
+ if (src[3])
+ return -1;
+
+ if (current) {
+ if ((!oldtree && !newtree) || /* 4 and 5 */
+ (!oldtree && newtree &&
+ same(current, newtree)) || /* 6 and 7 */
+ (oldtree && newtree &&
+ same(oldtree, newtree)) || /* 14 and 15 */
+ (oldtree && newtree &&
+ !same(oldtree, newtree) && /* 18 and 19*/
+ same(current, newtree))) {
+ *dst++ = current;
+ return 1;
+ }
+ else if (oldtree && !newtree && same(current, oldtree)) {
+ /* 10 or 11 */
+ return deleted_entry(oldtree, current, dst);
+ }
+ else if (oldtree && newtree &&
+ same(current, oldtree) && !same(current, newtree)) {
+ /* 20 or 21 */
+ return merged_entry(newtree, current, dst);
+ }
+ else
+ /* all other failures */
+ return -1;
+ }
+ else if (newtree)
+ return merged_entry(newtree, current, dst);
+ else
+ return deleted_entry(oldtree, current, dst);
+}