main.go

package main

type mesa struct {
	height int
	width  int
}

type position struct {
	x         int
	y         int
	direction string
}

type rover struct {
	p position
}

func (r *rover) Move(m mesa) {
	switch r.p.direction {
	case "N":
		if r.p.y+1 <= m.height {
			r.p.y += 1
		}
	case "S":
		if r.p.y-1 >= 0 {
			r.p.y -= 1
		}
	case "E":
		if r.p.x+1 <= m.width {
			r.p.x += 1
		}
	case "W":
		if r.p.x-1 >= 0 {
			r.p.x -= 1
		}
	default:
		break
	}
}

main_test.go

package main

import (
	"testing"
)

func TestForward(t *testing.T) {
	m := mesa{10, 10}
	atBottom := rover{position{5, 0, "S"}}
	atBottom.Move(m)

	if atBottom.p.y < 0 {
		t.Error("Fell off the bottom")
	}

	atTop := rover{position{10, 0, "N"}}
	atTop.Move(m)

	if atTop.p.y > 10 {
		t.Error("Fell of the top")
	}

	inMiddleE := rover{position{5, 5, "E"}}
	inMiddleE.Move(m)

	if inMiddleE.p.x != 6 {
		t.Error("Why didn't you move East?")
	}

	inMiddleW := rover{position{5, 5, "W"}}
	inMiddleW.Move(m)

	if inMiddleW.p.x != 4 {
		t.Error("Why didn't you move West?  You're at", inMiddleW.p.x, inMiddleW.p.y)
	}
}

func TestRepeated(t *testing.T) {
	m := mesa{10, 10}
	inMiddleE := rover{position{5, 5, "E"}}

	moveTimes := func(r rover, times int) rover {
		for i := 0; i < times; i++ {
			r.Move(m)
		}

		return r
	}

	r2 := moveTimes(inMiddleE, 3)
	if r2.p.x != 8 {
		t.Error("Why didn't you move 3 right?", r2.p.x)
	}

	if inMiddleE.p.x != 5 {
		t.Error("We mutated")
	}
}