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Diffstat (limited to 'doc/codewalk')
| -rw-r--r-- | doc/codewalk/functions.xml | 115 | ||||
| -rw-r--r-- | doc/codewalk/pig.go | 124 |
2 files changed, 239 insertions, 0 deletions
diff --git a/doc/codewalk/functions.xml b/doc/codewalk/functions.xml new file mode 100644 index 000000000..986a017e1 --- /dev/null +++ b/doc/codewalk/functions.xml @@ -0,0 +1,115 @@ +<codewalk title="First-Class Functions in Go"> + +<step title="Introduction" src="doc/codewalk/pig.go"> + Go supports first class functions, higher-order functions, user-defined + function types, function literals, closures, and multiple return values. + <br/><br/> + + This rich feature set supports a functional programming style in a strongly + typed language. + <br/><br/> + + In this codewalk we will look at a simple program that simulates a dice game + called <a href="http://en.wikipedia.org/wiki/Pig_(dice)">Pig</a> and evaluates + basic strategies. +</step> + +<step title="Game overview" src="doc/codewalk/pig.go:/\/\/ A score/,/thisTurn int\n}/"> + Pig is a two-player game played with a 6-sided die. Each turn, you may roll or stay. + <ul> + <li> If you roll a 1, you lose all points for your turn and play passes to + your opponent. Any other roll adds its value to your turn score. </li> + <li> If you stay, your turn score is added to your total score, and play passes + to your opponent. </li> + </ul> + + The first person to reach 100 total points wins. + <br/><br/> + + The <code>score</code> type stores the scores of the current and opposing + players, in addition to the points accumulated during the current turn. +</step> + +<step title="User-defined function types" src="doc/codewalk/pig.go:/\/\/ An action/,/bool\)/"> + In Go, functions can be passed around just like any other value. A function's + type signature describes the types of its arguments and return values. + <br/><br/> + + The <code>action</code> type is a function that takes a <code>score</code> + and returns the resulting <code>score</code> and whether the current turn is + over. + <br/><br/> + + If the turn is over, the <code>player</code> and <code>opponent</code> fields + in the resulting <code>score</code> should be swapped, as it is now the other player's + turn. +</step> + +<step title="Multiple return values" src="doc/codewalk/pig.go:/\/\/ roll returns/,/stay.*true\n}/"> + Go functions can return multiple values. + <br/><br/> + + The functions <code>roll</code> and <code>stay</code> each return a pair of + values. They also match the <code>action</code> type signature. These + <code>action</code> functions define the rules of Pig. +</step> + +<step title="Higher-order functions" src="doc/codewalk/pig.go:/\/\/ A strategy/,/action\n/"> + A function can use other functions as arguments and return values. + <br/><br/> + + A <code>strategy</code> is a function that takes a <code>score</code> as input + and returns an <code>action</code> to perform. <br/> + (Remember, an <code>action</code> is itself a function.) +</step> + +<step title="Function literals and closures" src="doc/codewalk/pig.go:/return func/,/return roll\n\t}/"> + Anonymous functions can be declared in Go, as in this example. Function + literals are closures: they inherit the scope of the function in which they + are declared. + <br/><br/> + + One basic strategy in Pig is to continue rolling until you have accumulated at + least k points in a turn, and then stay. The argument <code>k</code> is + enclosed by this function literal, which matches the <code>strategy</code> type + signature. +</step> + +<step title="Simulating games" src="doc/codewalk/pig.go:/\/\/ play/,/currentPlayer\n}/"> + We simulate a game of Pig by calling an <code>action</code> to update the + <code>score</code> until one player reaches 100 points. Each + <code>action</code> is selected by calling the <code>strategy</code> function + associated with the current player. +</step> + +<step title="Comparing functions" src="doc/codewalk/pig.go:/if action/,/currentPlayer\)\)\n\t\t}/"> + Functions can be compared for equality in Go. From the + <a href="http://golang.org/doc/go_spec.html#Comparison_operators">language specification</a>: + Function values are equal if they refer to the same function or if both are <code>nil</code>. + <br/><br/> + + We enforce that a <code>strategy</code> function can only return a legal + <code>action</code>: either <code>roll</code> or <code>stay</code>. +</step> + +<step title="Simulating a tournament" src="doc/codewalk/pig.go:/\/\/ roundRobin/,/gamesPerStrategy\n}/"> + The <code>roundRobin</code> function simulates a tournament and tallies wins. + Each strategy plays each other strategy <code>gamesPerSeries</code> times. +</step> + +<step title="Variadic function declarations" src="doc/codewalk/pig.go:/\/\/ ratioS/,/string {/"> + Variadic functions like <code>ratioString</code> take a variable number of + arguments. These arguments are available as a slice inside the function. +</step> + +<step title="Simulation results" src="doc/codewalk/pig.go:/func main/,/\n}/"> + The <code>main</code> function defines 100 basic strategies, simulates a round + robin tournament, and then prints the win/loss record of each strategy. + <br/><br/> + + Among these strategies, staying at 25 is best, but the <a + href="http://www.google.com/search?q=optimal+play+pig">optimal strategy for + Pig</a> is much more complex. +</step> + +</codewalk> diff --git a/doc/codewalk/pig.go b/doc/codewalk/pig.go new file mode 100644 index 000000000..9e415f589 --- /dev/null +++ b/doc/codewalk/pig.go @@ -0,0 +1,124 @@ +// 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" + "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) + if action != roll && action != stay { + panic(fmt.Sprintf("Player %d is cheating", currentPlayer)) + } + 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])) + } +} |