Fixed Point Math (#269)

Updates our code to use the fixed point match library apd.Decimal.

coins/exchange_rate_test.go

@@ -0,0 +1,63 @@
+package coins
+
+import (
+	"testing"
+
+	"github.com/cockroachdb/apd/v3"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestExchangeRate_ToXMR(t *testing.T) {
+	rate := StrToExchangeRate("0.25") // 4 XMR * 0.25 = 1 ETH
+	ethAmount := StrToDecimal("1")
+	const expectedXMRAmount = "4"
+	xmrAmount, err := rate.ToXMR(ethAmount)
+	require.NoError(t, err)
+	assert.Equal(t, expectedXMRAmount, xmrAmount.String())
+}
+
+func TestExchangeRate_ToXMR_roundDown(t *testing.T) {
+	rate := StrToExchangeRate("0.333333")
+	ethAmount := StrToDecimal("3.1")
+
+	// 3.1/0.333333 calculated to 13 decimals is 9.3000093000093 (300009 repeats indefinitely)
+	// This calculator goes to 200 decimals: https://www.mathsisfun.com/calculator-precision.html
+	// XMR rounds at 12 decimal places to:
+	const expectedXMRAmount = "9.300009300009"
+
+	xmrAmount, err := rate.ToXMR(ethAmount)
+	require.NoError(t, err)
+	assert.Equal(t, expectedXMRAmount, xmrAmount.String())
+}
+
+func TestExchangeRate_ToXMR_roundUp(t *testing.T) {
+	rate := StrToExchangeRate("0.666666")
+	ethAmount := StrToDecimal("6.6")
+	// 6.6/0.666666 to 13 decimal places is 9.9000099000099 (900009 repeats indefinitely)
+	// The 9 in the 12th position goes to zero changing 11th position to 1:
+	const expectedXMRAmount = "9.90000990001" // only 11 decimal places shown as 12th is 0
+	xmrAmount, err := rate.ToXMR(ethAmount)
+	require.NoError(t, err)
+	assert.Equal(t, expectedXMRAmount, xmrAmount.String())
+}
+
+func TestExchangeRate_ToXMR_fail(t *testing.T) {
+	rateZero := ToExchangeRate(new(apd.Decimal)) // zero exchange rate
+	_, err := rateZero.ToXMR(StrToDecimal("0.1"))
+	require.ErrorContains(t, err, "division by zero")
+}
+
+func TestExchangeRate_ToETH(t *testing.T) {
+	rate := StrToExchangeRate("0.25") // 4 XMR * 0.25 = 1 ETH
+	xmrAmount := StrToDecimal("4")
+	const expectedETHAmount = "1"
+	ethAmount, err := rate.ToETH(xmrAmount)
+	require.NoError(t, err)
+	assert.Equal(t, expectedETHAmount, ethAmount.String())
+}
+
+func TestExchangeRate_String(t *testing.T) {
+	rate := ToExchangeRate(apd.New(3, -4)) // 0.0003
+	assert.Equal(t, "0.0003", rate.String())
+}