Bootstrap copied BFF and local accounts with public Sendico contracts (#34)

backend/pkg/moneybase/fx_rounding.go

@@ -0,0 +1,50 @@
+package moneybase
+
+import (
+	"math/big"
+	"strings"
+
+	decimalbase "git.sendico.io/tech/sxchnge/bff/pkg/decimal"
+	"git.sendico.io/tech/sxchnge/bff/pkg/merrors"
+)
+
+// SourceRoundedConversionMatches reports whether sourceAmount is the minimum
+// source-scale amount that covers targetAmount at rate.
+func SourceRoundedConversionMatches(sourceAmount, targetAmount, rate string, sourceScale uint32) (bool, error) {
+	if err := ValidateScale("source scale", sourceScale); err != nil {
+		return false, err
+	}
+	source, err := positiveRat(sourceAmount, "source amount")
+	if err != nil {
+		return false, err
+	}
+	target, err := positiveRat(targetAmount, "target amount")
+	if err != nil {
+		return false, err
+	}
+	rateRat, err := positiveRat(rate, "conversion rate")
+	if err != nil {
+		return false, err
+	}
+	exactSource := new(big.Rat).Quo(target, rateRat)
+	roundedSource := ceilDecimalToScale(exactSource, sourceScale)
+	return roundedSource.Cmp(source) == 0 && source.Cmp(exactSource) >= 0, nil
+}
+
+func positiveRat(value string, field string) (*big.Rat, error) {
+	parsed, err := decimalbase.RatFromString(strings.TrimSpace(value))
+	if err != nil || parsed.Sign() <= 0 {
+		return nil, merrors.InvalidArgument(field + " must be a positive decimal")
+	}
+	return parsed, nil
+}
+
+func ceilDecimalToScale(value *big.Rat, scale uint32) *big.Rat {
+	factor := new(big.Int).Exp(big.NewInt(10), big.NewInt(int64(scale)), nil)
+	scaled := new(big.Rat).Mul(value, new(big.Rat).SetInt(factor))
+	q, r := new(big.Int).QuoRem(scaled.Num(), scaled.Denom(), new(big.Int))
+	if r.Sign() > 0 {
+		q.Add(q, big.NewInt(1))
+	}
+	return new(big.Rat).SetFrac(q, factor)
+}