// Package todos reads and writes the Markdown todo files used by Pi's todo extension.
package todos

import (
	"bytes"
	"crypto/rand"
	"encoding/hex"
	"encoding/json"
	"errors"
	"fmt"
	"os"
	"path/filepath"
	"regexp"
	"strings"
	"time"
)

var idPattern = regexp.MustCompile(`^[a-f0-9]{8}$`)

// ErrLocked means another session owns the todo's .lock file. We never steal locks.
var ErrLocked = errors.New("todo is locked by another session")

// Todo matches the pi extension's JSON header and optional Markdown body.
type Todo struct {
	ID                string   `json:"id"`
	Title             string   `json:"title"`
	Tags              []string `json:"tags"`
	Status            string   `json:"status"`
	CreatedAt         string   `json:"created_at"`
	AssignedToSession string   `json:"assigned_to_session,omitempty"`
	Body              string   `json:"-"`
}

// Dir returns Pi's default todo directory for a project.
func Dir(project string) string { return filepath.Join(project, ".pi", "todos") }

func validateID(id string) error {
	if !idPattern.MatchString(id) {
		return fmt.Errorf("invalid todo ID %q: expected 8 lowercase hex digits", id)
	}
	return nil
}

// Parse reads a JSON object followed by an optional blank line and Markdown body.
// The filename is authoritative for the ID, as in the Pi extension.
func Parse(id string, data []byte) (Todo, error) {
	if err := validateID(id); err != nil {
		return Todo{}, err
	}
	if len(data) == 0 || data[0] != '{' {
		return Todo{}, fmt.Errorf("todo %s: missing JSON header", id)
	}
	var header struct {
		ID                string   `json:"id"`
		Title             string   `json:"title"`
		Tags              []string `json:"tags"`
		Status            string   `json:"status"`
		CreatedAt         string   `json:"created_at"`
		AssignedToSession string   `json:"assigned_to_session"`
	}
	decoder := json.NewDecoder(bytes.NewReader(data))
	if err := decoder.Decode(&header); err != nil {
		return Todo{}, fmt.Errorf("todo %s: invalid JSON header: %w", id, err)
	}
	if header.ID != "" && header.ID != id {
		return Todo{}, fmt.Errorf("todo %s: header ID %q differs from filename", id, header.ID)
	}
	rest := data[decoder.InputOffset():]
	if len(rest) > 0 && rest[0] != '\n' && !bytes.HasPrefix(rest, []byte("\r\n")) {
		return Todo{}, fmt.Errorf("todo %s: expected newline after JSON header", id)
	}
	body := strings.TrimLeft(string(rest), "\r\n")
	if header.Status == "" {
		header.Status = "open"
	}
	if header.Tags == nil {
		header.Tags = []string{}
	}
	return Todo{ID: id, Title: header.Title, Tags: header.Tags, Status: header.Status,
		CreatedAt: header.CreatedAt, AssignedToSession: header.AssignedToSession, Body: body}, nil
}

// Serialize emits the extension's JSON header and a blank line before any body.
func Serialize(todo Todo) ([]byte, error) {
	if err := validateID(todo.ID); err != nil {
		return nil, err
	}
	if todo.Tags == nil {
		todo.Tags = []string{}
	}
	if todo.Status == "" {
		todo.Status = "open"
	}
	header, err := json.MarshalIndent(todo, "", "  ")
	if err != nil {
		return nil, err
	}
	body := strings.TrimRight(strings.TrimLeft(todo.Body, "\r\n"), " \t\r\n")
	if body == "" {
		return append(header, '\n'), nil
	}
	return []byte(string(header) + "\n\n" + body + "\n"), nil
}

// Read loads one todo. It does not modify or acquire a lock on the file.
func Read(dir, id string) (Todo, error) {
	if err := validateID(id); err != nil {
		return Todo{}, err
	}
	data, err := os.ReadFile(filepath.Join(dir, id+".md"))
	if err != nil {
		return Todo{}, fmt.Errorf("read todo %s: %w", id, err)
	}
	return Parse(id, data)
}

