package agent

import (
	"bytes"
	"errors"
	"fmt"
	"os"
	"os/exec"
	"path/filepath"
	"strings"
)

// ensureDir creates dir (and parents) if it does not already exist.
func ensureDir(dir string) error {
	if dir == "" {
		return nil
	}
	return os.MkdirAll(dir, 0o755)
}

// Worktree is one entry from `git worktree list --porcelain`.
type Worktree struct {
	Path     string
	Head     string
	Branch   string // short branch name ("" when detached or bare)
	Bare     bool
	Detached bool
}

// runGit runs git with args, using dir as the working directory (empty = the
// process cwd). It returns trimmed stdout, or an error that includes stderr so
// failures like "branch already checked out" surface verbatim.
func runGit(dir string, args ...string) (string, error) {
	cmd := exec.Command("git", args...)
	if dir != "" {
		cmd.Dir = dir
	}
	var stdout, stderr bytes.Buffer
	cmd.Stdout = &stdout
	cmd.Stderr = &stderr
	if err := cmd.Run(); err != nil {
		msg := strings.TrimSpace(stderr.String())
		if msg == "" {
			msg = strings.TrimSpace(stdout.String())
		}
		return "", fmt.Errorf("git %s: %w: %s", strings.Join(args, " "), err, msg)
	}
	return strings.TrimSpace(stdout.String()), nil
}

// GitClone clones url into dir. Any globalArgs (e.g. "-c",
// "credential.helper=...") are passed before the clone subcommand so auth can be
// supplied without persisting it in the resulting checkout's config.
func GitClone(url, dir string, globalArgs ...string) error {
	if err := ensureDir(filepath.Dir(dir)); err != nil {
		return err
	}
	args := append(append([]string{}, globalArgs...), "clone", url, dir)
	_, err := runGit(filepath.Dir(dir), args...)
	return err
}

// GitFetch runs `git fetch <remote>` in repoDir. globalArgs are passed before
// the subcommand (used to inject an ephemeral credential helper).
func GitFetch(repoDir, remote string, globalArgs ...string) error {
	args := append(append([]string{}, globalArgs...), "fetch", remote)
	_, err := runGit(repoDir, args...)
	return err
}

// GitFetchPrune runs `git fetch --prune <remote>` in repoDir so remote-tracking
// refs for branches deleted on the remote (e.g. after a merge) disappear.
func GitFetchPrune(repoDir, remote string, globalArgs ...string) error {
	args := append(append([]string{}, globalArgs...), "fetch", "--prune", remote)
	_, err := runGit(repoDir, args...)
	return err
}

// GitRemoteDefaultBranch returns the short name of remote's default branch
// (e.g. "main") by resolving refs/remotes/<remote>/HEAD.
func GitRemoteDefaultBranch(repoDir, remote string) (string, error) {
	out, err := runGit(repoDir, "rev-parse", "--abbrev-ref", remote+"/HEAD")
	if err != nil {
		return "", err
	}
	return strings.TrimPrefix(out, remote+"/"), nil
}

// GitBranchExists reports whether a local branch exists.
func GitBranchExists(repoDir, branch string) bool {
	_, err := runGit(repoDir, "show-ref", "--verify", "--quiet", "refs/heads/"+branch)
	return err == nil
}

// GitRemoteURL returns the configured URL for a remote.
func GitRemoteURL(repoDir, remote string) (string, error) {
	return runGit(repoDir, "remote", "get-url", remote)
}

// GitRemoteBranchExists reports whether a remote-tracking ref for branch exists
// (accurate only after a pruning fetch).
func GitRemoteBranchExists(repoDir, remote, branch string) bool {
	_, err := runGit(repoDir, "show-ref", "--verify", "--quiet", "refs/remotes/"+remote+"/"+branch)
	return err == nil
}

// GitIsDirty reports whether the checkout at dir has uncommitted or untracked
// changes.
func GitIsDirty(dir string) (bool, error) {
	out, err := runGit(dir, "status", "--porcelain")
	if err != nil {
		return false, err
	}
	return strings.TrimSpace(out) != "", nil
}

// GitIsAncestor reports whether ancestor is reachable from descendant.
func GitIsAncestor(repoDir, ancestor, descendant string) (bool, error) {
	cmd := exec.Command("git", "merge-base", "--is-ancestor", ancestor, descendant)
	cmd.Dir = repoDir
	var stderr bytes.Buffer
	cmd.Stderr = &stderr
	if err := cmd.Run(); err != nil {
		// Exit 1 is the documented "not an ancestor" answer; anything else is a
		// real failure (bad ref, not a repo).
		var exitErr *exec.ExitError
		if errors.As(err, &exitErr) && exitErr.ExitCode() == 1 {
			return false, nil
		}
		return false, fmt.Errorf("git merge-base --is-ancestor %s %s: %w: %s",
			ancestor, descendant, err, strings.TrimSpace(stderr.String()))
	}
	return true, nil
}

