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