thodg/libgit2

diff --git a/include/git2/worktree.h b/include/git2/worktree.h
index c6ca30b..4b045ee 100644
--- a/include/git2/worktree.h
+++ b/include/git2/worktree.h
@@ -61,6 +61,21 @@ GIT_EXTERN(void) git_worktree_free(git_worktree *wt);
  */
 GIT_EXTERN(int) git_worktree_validate(const git_worktree *wt);
 
+/**
+ * Add a new working tree
+ *
+ * Add a new working tree for the repository, that is create the
+ * required data structures inside the repository and check out
+ * the current HEAD at `path`
+ *
+ * @param out Output pointer containing new working tree
+ * @param repo Repository to create working tree for
+ * @param name Name of the working tree
+ * @param path Path to create working tree at
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_worktree_add(git_worktree **out, git_repository *repo, const char *name, const char *path);
+
 /** @} */
 GIT_END_DECL
 #endif
diff --git a/src/worktree.c b/src/worktree.c
index 2852c18..3c6cfee 100644
--- a/src/worktree.c
+++ b/src/worktree.c
@@ -5,9 +5,12 @@
  * a Linking Exception. For full terms see the included COPYING file.
  */
 
+#include "common.h"
+
+#include "git2/branch.h"
+#include "git2/commit.h"
 #include "git2/worktree.h"
 
-#include "common.h"
 #include "repository.h"
 #include "worktree.h"
 
@@ -90,6 +93,25 @@ err:
 	return NULL;
 }
 
+static int write_wtfile(const char *base, const char *file, const git_buf *buf)
+{
+	git_buf path = GIT_BUF_INIT;
+	int err;
+
+	assert(base && file && buf);
+
+	if ((err = git_buf_joinpath(&path, base, file)) < 0)
+		goto out;
+
+	if ((err = git_futils_writebuffer(buf, path.ptr, O_CREAT|O_EXCL|O_WRONLY, 0644)) < 0)
+		goto out;
+
+out:
+	git_buf_free(&path);
+
+	return err;
+}
+
 int git_worktree_lookup(git_worktree **out, git_repository *repo, const char *name)
 {
 	git_buf path = GIT_BUF_INIT;
@@ -183,3 +205,81 @@ out:
 
 	return err;
 }
+
+int git_worktree_add(git_worktree **out, git_repository *repo, const char *name, const char *worktree)
+{
+	git_buf path = GIT_BUF_INIT, buf = GIT_BUF_INIT;
+	git_reference *ref = NULL, *head = NULL;
+	git_commit *commit = NULL;
+	git_repository *wt = NULL;
+	git_checkout_options coopts = GIT_CHECKOUT_OPTIONS_INIT;
+	int err;
+
+	assert(out && repo && name && worktree);
+
+	*out = NULL;
+
+	/* Create worktree related files in commondir */
+	if ((err = git_buf_joinpath(&path, repo->commondir, "worktrees")) < 0)
+		goto out;
+	if (!git_path_exists(path.ptr))
+		if ((err = git_futils_mkdir(path.ptr, 0755, GIT_MKDIR_EXCL)) < 0)
+			goto out;
+	if ((err = git_buf_joinpath(&path, path.ptr, name)) < 0)
+		goto out;
+	if ((err = git_futils_mkdir(path.ptr, 0755, GIT_MKDIR_EXCL)) < 0)
+		goto out;
+
+	/* Create worktree work dir */
+	if ((err = git_futils_mkdir(worktree, 0755, GIT_MKDIR_EXCL)) < 0)
+		goto out;
+
+	/* Create worktree .git file */
+	if ((err = git_buf_printf(&buf, "gitdir: %s\n", path.ptr)) < 0)
+		goto out;
+	if ((err = write_wtfile(worktree, ".git", &buf)) < 0)
+		goto out;
+
+	/* Create commondir files */
+	if ((err = git_buf_sets(&buf, repo->commondir)) < 0
+	    || (err = git_buf_putc(&buf, '\n')) < 0
+	    || (err = write_wtfile(path.ptr, "commondir", &buf)) < 0)
+		goto out;
+	if ((err = git_buf_joinpath(&buf, worktree, ".git")) < 0
+	    || (err = git_buf_putc(&buf, '\n')) < 0
+	    || (err = write_wtfile(path.ptr, "gitdir", &buf)) < 0)
+		goto out;
+
+	/* Create new branch */
+	if ((err = git_repository_head(&head, repo)) < 0)
+		goto out;
+	if ((err = git_commit_lookup(&commit, repo, &head->target.oid)) < 0)
+		goto out;
+	if ((err = git_branch_create(&ref, repo, name, commit, false)) < 0)
+		goto out;
+
+	/* Set worktree's HEAD */
+	if ((err = git_repository_create_head(path.ptr, name)) < 0)
+		goto out;
+	if ((err = git_repository_open(&wt, worktree)) < 0)
+		goto out;
+
+	/* Checkout worktree's HEAD */
+	coopts.checkout_strategy = GIT_CHECKOUT_FORCE;
+	if ((err = git_checkout_head(wt, &coopts)) < 0)
+		goto out;
+
+	/* Load result */
+	if ((err = git_worktree_lookup(out, repo, name)) < 0)
+		goto out;
+
+out:
+	git_buf_free(&path);
+	git_buf_free(&buf);
+	git_reference_free(ref);
+	git_reference_free(head);
+	git_commit_free(commit);
+	git_repository_free(wt);
+
+	return err;
+}
diff --git a/tests/worktree/worktree.c b/tests/worktree/worktree.c
index 7e9cd25..8154baa 100644
--- a/tests/worktree/worktree.c
+++ b/tests/worktree/worktree.c
@@ -204,6 +204,89 @@ void test_worktree_worktree__open_invalid_parent(void)
 	git_worktree_free(wt);
 }
 
