go-by-test/clockface/clockface.go

package clockface

import (
	"fmt"
	"io"
	"math"
	"time"
)

// Point represents a two-dimentional Cartesian coordinate
type Point struct {
	X float64
	Y float64
}

const (
	secondHandLength = 90
	clockCentreX     = 150
	clockCentreY     = 150
)

const svgStart = `<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<svg xmlns="http://www.w3.org/2000/svg"
     width="100%"
     height="100%"
     viewBox="0 0 300 300"
     version="2.0">`

const bezel = `<circle cx="150" cy="150" r="100" style="fill:#fff;stroke:#000;stroke-width:5px;"/>`

const svgEnd = `</svg>`

func SVGWriter(w io.Writer, t time.Time) {
	io.WriteString(w, svgStart)
	io.WriteString(w, bezel)
	SecondHand(w, t)
	io.WriteString(w, svgEnd)
}

// SecondHand is the unit vector of the second hand of an analogue clock at the time `t` represented as a Point
func SecondHand(w io.Writer, t time.Time) {
	p := secondHandPoint(t)
	p = Point{p.X * secondHandLength, p.Y * secondHandLength} // scale
	p = Point{p.X, -p.Y}                                      // flip
	p = Point{p.X + clockCentreX, p.Y + clockCentreY}         // translate
	fmt.Fprintf(
		w,
		`<line x1="150" y1="150" x2="%f" y2="%f" style="fill:none;stroke:#f00;stroke-width:3px;"/>`,
		p.X,
		p.Y,
	)
}

func secondsInRadians(t time.Time) float64 {
	// XXX:Wanted 3.141592653589793 radians, but got 3.1415926535897936
	// return float64(t.Second()) * (math.Pi / 30)

	return math.Pi / (30 / float64(t.Second()))
}

func secondHandPoint(t time.Time) Point {
	angle := secondsInRadians(t)
	x := math.Sin(angle)
	y := math.Cos(angle)

	// XXX: Wanted {0 -1} Point, but got {1.2246467991473515e-16 -1}
	return Point{x, y}
}