fix(decode): allow float64-encoded values to decode into float32 (#12)

DecodeFloat32 now handles msgpcode.Double by narrowing to float32 with
an overflow check that rejects values exceeding math.MaxFloat32. NaN
and subnormals pass through correctly.

Closes #12

Author: Ignacio Van Droogenbroeck

decode_number.go

@@ -171,12 +171,22 @@ func (d *Decoder) DecodeFloat32() (float32, error) {
 }
 
 func (d *Decoder) float32(c byte) (float32, error) {
-	if c == msgpcode.Float {
+	switch c {
+	case msgpcode.Float:
 		n, err := d.uint32()
 		if err != nil {
 			return 0, err
 		}
 		return math.Float32frombits(n), nil
+	case msgpcode.Double:
+		n, err := d.float64(c)
+		if err != nil {
+			return 0, err
+		}
+		if n > math.MaxFloat32 || n < -math.MaxFloat32 {
+			return 0, fmt.Errorf("msgpack: float64 %v overflows float32", n)
+		}
+		return float32(n), nil
 	}
 
 	n, err := d.int(c)

types_test.go

@@ -1013,6 +1013,21 @@ func TestInt64(t *testing.T) {
 	}
 }
 
+func TestDecodeFloat32FromFloat64(t *testing.T) {
+	// Issue #12: float32 decode should accept Double code with narrowing.
+	b, err := msgpack.Marshal(float64(3.14))
+	require.NoError(t, err)
+
+	var f float32
+	require.NoError(t, msgpack.Unmarshal(b, &f))
+	require.InDelta(t, float32(3.14), f, 0.001)
+
+	// Overflow should error.
+	b, err = msgpack.Marshal(math.MaxFloat64)
+	require.NoError(t, err)
+	require.Error(t, msgpack.Unmarshal(b, &f))
+}
+
 func TestFloat32(t *testing.T) {
 	tests := []struct {
 		in     float32