// Package guestbook keeps the messages, checks new ones, and saves them to a
// JSON file. It knows nothing about the web.

package guestbook

import "errors"

const MaxNameLength = 40
const MaxTextLength = 500

// Message is one entry. The fields are exported because encoding/json and
// html/template can only see exported fields.
type Message struct {
	Name string
	Text string
}

// NewMessage trims and checks the values. Its error is safe to show a visitor.
func NewMessage(name string, text string) (Message, error) {
	// TODO: trim both values. An empty value or a value that is too long is an
	// error. Otherwise return the message and nil.
	return Message{}, errors.New("TODO: NewMessage is not written yet.")
}

// Book holds the messages. Make one with Open. Its methods may be called by
// many requests at the same time.
type Book struct {
	// TODO: add a sync.Mutex, the file name, and a slice of messages.
}

// Open loads the book from a file. A file that does not exist yet gives an empty
// book. A file that cannot be read or understood is an error.
func Open(filename string) (*Book, error) {
	// TODO: read the file, and treat os.ErrNotExist as an empty book.
	return &Book{}, nil
}

// Messages returns a copy of the messages, oldest first.
func (b *Book) Messages() []Message {
	// TODO: lock, and return a copy of the slice.
	return []Message{}
}

// Add saves a message. If saving fails, the book stays as it was.
func (b *Book) Add(message Message) error {
	// TODO: lock, make the longer slice, save it, and only then keep it.
	return errors.New("TODO: Add is not written yet")
}

// TODO: add a helper that writes the messages to a temporary file as JSON, and
// renames it over the real file.