+void test_worktree_worktree__init(void)
+{
+	git_worktree *wt;
+	git_repository *repo;
+	git_reference *branch;
+	git_buf path = GIT_BUF_INIT;
+
+	cl_git_pass(git_buf_joinpath(&path, fixture.repo->workdir, "../worktree-new"));
+	cl_git_pass(git_worktree_add(&wt, fixture.repo, "worktree-new", path.ptr));
+
+	/* Open and verify created repo */
+	cl_git_pass(git_repository_open(&repo, path.ptr));
+	cl_git_pass(git_branch_lookup(&branch, repo, "worktree-new", GIT_BRANCH_LOCAL));
+
+	git_buf_free(&path);
+	git_worktree_free(wt);
+	git_reference_free(branch);
+	git_repository_free(repo);
+}
+
+void test_worktree_worktree__init_existing_branch(void)
+{
+	git_reference *head, *branch;
+	git_commit *commit;
+	git_worktree *wt;
+	git_buf path = GIT_BUF_INIT;
+
+	cl_git_pass(git_repository_head(&head, fixture.repo));
+	cl_git_pass(git_commit_lookup(&commit, fixture.repo, &head->target.oid));
+	cl_git_pass(git_branch_create(&branch, fixture.repo, "worktree-new", commit, false));
+
+	cl_git_pass(git_buf_joinpath(&path, fixture.repo->workdir, "../worktree-new"));
+	cl_git_fail(git_worktree_add(&wt, fixture.repo, "worktree-new", path.ptr));
+
+	git_buf_free(&path);
+	git_commit_free(commit);
+	git_reference_free(head);
+	git_reference_free(branch);
+}
+
+void test_worktree_worktree__init_existing_worktree(void)
+{
+	git_worktree *wt;
+	git_buf path = GIT_BUF_INIT;
+
+	cl_git_pass(git_buf_joinpath(&path, fixture.repo->workdir, "../worktree-new"));
+	cl_git_fail(git_worktree_add(&wt, fixture.repo, "testrepo-worktree", path.ptr));
+
+	cl_git_pass(git_worktree_lookup(&wt, fixture.repo, "testrepo-worktree"));
+	cl_assert_equal_s(wt->gitlink_path, fixture.worktree->path_gitlink);
+
+	git_buf_free(&path);
+	git_worktree_free(wt);
+}
+
+void test_worktree_worktree__init_existing_path(void)
+{
+	const char *wtfiles[] = { "HEAD", "commondir", "gitdir", "index" };
+	git_worktree *wt;
+	git_buf path = GIT_BUF_INIT;
+	unsigned i;
+
+	/* Delete files to verify they have not been created by
+	 * the init call */
+	for (i = 0; i < ARRAY_SIZE(wtfiles); i++) {
+		cl_git_pass(git_buf_joinpath(&path,
+			    fixture.worktree->path_repository, wtfiles[i]));
+		cl_git_pass(p_unlink(path.ptr));
+	}
+
+	cl_git_pass(git_buf_joinpath(&path, fixture.repo->workdir, "../testrepo-worktree"));
+	cl_git_fail(git_worktree_add(&wt, fixture.repo, "worktree-new", path.ptr));
+
+	/* Verify files have not been re-created */
+	for (i = 0; i < ARRAY_SIZE(wtfiles); i++) {
+		cl_git_pass(git_buf_joinpath(&path,
+			    fixture.worktree->path_repository, wtfiles[i]));
+		cl_assert(!git_path_exists(path.ptr));
+	}
+
+	git_buf_free(&path);
+}
+
 void test_worktree_worktree__validate(void)
 {
 	git_worktree *wt;