package git

import (
	"errors"
	"fmt"
	"os"
	"path/filepath"
	"strings"
	"time"
)

// ErrMergeConflict is returned by Pull when the merge stopped on
// conflicts. The merge is left in progress so the caller can resolve
// per file (ours/theirs), commit, or abort.
var ErrMergeConflict = errors.New("MERGE_CONFLICT")

// ErrUnrelatedHistories is returned by Pull when local and remote do
// not share any history. The caller should offer a reset from remote.
var ErrUnrelatedHistories = errors.New("UNRELATED_HISTORIES")

// SetSSHKey configures the key and known_hosts file used for all
// remote operations. Host keys are accepted on first contact
// (accept-new) and pinned in knownHosts after that.
func (r *Repo) SetSSHKey(keyPath, knownHostsPath string) {
	r.mu.Lock()
	defer r.mu.Unlock()
	r.sshCommand = fmt.Sprintf(
		"ssh -i %q -o UserKnownHostsFile=%q -o StrictHostKeyChecking=accept-new -o IdentitiesOnly=yes",
		keyPath, knownHostsPath,
	)
}

// SetRemote points origin at url, creating the remote if needed.
func (r *Repo) SetRemote(url string) error {
	r.mu.Lock()
	defer r.mu.Unlock()
	if _, err := r.run("remote", "get-url", "origin"); err != nil {
		_, err = r.run("remote", "add", "origin", url)
		return err
	}
	_, err := r.run("remote", "set-url", "origin", url)
	return err
}

// Fetch updates all remote-tracking refs.
func (r *Repo) Fetch() error {
	r.mu.Lock()
	defer r.mu.Unlock()
	_, err := r.run("fetch", "origin", "--prune")
	return err
}

// CurrentBranch returns the checked-out branch name. It works on an
// unborn branch (fresh init) too, where rev-parse would fail.
func (r *Repo) CurrentBranch() (string, error) {
	r.mu.Lock()
	defer r.mu.Unlock()
	out, err := r.run("symbolic-ref", "--short", "HEAD")
	if err != nil {
		// Detached HEAD.
		out, err = r.run("rev-parse", "--short", "HEAD")
		if err != nil {
			return "", err
		}
	}
	return strings.TrimSpace(out), nil
}

// Branches lists local branch names plus remote branches that have no
// local counterpart yet (reported as "origin/<name>").
func (r *Repo) Branches() (local []string, remote []string, err error) {
	r.mu.Lock()
	defer r.mu.Unlock()
	local, remote = []string{}, []string{} // non-nil so they serialise as []
	out, err := r.run("for-each-ref", "--format=%(refname:short)", "refs/heads")
	if err != nil {
		return local, remote, err
	}
	for _, l := range strings.Split(strings.TrimSpace(out), "\n") {
		if l != "" {
			local = append(local, l)
		}
	}
	out, err = r.run("for-each-ref", "--format=%(refname:short)", "refs/remotes/origin")
	if err != nil {
		return local, remote, nil // no remote yet is fine
	}
	for _, l := range strings.Split(strings.TrimSpace(out), "\n") {
		name := strings.TrimPrefix(l, "origin/")
		if name == "" || name == "HEAD" || l == "origin" {
			continue
		}
		remote = append(remote, name)
	}
	return local, remote, nil
}

// CreateBranch creates a branch at the current HEAD and switches to it.
func (r *Repo) CreateBranch(name string) error {
	r.mu.Lock()
	defer r.mu.Unlock()
	_, err := r.run("checkout", "-b", name)
	return err
}

// SwitchBranch checks out an existing branch. A branch that only
// exists on the remote gets a local tracking branch.
func (r *Repo) SwitchBranch(name string) error {
	r.mu.Lock()
	defer r.mu.Unlock()
	if _, err := r.run("rev-parse", "--verify", "refs/heads/"+name); err == nil {
		_, err = r.run("checkout", name)
		return err
	}
	if _, err := r.run("rev-parse", "--verify", "refs/remotes/origin/"+name); err == nil {
		_, err = r.run("checkout", "-b", name, "--track", "origin/"+name)
		return err
	}
	return fmt.Errorf("branch %q not found locally or on origin", name)
}

// AheadBehind reports how many commits branch is ahead of and behind
// origin/branch. hasUpstream is false when origin/branch is missing.
func (r *Repo) AheadBehind(branch string) (ahead, behind int, hasUpstream bool) {
	r.mu.Lock()
	defer r.mu.Unlock()
	out, err := r.run("rev-list", "--left-right", "--count",
		branch+"...origin/"+branch)
	if err != nil {
		return 0, 0, false
	}
	fmt.Sscanf(strings.TrimSpace(out), "%d\t%d", &ahead, &behind)
	return ahead, behind, true
}

