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)
}