add Cosine Distance

metrics/pairwise/cosine.go

@@ -0,0 +1,34 @@
+package pairwise
+
+import (
+	"math"
+
+	"github.com/gonum/matrix/mat64"
+)
+
+type Cosine struct{}
+
+func NewCosine() *Cosine {
+	return &Cosine{}
+}
+
+// Dot computes dot value of vectorX and vectorY.
+func (c *Cosine) Dot(vectorX *mat64.Dense, vectorY *mat64.Dense) float64 {
+	subVector := mat64.NewDense(0, 0, nil)
+	subVector.MulElem(vectorX, vectorY)
+	result := mat64.Sum(subVector)
+
+	return result
+}
+
+// Distance computes Cosine distance.
+// It will return distance which represented as 1-cos() (ranged from 0 to 2).
+func (c *Cosine) Distance(vectorX *mat64.Dense, vectorY *mat64.Dense) float64 {
+        dotXY := c.Dot(vectorX, vectorY)
+        lengthX := math.Sqrt(c.Dot(vectorX, vectorX))
+        lengthY := math.Sqrt(c.Dot(vectorY, vectorY))
+
+        cos := dotXY / (lengthX*lengthY)
+
+	return 1-cos
+}

metrics/pairwise/cosine_test.go

@@ -0,0 +1,36 @@
+package pairwise
+
+import (
+	"testing"
+
+	"github.com/gonum/matrix/mat64"
+	. "github.com/smartystreets/goconvey/convey"
+)
+
+func TestCosine(t *testing.T) {
+	var vectorX, vectorY *mat64.Dense
+	cosine := NewCosine()
+
+	Convey("Given two vectors", t, func() {
+		vectorX = mat64.NewDense(3, 1, []float64{1, 2, 3})
+		vectorY = mat64.NewDense(3, 1, []float64{2, 4, 6})
+
+		Convey("When doing inner Dot", func() {
+			result := cosine.Dot(vectorX, vectorY)
+
+			Convey("The result should be 25", func() {
+				So(result, ShouldEqual, 28)
+			})
+		})
+
+		Convey("When calculating distance", func() {
+			result := cosine.Distance(vectorX, vectorY)
+
+			Convey("The result should be 0", func() {
+				So(result, ShouldEqual, 0)
+			})
+
+		})
+
+	})
+}