Fix HexNumber MarshalJSON (#139)

* Fix HexNumber MarshalJSON

* Add more test cases

types/types.go

@@ -10,7 +10,7 @@ import (
 type HexNumber big.Int
 
 func (i HexNumber) MarshalJSON() ([]byte, error) {
-	hexNumber := fmt.Sprintf("\"0x%x\"", (*big.Int)(&i).Uint64())
+	hexNumber := fmt.Sprintf("\"0x%x\"", (*big.Int)(&i))
 
 	return []byte(hexNumber), nil
 }

types/types_test.go

@@ -0,0 +1,66 @@
+package types
+
+import (
+	"encoding/json"
+	"math/big"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestHex_UnmarshalAndMarshalJSON(t *testing.T) {
+	tests := []struct {
+		name    string
+		input   []byte
+		result  string
+		isError bool
+	}{
+		{
+			name:    "value greater than 2^64 -1",
+			input:   []byte(`{"value":"0x850a9af493d065b6c"}`),
+			result:  "153386322112866048876",
+			isError: false,
+		},
+		{
+			name:    "value less than 2^64 -1",
+			input:   []byte(`{"value":"0x746a528800"}`),
+			result:  "500000000000",
+			isError: false,
+		},
+		{
+			name:    "error case: not hex (string)",
+			input:   []byte(`{"value":"error_value"}`),
+			result:  "",
+			isError: true,
+		},
+		{
+			name:    "error case: not hex (octal)",
+			input:   []byte(`{"value":"20502515364447501455554"}`),
+			result:  "",
+			isError: true,
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			type req struct {
+				Value *HexNumber `json:"value"`
+			}
+
+			var v req
+
+			err := json.Unmarshal(tc.input, &v)
+			if tc.isError {
+				return
+			}
+			assert.NoError(t, err)
+
+			output := (*big.Int)(v.Value)
+			assert.Equal(t, tc.result, output.String())
+
+			bytes, err := json.Marshal(v)
+			assert.NoError(t, err)
+			assert.Equal(t, tc.input, bytes)
+		})
+	}
+}