// List reads todos in filename order. Malformed files are reported, not overwritten.
func List(dir string) ([]Todo, error) {
	entries, err := os.ReadDir(dir)
	if errors.Is(err, os.ErrNotExist) {
		return []Todo{}, nil
	}
	if err != nil {
		return nil, fmt.Errorf("read todo directory: %w", err)
	}
	result := make([]Todo, 0)
	for _, entry := range entries {
		if entry.IsDir() || !strings.HasSuffix(entry.Name(), ".md") {
			continue
		}
		id := strings.TrimSuffix(entry.Name(), ".md")
		if err := validateID(id); err != nil {
			continue // unrelated Markdown file
		}
		todo, err := Read(dir, id)
		if err != nil {
			return nil, err
		}
		result = append(result, todo)
	}
	return result, nil
}

func withLock(dir, id string, fn func() error) error {
	lockPath := filepath.Join(dir, id+".lock")
	lock, err := os.OpenFile(lockPath, os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0600)
	if errors.Is(err, os.ErrExist) {
		return fmt.Errorf("todo %s: %w", id, ErrLocked)
	}
	if err != nil {
		return fmt.Errorf("lock todo %s: %w", id, err)
	}
	// Include the fields Pi's extension expects, without claiming a Pi session.
	info, _ := json.Marshal(map[string]any{"id": id, "pid": os.Getpid(), "created_at": time.Now().UTC().Format(time.RFC3339Nano)})
	if _, err := lock.Write(info); err != nil {
		lock.Close()
		os.Remove(lockPath)
		return fmt.Errorf("write lock for todo %s: %w", id, err)
	}
	lock.Close()
	defer os.Remove(lockPath)
	return fn()
}

func writeTemp(dir string, data []byte, perm os.FileMode) (string, error) {
	temp, err := os.CreateTemp(dir, ".todoapp-*")
	if err != nil {
		return "", err
	}
	name := temp.Name()
	defer func() {
		temp.Close()
		if err != nil {
			os.Remove(name)
		}
	}()
	if err = temp.Chmod(perm); err != nil {
		return "", err
	}
	if _, err = temp.Write(data); err != nil {
		return "", err
	}
	if err = temp.Sync(); err != nil {
		return "", err
	}
	if err = temp.Close(); err != nil {
		return "", err
	}
	return name, nil
}

// Create allocates a random Pi-compatible ID and never replaces an existing file.
func Create(dir string, todo Todo) (Todo, error) {
	if err := os.MkdirAll(dir, 0700); err != nil {
		return Todo{}, fmt.Errorf("create todo directory: %w", err)
	}
	for range 10 {
		var random [4]byte
		if _, err := rand.Read(random[:]); err != nil {
			return Todo{}, err
		}
		todo.ID = hex.EncodeToString(random[:])
		todo.CreatedAt = time.Now().UTC().Format("2006-01-02T15:04:05.000Z")
		if todo.Status == "" {
			todo.Status = "open"
		}
		data, err := Serialize(todo)
		if err != nil {
			return Todo{}, err
		}
		err = withLock(dir, todo.ID, func() error {
			name, err := writeTemp(dir, data, 0600)
			if err != nil {
				return err
			}
			defer os.Remove(name)
			return os.Link(name, filepath.Join(dir, todo.ID+".md")) // atomic, no overwrite
		})
		if errors.Is(err, os.ErrExist) || errors.Is(err, ErrLocked) {
			continue // rare ID collision
		}
		if err != nil {
			return Todo{}, fmt.Errorf("create todo %s: %w", todo.ID, err)
		}
		return todo, nil
	}
	return Todo{}, errors.New("failed to allocate an unused todo ID")
}

// Update edits a todo under a Pi-compatible lock and atomically replaces the file.
// ID, creation time, and assignment always come from the existing file.
func Update(dir string, todo Todo) error {
	if err := validateID(todo.ID); err != nil {
		return err
	}
	return withLock(dir, todo.ID, func() error {
		old, err := Read(dir, todo.ID)
		if err != nil {
			return err
		}
		todo.CreatedAt = old.CreatedAt
		todo.AssignedToSession = old.AssignedToSession
		if todo.Status == "" {
			todo.Status = old.Status
		}
		if todo.Tags == nil {
			todo.Tags = old.Tags
		}
		data, err := Serialize(todo)
		if err != nil {
			return err
		}
		path := filepath.Join(dir, todo.ID+".md")
		info, err := os.Stat(path)
		if err != nil {
			return err
		}
		name, err := writeTemp(dir, data, info.Mode().Perm())
		if err != nil {
			return err
		}
		defer os.Remove(name)
		if err := os.Rename(name, path); err != nil {
			return fmt.Errorf("replace todo %s: %w", todo.ID, err)
		}
		return nil
	})
}