doc: add Game of Life to playground toys

R=golang-dev, dsymonds, r
CC=golang-dev
https://golang.org/cl/9961043

doc/play/life.go

+// An implementation of Conway's Game of Life.
+package main
+
+import (
+	"bytes"
+	"fmt"
+	"math/rand"
+	"time"
+)
+
+// Field represents a two-dimensional field of cells.
+type Field struct {
+	s    [][]bool
+	w, h int
+}
+
+// NewField returns an empty field of the specified width and height.
+func NewField(w, h int) *Field {
+	s := make([][]bool, h)
+	for i := range s {
+		s[i] = make([]bool, w)
+	}
+	return &Field{s: s, w: w, h: h}
+}
+
+// Set sets the state of the specified cell to the given value.
+func (f *Field) Set(x, y int, b bool) {
+	f.s[y][x] = b
+}
+
+// Alive returns whether the specified cell is alive.
+// If the x or y coordinates are outside the field boundaries they are wrapped
+// toroidally. For instance, an x value of -1 is treated as width-1.
+func (f *Field) Alive(x, y int) bool {
+	x += f.w
+	x %= f.w
+	y += f.h
+	y %= f.h
+	return f.s[y][x]
+}
+
+// Next returns the state of the specified cell at the next time step.
+func (f *Field) Next(x, y int) bool {
+	// Count the adjacent cells that are alive.
+	alive := 0
+	for i := -1; i <= 1; i++ {
+		for j := -1; j <= 1; j++ {
+			if (j != 0 || i != 0) && f.Alive(x+i, y+j) {
+				alive++
+			}
+		}
+	}
+	// Return next state according to the game rules:
+	//   exactly 3 neighbors: on,
+	//   exactly 2 neighbors: maintain current state,
+	//   otherwise: off.
+	return alive == 3 || alive == 2 && f.Alive(x, y)
+}
+
+// Life stores the state of a round of Conway's Game of Life.
+type Life struct {
+	a, b *Field
+	w, h int
+}
+
+// NewLife returns a new Life game state with a random initial state.
+func NewLife(w, h int) *Life {
+	a := NewField(w, h)
+	for i := 0; i < (w * h / 4); i++ {
+		a.Set(rand.Intn(w), rand.Intn(h), true)
+	}
+	return &Life{
+		a: a, b: NewField(w, h),
+		w: w, h: h,
+	}
+}
+
+// Step advances the game by one instant, recomputing and updating all cells.
+func (l *Life) Step() {
+	// Update the state of the next field (b) from the current field (a).
+	for y := 0; y < l.h; y++ {
+		for x := 0; x < l.w; x++ {
+			l.b.Set(x, y, l.a.Next(x, y))
+		}
+	}
+	// Swap fields a and b.
+	l.a, l.b = l.b, l.a
+}
+
+// String returns the game board as a string.
+func (l *Life) String() string {
+	var buf bytes.Buffer
+	for y := 0; y < l.h; y++ {
+		for x := 0; x < l.w; x++ {
+			b := byte(' ')
+			if l.a.Alive(x, y) {
+				b = '*'
+			}
+			buf.WriteByte(b)
+		}
+		buf.WriteByte('\n')
+	}
+	return buf.String()
+}
+
+func main() {
+	l := NewLife(40, 15)
+	for i := 0; i < 300; i++ {
+		l.Step()
+		fmt.Print("\x0c", l) // Clear screen and print field.
+		time.Sleep(time.Second / 30)
+	}
+}

doc/root.html

@@ -31,6 +31,7 @@ Hello, 世界
 <div class="toys">
 <select>
 	<option value="hello.go">Hello, World!</option>
+	<option value="life.go">Conway's Game of Life</option>
 	<option value="fib.go">Fibonacci Closure</option>
 	<option value="peano.go">Peano Integers</option>
 	<option value="pi.go">Concurrent pi</option>