Cribbage

Goal

Be the first to move your peg all the way to the end of the board: 121 points. The game stops as soon as this total is reached, even in the middle of a hand.

The deal and the crib

• Each player receives six cards and discards two, face down: these four cards form the crib, an extra hand that belongs to the dealer.
• The dealer’s opponent cuts the deck; the dealer turns up the starter card. If it is a jack, the dealer scores 2 points: “his heels”.
• The role of dealer alternates every hand.

The play (up to 31)

• Taking turns, starting with the dealer’s opponent, each player lays down a card and announces the running total, never going over 31. Face cards = 10, ace = 1.
• Fifteen: 2 points. Thirty-one: 2 points.
• Pair (same rank as the previous card): 2; pair royal (three of a kind): 6; double pair royal (four of a kind): 12.
• Run: the last cards played form a sequence of 3 or more, in any order: 1 point per card.
• If you cannot play without going over 31, you say “Go”. Your opponent carries on alone; the last player to lay a card scores 1 point (unless they made 31). The count then starts again from zero.
• The very last card played also scores 1 point.

The show

The dealer’s opponent counts their hand first (an advantage late in the game), then the dealer counts their hand, then their crib. Each four-card hand is counted together with the starter card:

• Fifteens: every combination of cards adding up to 15 scores 2 points.
• Pairs: 2 points per pair (three of a kind makes three pairs, so 6; four of a kind, 12).
• Runs: 1 point per card, each distinct combination counting separately (4-5-5-6 contains two runs of three).
• Flush: four cards of the hand in the same suit, 4 points (5 if the starter matches). For the crib, only a five-card flush counts.
• “Nobs”: the jack of the same suit as the starter card, 1 point.

The best possible hand is worth 29: three 5s and the jack, with the fourth 5 of the jack’s suit as the starter card.

The board

Each player has two pegs: the rear one leaps over the front one by the number of points scored. That way you can always see your latest gain. If the loser has not passed 91 points, they are skunked; under 61, double skunked.

Controls

• Tap two cards then Put in the crib (Enter). Tap a playable card during the play, or use keys 1 to 6.
• The count is shown in detail on screen, combination by combination.

About the AI

For its discard, the AI evaluates the fifteen ways of keeping four cards: the exact expected value of its hand is calculated over the 46 possible starter cards, and the value of the crib comes from an expectation table computed by full enumeration. During the play, it draws random plausible opposing hands from the cards it has not seen (up to 200 at the Master level) and, for each one, solves the rest of the play exactly by minimax, then plays the card with the best average outcome. It never sees your cards or those in the crib.