// Copyright 2011 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package main
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import (
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"fmt"
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"math/rand"
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)
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const (
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win = 100 // The winning score in a game of Pig
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gamesPerSeries = 10 // The number of games per series to simulate
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)
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// A score includes scores accumulated in previous turns for each player,
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// as well as the points scored by the current player in this turn.
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type score struct {
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player, opponent, thisTurn int
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}
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// An action transitions stochastically to a resulting score.
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type action func(current score) (result score, turnIsOver bool)
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// roll returns the (result, turnIsOver) outcome of simulating a die roll.
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// If the roll value is 1, then thisTurn score is abandoned, and the players'
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// roles swap. Otherwise, the roll value is added to thisTurn.
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func roll(s score) (score, bool) {
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outcome := rand.Intn(6) + 1 // A random int in [1, 6]
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if outcome == 1 {
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return score{s.opponent, s.player, 0}, true
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}
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return score{s.player, s.opponent, outcome + s.thisTurn}, false
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}
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// stay returns the (result, turnIsOver) outcome of staying.
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// thisTurn score is added to the player's score, and the players' roles swap.
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func stay(s score) (score, bool) {
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return score{s.opponent, s.player + s.thisTurn, 0}, true
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}
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// A strategy chooses an action for any given score.
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type strategy func(score) action
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// stayAtK returns a strategy that rolls until thisTurn is at least k, then stays.
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func stayAtK(k int) strategy {
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return func(s score) action {
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if s.thisTurn >= k {
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return stay
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}
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return roll
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}
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}
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// play simulates a Pig game and returns the winner (0 or 1).
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func play(strategy0, strategy1 strategy) int {
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strategies := []strategy{strategy0, strategy1}
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var s score
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var turnIsOver bool
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currentPlayer := rand.Intn(2) // Randomly decide who plays first
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for s.player+s.thisTurn < win {
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action := strategies[currentPlayer](s)
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s, turnIsOver = action(s)
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if turnIsOver {
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currentPlayer = (currentPlayer + 1) % 2
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}
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}
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return currentPlayer
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}
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// roundRobin simulates a series of games between every pair of strategies.
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func roundRobin(strategies []strategy) ([]int, int) {
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wins := make([]int, len(strategies))
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for i := 0; i < len(strategies); i++ {
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for j := i + 1; j < len(strategies); j++ {
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for k := 0; k < gamesPerSeries; k++ {
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winner := play(strategies[i], strategies[j])
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if winner == 0 {
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wins[i]++
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} else {
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wins[j]++
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}
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}
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}
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}
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gamesPerStrategy := gamesPerSeries * (len(strategies) - 1) // no self play
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return wins, gamesPerStrategy
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}
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// ratioString takes a list of integer values and returns a string that lists
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// each value and its percentage of the sum of all values.
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// e.g., ratios(1, 2, 3) = "1/6 (16.7%), 2/6 (33.3%), 3/6 (50.0%)"
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func ratioString(vals ...int) string {
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total := 0
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for _, val := range vals {
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total += val
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}
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s := ""
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for _, val := range vals {
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if s != "" {
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s += ", "
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}
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pct := 100 * float64(val) / float64(total)
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s += fmt.Sprintf("%d/%d (%0.1f%%)", val, total, pct)
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}
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return s
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}
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func main() {
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strategies := make([]strategy, win)
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for k := range strategies {
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strategies[k] = stayAtK(k + 1)
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}
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wins, games := roundRobin(strategies)
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for k := range strategies {
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fmt.Printf("Wins, losses staying at k =% 4d: %s\n",
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k+1, ratioString(wins[k], games-wins[k]))
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}
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}
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