Five Card Draw for Gophers

kevburnsjr profile image Kevin Burns ・4 min read

So, I'm on this airplane.


It's an 8 hour flight with an entertainment system. Before starting a movie, I check out the games section and come across this poker game.


It's five card draw. One player. In Spanish. Buy in is $200 fake dollars and my balance after 30 minutes is $1,585. Ya, this is not a hard game.

In fact, this game is too easy. Annoyingly easy. Like it's letting me win. Is it possible that this game is actually random and I'm just some genius poker god? Or is the game feeding me cards to let me win? This question bothers me. It begs to be answered.

Hey, wait a minute. I'm a software engineer. I can just clone the game and fiddle with the internals to determine why this implementation seems so unbalanced.

Let's do it!

Here's the game: http://stupid-poker.kevburnsjr.com


It starts with a deck

package poker

type deck [52]string

var suits = []string{"S", "H", "D", "C"}
var cards = []string{"A", "2", "3", "4", "5", "6", "7", "8", "9", "T", "J", "Q", "K"}

func newDeck() *deck {
    d := &deck{}
    for i, s := range suits {
        for j, c := range cards {
            d[i*13+j] = c + s
    return d

An ordered deck isn't very useful, so let's make it shuffle.

// shuffles the deck in place
func (d *deck) shuffle() {
    for i := len(d) - 1; i > 0; i-- {
        j := rand.Intn(i + 1)
        d[i], d[j] = d[j], d[i]

Now that we have a deck, let's create the game

const (
    stateReady = iota

type Game interface {
    // Deal initializes the game, returning the player's initial hand and new balance
    Deal() (Hand, int, error)

    // Exchange accepts a slice of integers corresponding to indicies for cards in the given hand
    // It discards those cards from the hand and replaces them with new cards from the deck
    // It returns the final hand and the new balance
    Exchange([]int) (Hand, int)

The game only has 2 states. Either you are dealing a hand, or you are drawing cards and the hand is over.

type game struct {
    deck    *deck
    state   int
    anty    int
    balance int
    hand    Hand
    mutex   *sync.Mutex

func NewGame(anty int, balance int) Game {
    return &game{
        deck:    newDeck(),
        state:   stateReady,
        anty:    anty,
        balance: balance,
        mutex:   &sync.Mutex{},

func (g *game) Deal() (Hand, int, error) {
    defer g.mutex.Unlock()
    if g.balance < g.anty {
        return Hand{}, g.balance, ErrNoBalance
    g.balance -= g.anty
    g.hand = newHand(g.deck[0:5])
    g.state = stateDealt
    return g.hand, g.balance, nil

func (g *game) Exchange(cards []int) (Hand, int) {
    defer g.mutex.Unlock()
    if g.state != stateDealt {
        return g.hand, g.balance
    hand := g.hand
    for i, n := range cards {
        if n >= len(hand) {
        hand[n] = g.deck[i+5]
    res := hand.Score()
    g.balance += payout[res]
    if payout[res] > 0 {
        g.balance += g.anty
    g.state = stateReady
    return hand, g.balance

The hand is also an array of strings.

const (
    Hand_RoyalFlush    = "Royal Flush"
    Hand_StraightFlush = "Straight Flush"
    Hand_Quads         = "Four of a Kind"
    Hand_FullHouse     = "Full House"
    Hand_Flush         = "Flush"
    Hand_Straight      = "Straight"
    Hand_Trips         = "Three of a Kind"
    Hand_TwoPair       = "Two Pair"
    Hand_Jacks         = "Jacks or Higher"
    Hand_Nothing       = "Nothing"

type Hand [5]string

// Score scores the hand
func (h Hand) Score() string {
    if len(h[0]) < 1 {
        return Hand_Nothing
    if h.isFlush() {
        if h.isRoyal() {
            return Hand_RoyalFlush
        if h.isStraight() {
            return Hand_StraightFlush
        return Hand_Flush
    if h.isStraight() {
        return Hand_Straight
    pairs, trips, quads, highPair := h.detectDupes()
    if quads > 0 {
        return Hand_Quads
    if trips > 0 {
        if pairs > 0 {
            return Hand_FullHouse
        return Hand_Trips
    if pairs > 1 {
        return Hand_TwoPair
    if pairs == 1 && highPair {
        return Hand_Jacks
    return Hand_Nothing

We'll need some tests to ensure the hand scoring method continues to work as expected.

// Table based test to ensure correct scoring of hands
func TestScoreHand(t *testing.T) {
    var tests = map[string]string{
        "AS KS QS JS TS": Hand_RoyalFlush,
        "JD TD AD QD KD": Hand_RoyalFlush,
        "KS QS JS TS 9S": Hand_StraightFlush,
        "KS KH KD KC 9S": Hand_Quads,
        "KS 9S KH KD KC": Hand_Quads,
        "KS 9S KH KD 9C": Hand_FullHouse,
        "TS 8S QS JS 4S": Hand_Flush,
        "AS KD QS JS TS": Hand_Straight,
        "AS AH AD JS TS": Hand_Trips,
        "AS AH JS AD 4S": Hand_Trips,
        "AS AH JS JD 4S": Hand_TwoPair,
        "AS JD 4S AH JS": Hand_TwoPair,
        "AS JD AH JS 4S": Hand_TwoPair,
        "AS AD 8H JS 4S": Hand_Jacks,
        "JS JD 8H 9S 4S": Hand_Jacks,
        "TS TD 8H 9S 4S": Hand_Nothing,
    for c, exp := range tests {
        h := newHand(strings.Split(c, " "))
        res := h.Score()
        if exp != res {
            t.Log("Unexpected Result", h, res, "!=", exp)

After that we just slap an interface on it and call it a game :)

You can find the full code on Github

So... The end result?

It's still pretty easy to win. That probably means the balance issue has nothing to do with the random number generator and has more to do with the payouts being too high in relation to the the odds.

We had fun and we built a game. It's not likely to earn us the Turing Award.

Posted on by:

kevburnsjr profile

Kevin Burns


A former victim of PHP. Presently I do all my stuff in Go. Looking for new job opportunities in the SF Bay Area.


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