Use Numeric in JSON output

postgres/numeric.go

@@ -1,11 +1,20 @@
 package postgres
 
-import "github.com/jackc/pgtype"
+import (
+	"fmt"
+	"math/big"
+
+	"github.com/jackc/pgtype"
+)
 
 type Numeric struct {
 	pgtype.Numeric
 }
 
+func NewZeroNumeric() Numeric {
+	return Numeric{pgtype.Numeric{Exp: 0, Int: big.NewInt(0), Status: pgtype.Present, NaN: false}}
+}
+
 func (n Numeric) GetFloat64() float64 {
 	if n.Status != pgtype.Present {
 		return 0
@@ -33,3 +42,83 @@ func (n Numeric) IsZero() bool {
 	float := n.GetFloat64()
 	return float == 0
 }
+
+func (n Numeric) MatchExp(exp int32) Numeric {
+	diffExp := exp - n.Exp
+	factor := big.NewInt(0).Exp(big.NewInt(10), big.NewInt(int64(diffExp)), nil)
+	return Numeric{pgtype.Numeric{
+		Exp:    exp,
+		Int:    big.NewInt(0).Mul(n.Int, factor),
+		Status: n.Status,
+		NaN:    n.NaN,
+	}}
+}
+
+func (n Numeric) Sub(o Numeric) Numeric {
+	if n.Exp > o.Exp {
+		o = o.MatchExp(n.Exp)
+	} else if n.Exp < o.Exp {
+		n = n.MatchExp(o.Exp)
+	}
+
+	if o.Exp == n.Exp {
+		return Numeric{pgtype.Numeric{
+			Exp: n.Exp,
+			Int: big.NewInt(0).Sub(o.Int, n.Int),
+		}}
+	}
+
+	panic("Cannot subtract with different exponents")
+}
+func (n Numeric) Add(o Numeric) Numeric {
+	fmt.Println("N", n, "O", o)
+	if n.Exp > o.Exp {
+		o = o.MatchExp(n.Exp)
+	} else if n.Exp < o.Exp {
+		n = n.MatchExp(o.Exp)
+	}
+
+	fmt.Println("NM", n, "OM", o)
+	if o.Exp == n.Exp {
+		return Numeric{pgtype.Numeric{
+			Exp: n.Exp,
+			Int: big.NewInt(0).Add(o.Int, n.Int),
+		}}
+	}
+
+	panic("Cannot add with different exponents")
+}
+
+func (n Numeric) MarshalJSON() ([]byte, error) {
+	if n.Int.Int64() == 0 {
+		return []byte("\"0\""), nil
+	}
+
+	s := fmt.Sprintf("%d", n.Int)
+	bytes := []byte(s)
+
+	exp := n.Exp
+	for exp > 0 {
+		bytes = append(bytes, byte('0'))
+		exp--
+	}
+
+	if exp == 0 {
+		return bytes, nil
+	}
+
+	length := int32(len(bytes))
+	var bytesWithSeparator []byte
+
+	exp = -exp
+	for length <= exp {
+		bytes = append(bytes, byte('0'))
+		length++
+	}
+
+	split := length - exp
+	bytesWithSeparator = append(bytesWithSeparator, bytes[:split]...)
+	bytesWithSeparator = append(bytesWithSeparator, byte('.'))
+	bytesWithSeparator = append(bytesWithSeparator, bytes[split:]...)
+	return bytesWithSeparator, nil
+}