// Pull merges origin/branch into the current branch. ffOnly restricts
// the merge to fast-forwards (used by background sync so it never
// creates surprise merges). On conflict the merge is left open and
// ErrMergeConflict is returned.
func (r *Repo) Pull(branch, author, email string, ffOnly bool) error {
	r.mu.Lock()
	defer r.mu.Unlock()
	args := []string{
		"-c", fmt.Sprintf("user.name=%s", author),
		"-c", fmt.Sprintf("user.email=%s", email),
		"merge", "--no-edit",
	}
	if ffOnly {
		args = append(args, "--ff-only")
	}
	args = append(args, "origin/"+branch)
	_, err := r.run(args...)
	if err == nil {
		return nil
	}
	if strings.Contains(err.Error(), "unrelated histories") {
		return ErrUnrelatedHistories
	}
	if files := r.conflictFiles(); len(files) > 0 {
		return ErrMergeConflict
	}
	return err
}

// Push publishes branch to origin and sets it as upstream.
func (r *Repo) Push(branch string) error {
	r.mu.Lock()
	defer r.mu.Unlock()
	_, err := r.run("push", "-u", "origin", branch)
	return err
}

// MergeInProgress reports whether a merge has been started but not
// yet committed or aborted.
func (r *Repo) MergeInProgress() bool {
	_, err := os.Stat(filepath.Join(r.root, ".git", "MERGE_HEAD"))
	return err == nil
}

// ConflictFiles lists paths that are in conflicted state.
func (r *Repo) ConflictFiles() []string {
	r.mu.Lock()
	defer r.mu.Unlock()
	return r.conflictFiles()
}

func (r *Repo) conflictFiles() []string {
	files := []string{} // non-nil so it serialises as [] not null
	out, err := r.run("diff", "--name-only", "--diff-filter=U")
	if err != nil {
		return files
	}
	for _, l := range strings.Split(strings.TrimSpace(out), "\n") {
		if l != "" {
			files = append(files, l)
		}
	}
	return files
}

// ResolveConflicts settles conflicted paths by taking "ours" (local)
// or "theirs" (remote). Empty paths means every conflicted file. When
// no conflicts remain the merge is committed.
func (r *Repo) ResolveConflicts(strategy string, paths []string, author, email string) error {
	if strategy != "ours" && strategy != "theirs" {
		return fmt.Errorf("unknown strategy %q (want ours or theirs)", strategy)
	}
	r.mu.Lock()
	defer r.mu.Unlock()
	if len(paths) == 0 {
		paths = r.conflictFiles()
	}
	for _, p := range paths {
		if _, err := r.run("checkout", "--"+strategy, "--", p); err != nil {
			// Delete/modify conflicts have no blob on one side; taking
			// the missing side means removing the file.
			if _, rmErr := r.run("rm", "--", p); rmErr != nil {
				return err
			}
			continue
		}
		if _, err := r.run("add", "--", p); err != nil {
			return err
		}
	}
	if len(r.conflictFiles()) > 0 {
		return nil // partial resolve; merge stays open
	}
	_, err := r.run(
		"-c", fmt.Sprintf("user.name=%s", author),
		"-c", fmt.Sprintf("user.email=%s", email),
		"commit", "--no-edit",
	)
	return err
}

// AbortMerge throws away an in-progress merge.
func (r *Repo) AbortMerge() error {
	r.mu.Lock()
	defer r.mu.Unlock()
	_, err := r.run("merge", "--abort")
	return err
}

// ResetToRemote force-matches branch to origin/branch. Any local-only
// state is kept in a backup branch first, so the operation is
// recoverable. Returns the backup branch name ("" when the repo had
// no commits to back up).
func (r *Repo) ResetToRemote(branch, author, email string) (backup string, err error) {
	r.mu.Lock()
	defer r.mu.Unlock()
	if r.MergeInProgress() {
		if _, err := r.run("merge", "--abort"); err != nil {
			return "", err
		}
	}
	// Snapshot uncommitted edits so the backup branch is complete.
	if out, _ := r.run("status", "--porcelain"); strings.TrimSpace(out) != "" {
		r.run("add", "-A")
		r.run(
			"-c", fmt.Sprintf("user.name=%s", author),
			"-c", fmt.Sprintf("user.email=%s", email),
			"commit", "-m", "Snapshot före reset från remote",
		)
	}
	if _, err := r.run("rev-parse", "--verify", "HEAD"); err == nil {
		backup = "backup/" + time.Now().Format("20060102-150405")
		if _, err := r.run("branch", backup); err != nil {
			return "", err
		}
	}
	if _, err := r.run("fetch", "origin", "--prune"); err != nil {
		return backup, err
	}
	if _, err := r.run("checkout", "-B", branch, "origin/"+branch); err != nil {
		return backup, err
	}
	_, err = r.run("branch", "--set-upstream-to", "origin/"+branch, branch)
	return backup, err
}