Incorrect min amount for multiasset transaction

[visopsys]

I think this min value calculation for multiasset transaction is incorrect:

func (tb *TxBuilder) MinCoinsForTxOut(txOut *TxOutput) Coin {
	var size uint
	if txOut.Amount.OnlyCoin() {
		size = 1
	} else {
		numAssets := txOut.Amount.MultiAsset.numAssets()
		assetsLength := txOut.Amount.MultiAsset.assetsLength()
		numPIDs := txOut.Amount.MultiAsset.numPIDs()

		size = 6 + uint(math.Floor(
			float64(numAssets*12+assetsLength+numPIDs*28+7)/8,
		))
	}
	return Coin(utxoEntrySizeWithoutVal+size) * tb.protocol.CoinsPerUTXOWord
}

I have a multi-asset input and my values are as follow:

numAssets = 1, assetsLength = 8, numPIDs = 1, utxoEntrySizeWithoutVal = 27, tb.protocol.CoinsPerUTXOWord
 = 4310

Total coin amount = 168090 (or 0.16809 ADA)

This number is clearly smaller than min 1.3 ADA required to transfer multiasset. I got BabbageOutputTooSmallUTxO error when submitting this. Please check this function again for multi-asset

[visopsys]

How do you come up with the calculation formula?

[echovl]

The equation comes from the ledger spec, let me check this out

[core-dev-crypto]

Hi @echovl,

I'll check the general formula and it seem corret. I think that the error is in the rounding of size calculation:

size = 6 + uint(math.Floor( float64(numAssets*12+assetsLength+numPIDs*28+7)/8, ))

The +7 is used in case of truncation to flooring the number. So

size = 6 + uint(math.Trunc( float64(numAssets*12+assetsLength+numPIDs*28+7)/8, ))

or

size = 6 + uint(math.Floor( float64(numAssets*12+assetsLength+numPIDs*28)/8, ))

are the possible solutions.

[echovl]

@core-dev-crypto can u make a PR?, I'm really busy these days, will check it out later

[core-dev-crypto]

Yes, sure.