// GitUnmergedCommits counts commits on head whose patch has no equivalent on
// upstream, using `git cherry` so squash- and rebase-merged work is recognised
// despite its rewritten SHAs.
func GitUnmergedCommits(repoDir, upstream, head string) (int, error) {
	out, err := runGit(repoDir, "cherry", upstream, head)
	if err != nil {
		return 0, err
	}
	n := 0
	for _, line := range strings.Split(out, "\n") {
		if strings.HasPrefix(strings.TrimSpace(line), "+") {
			n++
		}
	}
	return n, nil
}

// GitWorktreeAdd adds a worktree at path checked out to branch. When the branch
// already exists it is reused; otherwise it is created from startPoint.
func GitWorktreeAdd(repoDir, path, branch, startPoint string) error {
	if err := ensureDir(filepath.Dir(path)); err != nil {
		return err
	}
	var args []string
	if GitBranchExists(repoDir, branch) {
		args = []string{"worktree", "add", path, branch}
	} else {
		args = []string{"worktree", "add", path, "-b", branch, startPoint}
	}
	_, err := runGit(repoDir, args...)
	return err
}

// GitWorktreeRemove removes the worktree at path (force skips the dirty check).
func GitWorktreeRemove(repoDir, path string, force bool) error {
	args := []string{"worktree", "remove", path}
	if force {
		args = append(args, "--force")
	}
	_, err := runGit(repoDir, args...)
	return err
}

// GitWorktreePrune prunes stale worktree administrative entries.
func GitWorktreePrune(repoDir string) error {
	_, err := runGit(repoDir, "worktree", "prune")
	return err
}

// GitWorktreeList returns the worktrees registered for repoDir.
func GitWorktreeList(repoDir string) ([]Worktree, error) {
	out, err := runGit(repoDir, "worktree", "list", "--porcelain")
	if err != nil {
		return nil, err
	}
	return ParseWorktreeList(out), nil
}

// GitDeleteBranch deletes a local branch (force uses -D).
func GitDeleteBranch(repoDir, branch string, force bool) error {
	flag := "-d"
	if force {
		flag = "-D"
	}
	_, err := runGit(repoDir, "branch", flag, branch)
	return err
}

// GitConfigSet sets a config key in repoDir. When worktree is true the value is
// written to the per-worktree config (extensions.worktreeConfig must be enabled)
// so it does not touch the shared checkout's config.
func GitConfigSet(repoDir string, worktree bool, key, value string) error {
	args := []string{"config"}
	if worktree {
		args = append(args, "--worktree")
	}
	args = append(args, key, value)
	_, err := runGit(repoDir, args...)
	return err
}

// GitCommonDir returns the absolute path to the shared .git directory for the
// checkout at dir (a worktree's common dir points back at its source repo).
func GitCommonDir(dir string) (string, error) {
	out, err := runGit(dir, "rev-parse", "--path-format=absolute", "--git-common-dir")
	if err != nil {
		return "", err
	}
	return out, nil
}

// GitCurrentBranch returns the short branch name checked out at dir.
func GitCurrentBranch(dir string) (string, error) {
	return runGit(dir, "rev-parse", "--abbrev-ref", "HEAD")
}

// SourceRepoDir maps a worktree checkout to its source repo directory by walking
// from the shared .git common dir up to the repo root.
func SourceRepoDir(worktreeDir string) (string, error) {
	common, err := GitCommonDir(worktreeDir)
	if err != nil {
		return "", err
	}
	// common is ".../<repo>/.git"; the repo dir is its parent.
	return filepath.Dir(common), nil
}

// ParseWorktreeList parses the output of `git worktree list --porcelain`.
func ParseWorktreeList(out string) []Worktree {
	var wts []Worktree
	var cur *Worktree
	flush := func() {
		if cur != nil {
			wts = append(wts, *cur)
			cur = nil
		}
	}
	for _, line := range strings.Split(out, "\n") {
		line = strings.TrimRight(line, "\r")
		if line == "" {
			flush()
			continue
		}
		key, val, _ := strings.Cut(line, " ")
		switch key {
		case "worktree":
			flush()
			cur = &Worktree{Path: val}
		case "HEAD":
			if cur != nil {
				cur.Head = val
			}
		case "branch":
			if cur != nil {
				cur.Branch = strings.TrimPrefix(val, "refs/heads/")
			}
		case "bare":
			if cur != nil {
				cur.Bare = true
			}
		case "detached":
			if cur != nil {
				cur.Detached = true
			}
		}
	}
	flush()
	return wts
}

// SanitizeBranch turns a branch name into a filesystem-safe path segment by
// replacing separators that would otherwise create nested directories.
func SanitizeBranch(branch string) string {
	r := strings.NewReplacer("/", "-", "\\", "-", ":", "-", " ", "-")
	return r.Replace(strings.TrimSpace(branch))
}

// WorktreeDirName is the directory name (under the worktree root) for a repo's
// branch worktree: "<repo>__<sanitized-branch>".
func WorktreeDirName(repo, branch string) string {
	return repo + "__" + SanitizeBranch(branch)
}