tpl/math: Add log function

It might be very useful for building tag clouds.

tpl/math/math.go

@@ -15,7 +15,10 @@ package math
 
 import (
 	"errors"
+	"math"
 	"reflect"
+
+	"github.com/spf13/cast"
 )
 
 // New returns a new instance of the math-namespaced template functions.
@@ -34,6 +37,16 @@ func (ns *Namespace) Div(a, b interface{}) (interface{}, error) {
 	return DoArithmetic(a, b, '/')
 }
 
+func (ns *Namespace) Log(a interface{}) (float64, error) {
+	af, err := cast.ToFloat64E(a)
+
+	if err != nil {
+		return 0, errors.New("Log operator can't be used with non integer or float value")
+	}
+
+	return math.Log(af), nil
+}
+
 // Mod returns a % b.
 func (ns *Namespace) Mod(a, b interface{}) (int64, error) {
 	av := reflect.ValueOf(a)