package main

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"os"
	"path/filepath"
	"syscall"
	"time"
	"unicode/utf8"
)

// errLocked tells that another process owns the chat.
var errLocked = errors.New("another process owns the chat")

// kinds holds the kinds that a message can have.
var kinds = map[string]bool{
	"user":  true,
	"reply": true,
	"tool":  true,
	"echo":  true,
	"work":  true,
	"note":  true,
}

// Message is one entry of the chat log. ID is the permanent position of the
// message in the chat, from 0. Size is the UTF-8 byte length of Text. Date is
// the time of the write in RFC 3339 format.
type Message struct {
	ID   int    `json:"i"`
	Kind string `json:"kind"`
	Text string `json:"text"`
	Size int    `json:"size"`
	Date string `json:"date"`
}

// Log is the append-only message log of one chat. It writes one JSON line for
// each message to main/YYYY-MM-DD.jsonl, named for the local day of the write.
// It never edits or deletes a line. Each line goes to the operating system
// before Append returns, so a crash of the process loses no message. An
// exclusive lock on the lock file makes sure that only one process writes the
// log. After a failed write, the end of the file is not known, so the log
// refuses all writes until it is opened again. The caller serializes all calls.
type Log struct {
	clock    func() time.Time
	lock     *os.File
	journal  journal
	messages []Message
	failure  error
}

// openLog locks the chat in root and loads all of its messages. The clock gives
// the date of each new message.
func openLog(ctx context.Context, root string, clock func() time.Time) (*Log, error) {
	err := os.MkdirAll(filepath.Join(root, "main"), 0o700)
	if err != nil {
		return nil, fmt.Errorf("create message directory: %w", err)
	}

	lock, err := lockFile(filepath.Join(root, "lock"))
	if err != nil {
		return nil, err
	}

	l := &Log{
		clock: clock,
		lock:  lock,
		journal: journal{
			dir: filepath.Join(root, "main"),
		},
	}

	err = l.load(ctx)
	if err != nil {
		return nil, errors.Join(fmt.Errorf("load message log: %w", err), l.Close())
	}

	return l, nil
}

func (l *Log) load(ctx context.Context) error {
	return readLines(ctx, l.journal.dir, func(message Message) error {
		if message.ID != len(l.messages) {
			return fmt.Errorf("expected message %d, found %d", len(l.messages), message.ID)
		}

		err := validateMessage(message)
		if err != nil {
			return fmt.Errorf("message %d: %w", message.ID, err)
		}

		l.messages = append(l.messages, message)

		return nil
	})
}

// Close releases the lock. The log refuses all calls after Close.
func (l *Log) Close() error {
	err := l.journal.Close()
	if l.lock != nil {
		err = errors.Join(err, l.lock.Close())
		l.lock = nil
	}

	return err
}

// Append writes a new message to the log and returns it with its ID, size, and
// date.
func (l *Log) Append(kind, text string) (Message, error) {
	if l.lock == nil {
		return Message{}, os.ErrClosed
	}

	if l.failure != nil {
		return Message{}, fmt.Errorf("log needs a reopen after a failed write: %w", l.failure)
	}

	now := l.clock()
	message := Message{
		ID:   len(l.messages),
		Kind: kind,
		Text: text,
		Size: len(text),
		Date: now.Format(time.RFC3339Nano),
	}

	err := validateMessage(message)
	if err != nil {
		return Message{}, err
	}

	line, err := json.Marshal(message)
	if err != nil {
		return Message{}, fmt.Errorf("encode message: %w", err)
	}

	err = l.journal.Write(now.Format(time.DateOnly), append(line, '\n'))
	if err != nil {
		l.failure = err
		return Message{}, fmt.Errorf("write message %d: %w", message.ID, err)
	}

	l.messages = append(l.messages, message)

	return message, nil
}

// Len returns the number of messages in the log.
func (l *Log) Len() int {
	return len(l.messages)
}

// Message returns the message with the given ID.
func (l *Log) Message(id int) Message {
	return l.messages[id]
}

// validateMessage makes sure that a message has a known kind, valid UTF-8 text,
// the size of its text, and a valid date.
func validateMessage(message Message) error {
	if !kinds[message.Kind] {
		return fmt.Errorf("unknown message kind %q", message.Kind)
	}

	if !utf8.ValidString(message.Text) {
		return errors.New("message text is not valid UTF-8")
	}

	if message.Size != len(message.Text) {
		return fmt.Errorf("message size %d is not the text size %d", message.Size, len(message.Text))
	}

	_, err := time.Parse(time.RFC3339Nano, message.Date)
	if err != nil {
		return fmt.Errorf("parse message date: %w", err)
	}

	return nil
}

// lockFile opens the file at path and takes an exclusive lock on it. The lock
// stays until the file closes or the process ends.
func lockFile(path string) (*os.File, error) {
	file, err := os.OpenFile(path, os.O_CREATE|os.O_RDWR, 0o600)
	if err != nil {
		return nil, fmt.Errorf("open lock file: %w", err)
	}

	err = syscall.Flock(int(file.Fd()), syscall.LOCK_EX|syscall.LOCK_NB)
	if errors.Is(err, syscall.EWOULDBLOCK) {
		err = errLocked
	}

	if err != nil {
		return nil, errors.Join(fmt.Errorf("lock chat: %w", err), file.Close())
	}

	return file, nil
}