//! Git-toplevel probe for the spawn-form worktree section (#25).
//!
//! When the user picks a working directory, the spawn form wants to offer a
//! "git worktree" toggle that needs only a branch name — the `base_repo` is
//! *inferred* from the chosen dir rather than typed. This module answers that
//! one question: "what git repository, if any, encloses this path?"
//!
//! [`find_root`] walks up from a path to the nearest ancestor containing a
//! `.git` entry and returns that ancestor (the repo toplevel). A regular `.git`
//! directory and a `.git` *file* (the linked-worktree / submodule form) both
//! count, so a dir that is itself inside a checked-out worktree still resolves
//! to its worktree toplevel.
//!
//! It's a pure filesystem walk with no `git` subprocess — same shape as
//! [`crate::dirlist`]: unit-tested in isolation, and the HTTP handler /
//! HTTP layer (#26) just wraps its `Option` for the wire.

use std::path::{Path, PathBuf};

/// Walk up from `start` to the nearest ancestor that holds a `.git` entry and
/// return that ancestor — the enclosing repository's toplevel. Returns `None`
/// when no ancestor is a git repo.
///
/// Both a `.git` directory (ordinary checkout) and a `.git` file (linked
/// worktree or submodule, which stores a `gitdir:` pointer) count as a repo
/// marker, matching what `git rev-parse --show-toplevel` would resolve. `start`
/// itself is checked first, so passing a repo root returns that root.
///
/// The input is used as-is: callers that accept abbreviated client paths
/// should resolve them (e.g. via [`crate::dirlist`]'s home-dir expansion)
/// before probing, so an empty or `~` path doesn't walk up from the process
/// working directory.
pub fn find_root(start: &Path) -> Option<PathBuf> {
    // `ancestors()` yields `start`, its parent, grandparent, … up to the root,
    // so a repo toplevel is matched at the first hit (closest enclosing repo).
    start
        .ancestors()
        .find(|dir| is_git_root(dir))
        .map(Path::to_path_buf)
}

/// Whether `dir` holds a `.git` entry — directory or file. A bare `exists()`
/// check is deliberate: we don't open or parse it, only detect the marker.
fn is_git_root(dir: &Path) -> bool {
    dir.join(".git").exists()
}

#[cfg(test)]
mod tests {
    use super::*;
    use std::fs;

    /// A temp repo: `<root>/.git/` plus a nested `a/b/c` subtree. Returns the
    /// temp dir (its `Drop` cleans up) so callers keep it alive for the test.
    fn repo_fixture() -> tempfile::TempDir {
        let tmp = tempfile::tempdir().unwrap();
        let root = tmp.path();
        fs::create_dir(root.join(".git")).unwrap();
        fs::create_dir_all(root.join("a/b/c")).unwrap();
        tmp
    }

    #[test]
    fn dir_inside_repo_resolves_to_toplevel() {
        let tmp = repo_fixture();
        // The repo root itself resolves to the repo root.
        assert_eq!(find_root(tmp.path()).as_deref(), Some(tmp.path()));
        // An immediate subdir resolves to the toplevel, not itself.
        assert_eq!(
            find_root(&tmp.path().join("a")).as_deref(),
            Some(tmp.path())
        );
    }

    #[test]
    fn nested_subdir_resolves_to_toplevel() {
        let tmp = repo_fixture();
        let deep = tmp.path().join("a/b/c");
        assert_eq!(find_root(&deep).as_deref(), Some(tmp.path()));
    }

    #[test]
    fn non_repo_dir_is_none() {
        // A temp dir with no `.git` anywhere up the chain. Use the temp root so
        // the walk can't accidentally hit a real repo above the test's cwd.
        let tmp = tempfile::tempdir().unwrap();
        let sub = tmp.path().join("plain/dir");
        fs::create_dir_all(&sub).unwrap();
        assert_eq!(find_root(&sub), None);
    }

    #[test]
    fn git_file_worktree_marker_counts_as_root() {
        // Linked worktrees and submodules store `.git` as a *file* holding a
        // `gitdir:` pointer rather than a directory. It must still resolve.
        let tmp = tempfile::tempdir().unwrap();
        let root = tmp.path();
        fs::write(root.join(".git"), b"gitdir: /somewhere/.git/worktrees/wt\n").unwrap();
        let sub = root.join("src");
        fs::create_dir(&sub).unwrap();
        assert_eq!(find_root(&sub).as_deref(), Some(root));
    }

    #[test]
    fn nearest_enclosing_repo_wins() {
        // An inner repo nested inside an outer repo resolves to the *inner*
        // toplevel — the walk stops at the first `.git` it meets going up.
        let tmp = tempfile::tempdir().unwrap();
        let outer = tmp.path();
        fs::create_dir(outer.join(".git")).unwrap();
        let inner = outer.join("vendored");
        fs::create_dir(&inner).unwrap();
        fs::create_dir(inner.join(".git")).unwrap();
        let inner_sub = inner.join("src");
        fs::create_dir(&inner_sub).unwrap();
        assert_eq!(find_root(&inner_sub).as_deref(), Some(inner.as_path()));
    }
}