Add unit tests for Unit method of XY

It was untested by any existing unit tests (and is not used internally
anywhere). We don't want to remove it though, since it's part of the
public API. So I added some unit tests instead.

geom/util_test.go

@@ -2,6 +2,7 @@ package geom_test
 
 import (
 	"bytes"
+	"math"
 	"testing"
 
 	. "github.com/peterstace/simplefeatures/geom"
@@ -86,6 +87,12 @@ func expectXYEq(t *testing.T, got, want XY) {
 		t.Errorf("\ngot:  %v\nwant: %v\n", got, want)
 	}
 }
+func expectXYWithinTolerance(t *testing.T, got, want XY, tolerance float64) {
+	t.Helper()
+	if delta := math.Abs(got.Sub(want).Length()); delta > tolerance {
+		t.Errorf("\ngot:  %v\nwant: %v\n", got, want)
+	}
+}
 
 func expectCoordinatesTypeEq(t *testing.T, got, want CoordinatesType) {
 	t.Helper()

geom/xy.go

@@ -75,11 +75,6 @@ func (w XY) Less(o XY) bool {
 	return w.Y < o.Y
 }
 
-func (w XY) distanceTo(o XY) float64 {
-	delta := o.Sub(w)
-	return math.Sqrt(delta.Dot(delta))
-}
-
 func (w XY) distanceSquaredTo(o XY) float64 {
 	delta := o.Sub(w)
 	return delta.Dot(delta)

geom/xy_test.go

@@ -0,0 +1,54 @@
+package geom_test
+
+import (
+	"math"
+	"testing"
+
+	"github.com/peterstace/simplefeatures/geom"
+)
+
+func TestXYUnit(t *testing.T) {
+	sqrt2 := math.Sqrt(2)
+	sqrt5 := math.Sqrt(5)
+	for _, tc := range []struct {
+		description string
+		input       geom.XY
+		output      geom.XY
+	}{
+		{
+			description: "+ve unit in X",
+			input:       geom.XY{X: 1},
+			output:      geom.XY{X: 1},
+		},
+		{
+			description: "+ve unit in Y",
+			input:       geom.XY{Y: 1},
+			output:      geom.XY{Y: 1},
+		},
+		{
+			description: "-ve unit in X",
+			input:       geom.XY{X: -1},
+			output:      geom.XY{X: -1},
+		},
+		{
+			description: "-ve unit in Y",
+			input:       geom.XY{Y: -1},
+			output:      geom.XY{Y: -1},
+		},
+		{
+			description: "non-aligned unit",
+			input:       geom.XY{X: -1 / sqrt2, Y: 1 / sqrt2},
+			output:      geom.XY{X: -1 / sqrt2, Y: 1 / sqrt2},
+		},
+		{
+			description: "non-unit",
+			input:       geom.XY{X: 1, Y: -2},
+			output:      geom.XY{X: 1 / sqrt5, Y: -2 / sqrt5},
+		},
+	} {
+		t.Run(tc.description, func(t *testing.T) {
+			got := tc.input.Unit()
+			expectXYWithinTolerance(t, got, tc.output, 0.0000001)
+		})
+	}
+}