add UnmarshalJSON and MarshalJSON to Decimal (#191)

* add UnmarshalJSON and MarshalJSON to Decimal

* add more tests

ion/decimal.go

@@ -390,3 +390,51 @@ func (d *Decimal) String() string {
 		return b.String()
 	}
 }
+
+// UnmarshalJSON implements the json.Unmarshaler interface.
+func (d *Decimal) UnmarshalJSON(decimalBytes []byte) error {
+	str := string(decimalBytes)
+	if str == "null" {
+		return nil
+	}
+	str = strings.Replace(str, "E", "D", 1)
+	str = strings.Replace(str, "e", "d", 1)
+	parsed, err := ParseDecimal(str)
+	if err != nil {
+		return fmt.Errorf("error unmarshalling decimal '%s': %w", str, err)
+	}
+	*d = *parsed
+	return nil
+}
+
+// MarshalJSON implements the json.Marshaler interface.
+func (d *Decimal) MarshalJSON() ([]byte, error) {
+	absN := new(big.Int).Abs(d.n).String()
+	scale := int(-d.scale)
+	sign := d.n.Sign()
+
+	var str string
+	if scale == 0 {
+		str = absN
+	} else if scale > 0 {
+		// add zeroes to the right
+		str = absN + strings.Repeat("0", scale)
+	} else {
+		// add zeroes to the left
+		absScale := -scale
+		nLen := len(absN)
+
+		if absScale >= nLen {
+			str = "0." + strings.Repeat("0", absScale-nLen) + absN
+		} else {
+			str = absN[:nLen-absScale] + "." + absN[nLen-absScale:]
+		}
+		str = strings.TrimRight(str, "0")
+		str = strings.TrimSuffix(str, ".")
+	}
+
+	if sign == -1 {
+		str = "-" + str
+	}
+	return []byte(str), nil
+}

ion/decimal_test.go

@@ -16,6 +16,7 @@
 package ion
 
 import (
+	"encoding/json"
 	"fmt"
 	"math/big"
 	"testing"
@@ -329,3 +330,72 @@ func TestUpscale(t *testing.T) {
 	actual := d.upscale(4).String()
 	assert.Equal(t, "10.0000", actual)
 }
+
+func TestMarshalJSON(t *testing.T) {
+	test := func(a string, expected string) {
+		t.Run("("+a+")", func(t *testing.T) {
+			ad, err := ParseDecimal(a)
+			require.NoError(t, err)
+
+			am, err := ad.MarshalJSON()
+			require.NoError(t, err)
+
+			assert.Equal(t, []byte(expected), am)
+		})
+	}
+	test("123000", "123000")
+
+	test("1.01", "1.01")
+	test("0.01", "0.01")
+	test("0.0", "0")
+	test("0.123456789012345678901234567890", "0.12345678901234567890123456789")                                                           // Trims trailing zeros
+	test("123456789012345678901234567890.123456789012345678901234567890", "123456789012345678901234567890.12345678901234567890123456789") // Trims trailing zeros
+
+	test("1d-2", "0.01")
+	test("1d-3", "0.001")
+	test("1d2", "100")
+
+	test("-1d3", "-1000")
+	test("-1d-3", "-0.001")
+	test("-0.0", "0")
+	test("-0.1", "-0.1")
+}
+
+func TestUnmarshalJSON(t *testing.T) {
+	test := func(a string, expected string) {
+		t.Run("("+a+")", func(t *testing.T) {
+			expectedDec := MustParseDecimal(expected)
+
+			var r struct {
+				D *Decimal `json:"d"`
+			}
+			err := json.Unmarshal([]byte(`{"d":`+a+`}`), &r)
+			require.NoError(t, err)
+
+			assert.Truef(t, expectedDec.Equal(r.D), "expected %v, got %v", expected, r.D)
+		})
+	}
+
+	test("123000", "123000")
+	test("123.1", "123.1")
+	test("123.10", "123.1")
+	test("-123000", "-123000")
+	test("-123.1", "-123.1")
+	test("-123.10", "-123.1")
+
+	test("1e+2", "100")
+	test("1e2", "100")
+	test("1E2", "100")
+	test("1E+2", "100")
+
+	test("-1e+2", "-100")
+	test("-1e2", "-100")
+	test("-1E2", "-100")
+	test("-1E+2", "-100")
+
+	test("1e-2", "0.01")
+	test("1E-2", "0.01")
+
+	test("-1e-2", "-0.01")
+	test("-1E-2", "-0.01")
+}