Domain Driven Design and Go: Part 2, Domain Modeling
Nov 13, 2018

## Introduction

Last time we talked about how awesome Lotería is and we defined the concrete rules of the game. What’s next is not a surprise: we are going to translate those rules. We will convert them into something the machine can understand.

Let’s go through each rule and “translate it” to technical terms.

There is a deck with 54 cards,

The deck is represented with an array of 54 `Card`s,

There are N players, each player is given one Tabla (Board),

We have N `Player`s, each of them has one `Board`, the players can not choose those said boards, they are randomly assigned,

Tablas always have a randomly created 4x4 grid of cards,

Given tablas are unique during the duration of the game,

`Board`s are random and unique per game session, meaning no player will be using the same permutation of values on the board of any other board already created,

There’s a Cantor (Announcer), who is in charge of randomly selecting cards from the deck, Players have to listen to the announced cards and mark them on their boards,

Those two requirements are really clear, however we still need a bit more of time to provide a better technical implementation. Let’s move those to the parking lot.

The winner is determined by the first player who shouts “LOTERÍA!” and (more importantly) has four sequential marked cards in a horizontal, vertical or diagonal row.

We need a way to allow the players to shout they won and, obviously, we must validate that the result really matches the rules.

## Iteration #1

Initial iteration will consider the following three, of eight, requirements:

• There is a deck with 54 cards,
• Tablas always have a randomly created 4x4 grid of cards, and
• The winner has four sequential marked cards in a horizontal, vertical or diagonal row.

### There is a deck with 54 cards

Defining a `Card` type should clarify our goal, then we need an array of `Card`s with a length of 54, or even better we can define a new type `Deck` to explicitly indicate that:

``````type (
// Card defines the card, which is part of the board and also announced by
// the caller.
Card uint64

// Deck defines the type containing all the 54 Cards.
Deck [54]Card
)``````

Why is the card `uint64`? Because this type has 64 bits, which in practice can represent up to 64 different cards, so by using this type we can flag each bit to independently indicate a concrete card per bit; we only need 54 but we support up to 64. That being said… why `uint64` instead of `int64`? Well I personally like working with non negative numbers (not that any of this matters at all in this particular case).

Defining the concrete cards is a repetitive exercise, sort of like:

``````const (
RoosterCard Card = iota
DevilCard
DandyCard
// all other cards ...
)``````

### Tablas always have a randomly created 4x4 grid of cards

`Board`s have 16 `Card`s, defining them as a concrete type is the way to go because we are going to be interacting with then heavily during our game, so this makes sense:

``````type (
// Board defines a "tabla", which is 4x4 grid of 16 Cards.
Board struct {
cards map[Card]index
}
)``````

Notice there’s an `index` type, I will describe that below.

What about the randomly created requirement? For that, the best approach would be to create a concrete “constructor”, like so:

``````// NewRandomBoard returns a board with random Cards.
func NewRandomBoard() Board {
r := rand.New(rand.NewSource(time.Now().UnixNano()))

cards := map[Card]index{}
for len(cards) < 16 {
v := r.Intn(53)
if _, ok := cards[Card(v)]; !ok {
cards[Card(v)] = index(1) << uint16(len(cards))
}
}

return Board{cards: cards}
}``````

That function above randomizes 16 numbers between 0 and 53 and assigns them to the internal map. One really important thing to notice in this function is the value of `index` in the map, it represents the location of this card in the (to be added) `marked` attribute. Please continue reading.

### The winner has four sequential marked cards

This means we require having a way to mark cards on our `Board`:

``````// Mark marks off the card on the board.
func (b *Board) Mark(c Card) error {
index, ok := b.cards[c]
if !ok {
return ErrCardNotOnBoard
}

b.marked |= index

return nil
}``````

The method above introduces the field `marked` which was mentioned above, this attribute is used to keep track of the marked cards, using bit flagging. This same field, `marked`, is also used to determine if the `Board` is a winning one:

``````// Winner indicates whether the marked cards win the game.
func (b *Board) Winner() bool {
for _, pattern := range defaultWinningPatterns {
if (uint16(b.marked) & uint16(pattern)) == uint16(pattern) {
b.WinningPattern = pattern
return true
}
}
return false
}``````

Where those `defaultWinningPatterns` are predefined Winning Patterns we computed in advance.

It looks like we made significant progress!! Let’s take a break for now, digest everything and come back tomorrow.

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