implement suggested exchange rate (#262)

Pulls chainlink's ETH and XMR price feeds to create suggested exchange rates.
Co-authored-by: Dmitry Holodov <dimalinux@protonmail.com>

coins/coins.go

@@ -8,7 +8,6 @@ import (
 	"strconv"
 
 	"github.com/cockroachdb/apd/v3"
-	"github.com/ethereum/go-ethereum/log"
 )
 
 // PiconeroAmount represents some amount of piconero (the smallest denomination of monero)

coins/common.go

@@ -2,6 +2,7 @@ package coins
 
 import (
 	"github.com/cockroachdb/apd/v3"
+	logging "github.com/ipfs/go-log"
 )
 
 const (
@@ -19,6 +20,8 @@ const (
 var (
 	// DecimalCtx is the apd context used for math operations on our coins
 	decimalCtx = apd.BaseContext.WithPrecision(MaxCoinPrecision)
+
+	log = logging.Logger("coins")
 )
 
 // DecimalCtx clones and returns the apd.Context we use for coin math operations.

coins/common_test.go

@@ -3,9 +3,14 @@ package coins
 import (
 	"testing"
 
+	logging "github.com/ipfs/go-log"
 	"github.com/stretchr/testify/assert"
 )
 
+func init() {
+	logging.SetLogLevel("coins", "debug")
+}
+
 func TestDecimalCtx(t *testing.T) {
 	c := DecimalCtx()
 	assert.Equal(t, c.Precision, uint32(MaxCoinPrecision))

coins/exchange_rate.go

@@ -9,6 +9,21 @@ import (
 // ie. an ExchangeRate of 0.1 means that the node considers 1 ETH = 10 XMR.
 type ExchangeRate apd.Decimal
 
+// CalcExchangeRate computes and returns an exchange rate using ETH and XRM prices. The
+// price can be relative to USD, bitcoin or something else, but both values should be
+// relative to the same alternate currency.
+func CalcExchangeRate(xmrPrice *apd.Decimal, ethPrice *apd.Decimal) (*ExchangeRate, error) {
+	rate := new(apd.Decimal)
+	_, err := decimalCtx.Quo(rate, xmrPrice, ethPrice)
+	if err != nil {
+		return nil, err
+	}
+	if err = roundToDecimalPlace(rate, rate, MaxExchangeRateDecimals); err != nil {
+		return nil, err
+	}
+	return ToExchangeRate(rate), nil
+}
+
 // ToExchangeRate casts an *apd.Decimal to *ExchangeRate
 func ToExchangeRate(rate *apd.Decimal) *ExchangeRate {
 	return (*ExchangeRate)(rate)

coins/exchange_rate_test.go

@@ -61,3 +61,18 @@ func TestExchangeRate_String(t *testing.T) {
 	rate := ToExchangeRate(apd.New(3, -4)) // 0.0003
 	assert.Equal(t, "0.0003", rate.String())
 }
+
+func TestCalcExchangeRate(t *testing.T) {
+	xmrPrice := StrToDecimal("200")
+	ethPrice := StrToDecimal("300")
+	rate, err := CalcExchangeRate(xmrPrice, ethPrice)
+	require.NoError(t, err)
+	assert.Equal(t, "0.666667", rate.String())
+}
+
+func TestCalcExchangeRate_fail(t *testing.T) {
+	xmrPrice := StrToDecimal("1.0")
+	ethPrice := StrToDecimal("0") // create a division by zero error
+	_, err := CalcExchangeRate(xmrPrice, ethPrice)
+	require.ErrorContains(t, err, "division by zero")
+}