Filling in `Accumulator` implementation and test client

as well as fixing the Canvas implementation so that it doesn't pop up an
X window on init.

algs4/src/go/Makefile

@@ -8,7 +8,9 @@ TARGETS := \
 	$(REPO_BASE)/algs4-randomseq \
 	$(REPO_BASE)/algs4-average \
 	$(REPO_BASE)/algs4-interval2d \
-	$(REPO_BASE)/algs4-date
+	$(REPO_BASE)/algs4-date \
+	$(REPO_BASE)/algs4-test-accumulator
+
 
 test: build
 	go test -x $(TARGETS)

algs4/src/go/algs4-test-accumulator/main.go

@@ -0,0 +1,34 @@
+package main
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"strconv"
+
+	"github.com/meatballhat/box-o-sand/algs4/src/go/algs4"
+)
+
+func die(err error) {
+	fmt.Fprintf(os.Stderr, "ERROR: %v\n", err)
+	os.Exit(1)
+}
+
+func main() {
+	if len(os.Args) < 2 {
+		die(errors.New("We need an int!"))
+	}
+
+	nValues, err := strconv.Atoi(os.Args[1])
+	if err != nil {
+		die(err)
+	}
+
+	a := algs4.NewAccumulator()
+	for t := 0; t < nValues; t++ {
+		a.AddDataValue(algs4.Random())
+	}
+
+	fmt.Println(a)
+	return
+}

algs4/src/go/algs4/accumulator.go

@@ -0,0 +1,28 @@
+package algs4
+
+import "fmt"
+
+type Accumulator struct {
+	total float64
+	n     int
+}
+
+func NewAccumulator() *Accumulator {
+	return &Accumulator{
+		float64(0.0),
+		0,
+	}
+}
+
+func (me *Accumulator) AddDataValue(val float64) {
+	me.n++
+	me.total += val
+}
+
+func (me *Accumulator) Mean() float64 {
+	return me.total / float64(me.n)
+}
+
+func (me *Accumulator) String() string {
+	return fmt.Sprintf("Mean (%d values): %7.5f", me.n, me.Mean())
+}

algs4/src/go/algs4/canvas.go

@@ -41,11 +41,6 @@ type Canvas struct {
 }
 
 func NewCanvas(height, width int, xmin, xmax, ymin, ymax float64) (*Canvas, error) {
-	window, err := x11.NewWindow()
-	if err != nil {
-		return nil, err
-	}
-
 	canvas := &Canvas{
 		Height: height,
 		Width:  width,
@@ -55,12 +50,27 @@ func NewCanvas(height, width int, xmin, xmax, ymin, ymax float64) (*Canvas, erro
 		Ymax:   ymax,
 
 		img:    image.NewRGBA(image.Rect(0, 0, height, width)),
-		window: window,
+		window: nil,
 	}
 
 	return canvas, nil
 }
 
+func (me *Canvas) ensureHasWindow() error {
+	if me.window != nil {
+		return nil
+	}
+
+	window, err := x11.NewWindow()
+	if err != nil {
+		return err
+	}
+
+	me.window = window
+
+	return nil
+}
+
 func (me *Canvas) Image() *image.RGBA {
 	return me.img
 }
@@ -98,14 +108,26 @@ func (me *Canvas) Rectangle(x, y, halfWidth, halfHeight float64) error {
 	return nil
 }
 
-func (me *Canvas) Pixel(x, y float64) {
+func (me *Canvas) Pixel(x, y float64) error {
+	err := me.ensureHasWindow()
+	if err != nil {
+		return err
+	}
+
 	originX, originY := int(me.scaleX(x)), int(me.scaleY(y))
 	draw.Draw(me.img, image.Rect(originX, originY, originX+1, originY+1),
 		&image.Uniform{image.White}, image.ZP, draw.Src)
 	me.window.FlushImage()
+
+	return nil
 }
 
 func (me *Canvas) Draw() error {
+	err := me.ensureHasWindow()
+	if err != nil {
+		return err
+	}
+
 	draw.Draw(me.window.Screen(), me.window.Screen().Bounds(), me.img, image.ZP, draw.Src)
 	me.window.FlushImage()
 

algs4/src/go/algs4/random.go

@@ -12,3 +12,7 @@ func init() {
 func RandomUniform(min, max float64) float64 {
 	return rand.Float64()*(max-min) + min
 }
+
+func Random() float64 {
+	return rand.Float64()
+}