// Package scoreboard keeps players and their scores, enforces the rules, and
// saves everything to a JSON file. It knows nothing about the web.

package scoreboard

import "errors"

const MaxNameLength = 30
const MaxChange = 1000
const MaxScore = 1000000

// ErrUnknownPlayer means that no player has the name asked for.
var ErrUnknownPlayer = errors.New("There is no player with that name.")

// ErrNotSaved means that the file could not be written. It is the server's
// problem and not the visitor's, and the board stays as it was.
var ErrNotSaved = errors.New("could not save")

// Player is one row of the board. The fields are exported because encoding/json
// and html/template can only see exported fields.
type Player struct {
	Name  string
	Score int
}

// Board holds the players. Make one with Open. Its methods may be called by many
// requests at the same time. Rules that must stay true: names are unique and not
// empty, and every score is from 0 to MaxScore.
type Board struct {
	// TODO: add a sync.Mutex, the file name, and a slice of players.
}

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

// TODO: add byScore here: the highest score first, and equal scores in order of
// name. It is a named function, for slices.SortFunc.

// Players returns a sorted copy of the players.
func (b *Board) Players() []Player {
	// TODO: lock, copy the slice, sort the copy with byScore, and return it.
	return []Player{}
}

// AddPlayer adds a player with a score of 0.
func (b *Board) AddPlayer(name string) error {
	// TODO: trim and check the name, lock, refuse a duplicate, and save.
	return errors.New("TODO: AddPlayer is not written yet.")
}

// AddPoints changes a player's score by points, which may be negative.
func (b *Board) AddPoints(name string, points int) error {
	// TODO: check points, lock, find the player or return ErrUnknownPlayer,
	// refuse a score outside the rules, and save.
	return errors.New("TODO: AddPoints is not written yet.")
}

// TODO: add a helper that saves a slice, and returns an error containing
// ErrNotSaved if that fails, and a helper that writes the JSON file.