Added NewFromRat implementation.

decimal.go

@@ -158,6 +158,18 @@ func NewFromFloatWithExponent(value float64, exp int32) Decimal {
 	}
 }
 
+// NewFromRat returns a decimal from a big.Rat representation.
+func NewFromRat(value *big.Rat) Decimal {
+	// fast path, where Rat is an int
+	if value.IsInt() {
+		f, _ := value.Float64()
+		return New(int64(math.Floor(f)), 0)
+	}
+
+	floatValue, _ := value.Float64()
+	return NewFromFloat(floatValue)
+}
+
 // rescale returns a rescaled version of the decimal. Returned
 // decimal may be less precise if the given exponent is bigger
 // than the initial exponent of the Decimal.

decimal_test.go

@@ -2,6 +2,7 @@ package decimal
 
 import (
 	"math"
+	"math/big"
 	"testing"
 )
 
@@ -59,6 +60,26 @@ func TestNewFromFloat(t *testing.T) {
 	}
 }
 
+func TestNewFromRat(t *testing.T) {
+	// add negatives
+	for f, s := range testTable {
+		if f > 0 {
+			testTable[-f] = "-" + s
+		}
+	}
+
+	for f, s := range testTable {
+		r := big.NewRat(0, 1)
+		r.SetFloat64(f)
+		d := NewFromRat(r)
+		if d.String() != s {
+			t.Errorf("expected %s, got %s (%s, %d)",
+				s, d.String(),
+				d.value.String(), d.exp)
+		}
+	}
+}
+
 func TestNewFromString(t *testing.T) {
 	// add negatives
 	for f, s := range testTable {