Improve accuracy of numeric to float

fixes #27

numeric.go

@@ -291,19 +291,16 @@ func (dst *Numeric) toBigInt() (*big.Int, error) {
 }
 
 func (src *Numeric) toFloat64() (float64, error) {
-	f, err := strconv.ParseFloat(src.Int.String(), 64)
+	buf := make([]byte, 0, 32)
+
+	buf = append(buf, src.Int.String()...)
+	buf = append(buf, 'e')
+	buf = append(buf, strconv.FormatInt(int64(src.Exp), 10)...)
+
+	f, err := strconv.ParseFloat(string(buf), 64)
 	if err != nil {
 		return 0, err
 	}
-	if src.Exp > 0 {
-		for i := 0; i < int(src.Exp); i++ {
-			f *= 10
-		}
-	} else if src.Exp < 0 {
-		for i := 0; i > int(src.Exp); i-- {
-			f /= 10
-		}
-	}
 	return f, nil
 }
 

numeric_test.go

@@ -266,6 +266,7 @@ func TestNumericAssignTo(t *testing.T) {
 		{src: &pgtype.Numeric{Int: big.NewInt(42), Status: pgtype.Present}, dst: &_i8, expected: _int8(42)},
 		{src: &pgtype.Numeric{Int: big.NewInt(0), Status: pgtype.Null}, dst: &pi8, expected: ((*int8)(nil))},
 		{src: &pgtype.Numeric{Int: big.NewInt(0), Status: pgtype.Null}, dst: &_pi8, expected: ((*_int8)(nil))},
+		{src: &pgtype.Numeric{Int: big.NewInt(1006), Exp: -2, Status: pgtype.Present}, dst: &f64, expected: float64(10.06)}, // https://github.com/jackc/pgtype/issues/27
 	}
 
 	for i, tt := range simpleTests {