fixedn: allow to parse big decimals

pkg/encoding/fixedn/decimal_test.go

@@ -0,0 +1,51 @@
+package fixedn
+
+import (
+	"math/big"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestDecimalFromStringGood(t *testing.T) {
+	var testCases = []struct {
+		bi   *big.Int
+		prec int
+		s    string
+	}{
+		{big.NewInt(123), 2, "1.23"},
+		{big.NewInt(12300), 2, "123"},
+		{big.NewInt(1234500000), 8, "12.345"},
+		{big.NewInt(-12345), 3, "-12.345"},
+		{big.NewInt(35), 8, "0.00000035"},
+		{big.NewInt(1230), 5, "0.0123"},
+		{big.NewInt(123456789), 20, "0.00000000000123456789"},
+	}
+	for _, tc := range testCases {
+		t.Run(tc.s, func(t *testing.T) {
+			s := ToString(tc.bi, tc.prec)
+			require.Equal(t, tc.s, s)
+
+			bi, err := FromString(s, tc.prec)
+			require.NoError(t, err)
+			require.Equal(t, tc.bi, bi)
+		})
+	}
+}
+
+func TestDecimalFromStringBad(t *testing.T) {
+	var errCases = []struct {
+		s    string
+		prec int
+	}{
+		{"12A", 1},
+		{"12.345", 2},
+		{"12.3A", 2},
+	}
+	for _, tc := range errCases {
+		t.Run(tc.s, func(t *testing.T) {
+			_, err := FromString(tc.s, tc.prec)
+			require.Error(t, err)
+		})
+	}
+}