[PATCH 03/12] merge-tree: add option parsing and initial shell for real merge function

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

From: Elijah Newren <newren@gmail.com>

Let merge-tree accept a `--write-tree` parameter for choosing real
merges instead of trivial merges, and accept an optional
`--trivial-merge` option to get the traditional behavior.  Note that
these accept different numbers of arguments, though, so these names
need not actually be used.

Note that real merges differ from trivial merges in that they handle:
  - three way content merges
  - recursive ancestor consolidation
  - renames
  - proper directory/file conflict handling
  - etc.
Basically all the stuff you'd expect from `git merge`, just without
updating the index and working tree.  The initial shell added here does
nothing more than die with "real merges are not yet implemented", but
that will be fixed in subsequent commits.

Signed-off-by: Elijah Newren <newren@gmail.com>
---
 builtin/merge-tree.c | 67 ++++++++++++++++++++++++++++++++++++++------
 git.c                |  2 +-
 2 files changed, 59 insertions(+), 10 deletions(-)

diff --git a/builtin/merge-tree.c b/builtin/merge-tree.c
index 914ec960b7e..33e47cc1534 100644
--- a/builtin/merge-tree.c
+++ b/builtin/merge-tree.c
@@ -3,13 +3,12 @@
 #include "tree-walk.h"
 #include "xdiff-interface.h"
 #include "object-store.h"
+#include "parse-options.h"
 #include "repository.h"
 #include "blob.h"
 #include "exec-cmd.h"
 #include "merge-blobs.h"
 
-static const char merge_tree_usage[] = "git merge-tree <base-tree> <branch1> <branch2>";
-
 struct merge_list {
 	struct merge_list *next;
 	struct merge_list *link;	/* other stages for this object */
@@ -366,15 +365,17 @@ static void *get_tree_descriptor(struct repository *r,
 	return buf;
 }
 
-static int trivial_merge(int argc, const char **argv)
+static int trivial_merge(const char *base,
+			 const char *branch1,
+			 const char *branch2)
 {
 	struct repository *r = the_repository;
 	struct tree_desc t[3];
 	void *buf1, *buf2, *buf3;
 
-	buf1 = get_tree_descriptor(r, t+0, argv[1]);
-	buf2 = get_tree_descriptor(r, t+1, argv[2]);
-	buf3 = get_tree_descriptor(r, t+2, argv[3]);
+	buf1 = get_tree_descriptor(r, t+0, base);
+	buf2 = get_tree_descriptor(r, t+1, branch1);
+	buf3 = get_tree_descriptor(r, t+2, branch2);
 	trivial_merge_trees(t, "");
 	free(buf1);
 	free(buf2);
@@ -384,9 +385,57 @@ static int trivial_merge(int argc, const char **argv)
 	return 0;
 }
 
+struct merge_tree_options {
+	int real;
+	int trivial;
+};
+
+static int real_merge(struct merge_tree_options *o,
+		      const char *branch1, const char *branch2)
+{
+	die(_("real merges are not yet implemented"));
+}
+
 int cmd_merge_tree(int argc, const char **argv, const char *prefix)
 {
-	if (argc != 4)
-		usage(merge_tree_usage);
-	return trivial_merge(argc, argv);
+	struct merge_tree_options o = { 0 };
+	int expected_remaining_argc;
+
+	const char * const merge_tree_usage[] = {
+		N_("git merge-tree [--write-tree] <branch1> <branch2>"),
+		N_("git merge-tree [--trivial-merge] <base-tree> <branch1> <branch2>"),
+		NULL
+	};
+	struct option mt_options[] = {
+		OPT_BOOL(0, "write-tree", &o.real,
+			 N_("do a real merge instead of a trivial merge")),
+		OPT_BOOL(0, "trivial-merge", &o.trivial,
+			 N_("do a trivial merge only")),
+		OPT_END()
+	};
+
+	/* Check for a request for basic help */
+	if (argc == 2 && !strcmp(argv[1], "-h"))
+		usage_with_options(merge_tree_usage, mt_options);
+
+	/* Parse arguments */
+	argc = parse_options(argc, argv, prefix, mt_options,
+			     merge_tree_usage, 0);
+	if (o.real && o.trivial)
+		die(_("--write-tree and --trivial-merge are incompatible"));
+	if (o.real || o.trivial) {
+		expected_remaining_argc = (o.real ? 2 : 3);
+		if (argc != expected_remaining_argc)
+			usage_with_options(merge_tree_usage, mt_options);
+	} else {
+		if (argc < 2 || argc > 3)
+			usage_with_options(merge_tree_usage, mt_options);
+		o.real = (argc == 2);
+	}
+
+	/* Do the relevant type of merge */
+	if (o.real)
+		return real_merge(&o, argv[0], argv[1]);
+	else
+		return trivial_merge(argv[0], argv[1], argv[2]);
 }
diff --git a/git.c b/git.c
index 5ff21be21f3..6090a1289db 100644
--- a/git.c
+++ b/git.c
@@ -558,7 +558,7 @@ static struct cmd_struct commands[] = {
 	{ "merge-recursive-ours", cmd_merge_recursive, RUN_SETUP | NEED_WORK_TREE | NO_PARSEOPT },
 	{ "merge-recursive-theirs", cmd_merge_recursive, RUN_SETUP | NEED_WORK_TREE | NO_PARSEOPT },
 	{ "merge-subtree", cmd_merge_recursive, RUN_SETUP | NEED_WORK_TREE | NO_PARSEOPT },
-	{ "merge-tree", cmd_merge_tree, RUN_SETUP | NO_PARSEOPT },
+	{ "merge-tree", cmd_merge_tree, RUN_SETUP },
 	{ "mktag", cmd_mktag, RUN_SETUP | NO_PARSEOPT },
 	{ "mktree", cmd_mktree, RUN_SETUP },
 	{ "multi-pack-index", cmd_multi_pack_index, RUN_SETUP },
-- 
gitgitgadget