Add MsgPack Marshal and Unmarshal function

Mote: Marshal max digits is 30. it decimal.IntPart large than 30,it will be lose extra digits.

decimal_msgpack.go

@@ -0,0 +1,92 @@
+package decimal
+
+import (
+	"errors"
+)
+
+var (
+	errShortBytes = errors.New("msgp: too few bytes left to read object")
+)
+
+// MarshalMsg implements msgp.Marshaler
+func (d Decimal) MarshalMsg(b []byte) (o []byte, err error) {
+	o = require(b, d.Msgsize())
+	str := d.String()
+	sz := len(str)
+	// limit to 30 digits
+	// note, if d.IntPart size large than 30, will be lose.
+	if sz > 30 {
+		sz = 30
+		// if last char is '.' then limit to 20 digits
+		if str[29] == '.' {
+			sz = 29
+		}
+
+		str = str[:sz]
+	}
+
+	o, n := ensure(b, 1+sz)
+	o[n] = byte(0xa0 | sz)
+	n++
+
+	return o[:n+copy(o[n:], str)], nil
+}
+
+// UnmarshalMsg implements msgp.Unmarshaler
+func (d *Decimal) UnmarshalMsg(b []byte) (o []byte, err error) {
+	l := len(b)
+	if l < 1 {
+		return nil, errShortBytes
+	}
+
+	sz := int(b[0] & 0x1f)
+	if len(b[1:]) < sz {
+		err = errShortBytes
+		return
+	}
+	if *d, err = NewFromString(string(b[1 : sz+1])); err == nil {
+		o = b[sz:]
+	}
+	return
+}
+
+// Msgsize returns an upper bound estimate of the number of bytes occupied by the serialized message
+func (d Decimal) Msgsize() int {
+	return 31
+}
+
+// Require ensures that cap(old)-len(old) >= extra.
+func require(old []byte, extra int) []byte {
+	l := len(old)
+	c := cap(old)
+	r := l + extra
+	if c >= r {
+		return old
+	} else if l == 0 {
+		return make([]byte, 0, extra)
+	}
+	// the new size is the greater
+	// of double the old capacity
+	// and the sum of the old length
+	// and the number of new bytes
+	// necessary.
+	c <<= 1
+	if c < r {
+		c = r
+	}
+	n := make([]byte, l, c)
+	copy(n, old)
+	return n
+}
+
+// ensure 'sz' extra bytes in 'b' btw len(b) and cap(b)
+func ensure(b []byte, sz int) ([]byte, int) {
+	l := len(b)
+	c := cap(b)
+	if c-l < sz {
+		o := make([]byte, (2*c)+sz) // exponential growth
+		n := copy(o, b)
+		return o[:n+sz], n
+	}
+	return b[:l+sz], l
+}