// isDust returns whether or not the passed transaction output amount is
// considered dust or not based on the passed minimum transaction relay fee.
// Dust is defined in terms of the minimum transaction relay fee. In
// particular, if the cost to the network to spend coins is more than 1/3 of the
// minimum transaction relay fee, it is considered dust.
func isDust(txOut *wire.TxOut, minRelayTxFee dcrutil.Amount) bool {
// Unspendable outputs are considered dust.
if txscript.IsUnspendable(txOut.PkScript) {
return true
}
// The total serialized size consists of the output and the associated
// input script to redeem it. Since there is no input script
// to redeem it yet, use the minimum size of a typical input script.
//
// Pay-to-pubkey-hash bytes breakdown:
//
// Output to hash (38 bytes):
// 2 script version, 8 value, 1 script len, 25 script
// [1 OP_DUP, 1 OP_HASH_160, 1 OP_DATA_20, 20 hash,
// 1 OP_EQUALVERIFY, 1 OP_CHECKSIG]
//
// Input with compressed pubkey (165 bytes):
// 37 prev outpoint, 16 fraud proof, 1 script len,
// 107 script [1 OP_DATA_72, 72 sig, 1 OP_DATA_33,
// 33 compressed pubkey], 4 sequence
//
// Input with uncompressed pubkey (198 bytes):
// 37 prev outpoint, 16 fraud proof, 1 script len,
// 139 script [1 OP_DATA_72, 72 sig, 1 OP_DATA_65,
// 65 compressed pubkey], 4 sequence, 1 witness
// append
//
// Pay-to-pubkey bytes breakdown:
//
// Output to compressed pubkey (46 bytes):
// 2 script version, 8 value, 1 script len, 35 script
// [1 OP_DATA_33, 33 compressed pubkey, 1 OP_CHECKSIG]
//
// Output to uncompressed pubkey (76 bytes):
// 2 script version, 8 value, 1 script len, 67 script
// [1 OP_DATA_65, 65 pubkey, 1 OP_CHECKSIG]
//
// Input (133 bytes):
// 37 prev outpoint, 16 fraud proof, 1 script len, 73
// script [1 OP_DATA_72, 72 sig], 4 sequence, 1 witness
// append
//
// Theoretically this could examine the script type of the output script
// and use a different size for the typical input script size for
// pay-to-pubkey vs pay-to-pubkey-hash inputs per the above breakdowns,
// but the only combinination which is less than the value chosen is
// a pay-to-pubkey script with a compressed pubkey, which is not very
// common.
//
// The most common scripts are pay-to-pubkey-hash, and as per the above
// breakdown, the minimum size of a p2pkh input script is 165 bytes. So
// that figure is used.
totalSize := txOut.SerializeSize() + 165
// The output is considered dust if the cost to the network to spend the
// coins is more than 1/3 of the minimum free transaction relay fee.
// minFreeTxRelayFee is in Atom/KB, so multiply by 1000 to
// convert to bytes.
//
// Using the typical values for a pay-to-pubkey-hash transaction from
// the breakdown above and the default minimum free transaction relay
// fee of 5000000, this equates to values less than 546 atoms being
// considered dust.
//
// The following is equivalent to (value/totalSize) * (1/3) * 1000
// without needing to do floating point math.
return txOut.Value*1000/(3*int64(totalSize)) < int64(minRelayTxFee)
}
