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Sure! The trick is to rely every enter with its efficient worth relatively than its nominal worth.
Transactions are composed of three elements: the overhead, the inputs and the outputs.
Outputs:
The scale of the output scripts is decided by the recipient addresses, so their measurement is given by the transaction directions. We have no idea whether or not we are going to want a change output but, however we all know what measurement it will be if we created one.Overhead:
The transaction overhead is 10 bytes for non-segwit transactions, or 42 WU for segwit transactions¹. You’ll be able to in all probability guess which one you will want, however within the worst case you overestimate by 0.5 vB. We are able to due to this fact think about the overhead measurement additionally to be fastened.Inputs:
The tough half is the transaction inputs. You have no idea what number of you will want, and may not know what enter script sizes they’re going to have when you’ve got inputs of varied handle codecs. Moreover, signatures aren’t at all times the identical measurement. Nonetheless, we do know the utmost measurement of the signature, and the scale of the enter script for every UTXO. Along with the goal payment fee, we will calculate the efficient worth:effectiveValue = utxo.worth − feePerByte × bytesPerInput²
The choice goal begins out because the sum of the recipient quantities. We simplify the coin choice downside by including the fastened prices to the choice goal. We then carry out the choice by way of counting efficient values relatively than nominal values. Since every enter has paid for itself already when it’s chosen, we not want to fret about an enter’s payment impression after choice; as a substitute every efficient worth goes absolutely in the direction of the choice goal.
In regard to the change output, there are two methods. One, in case you are aiming to construct a transaction that avoids creating change, e.g. through the use of Department and Sure choice, you retain the goal as is: goal = recipient quantities + charges for fastened transaction elements. Two, in case you are utilizing a method that can create change, you add a buffer to the choice goal that’s giant sufficient to cowl the payment of the change output and depart sufficient for a good-sized change output: goal = recipient quantities + charges for fastened tx elements + change output value + minChange.
For the signatures, you estimate utilizing the utmost signature size. For the reason that signatures are a part of the witness for segwit inputs, this outcomes solely in a small measurement overestimation which is able to result in minutely overshooting the goal payment fee.³ Alternatively, you need to use signature grinding to save lots of an anticipated 0.5 bytes per signature and estimate the payment fee extra exactly.
¹ Assuming that the transaction has not more than 253 inputs and not more than 253 outputs. The enter/output counter will take 3 bytes as a substitute of 1 byte for larger enter/output counts.
² Efficient worth was proposed in part 5.3 of An Analysis of Coin Choice Methods, Erhardt 2016, disclosure: authored by yours really.
³ Overshooting the goal payment fee is preferable; it prices only some satoshis and barely will increase the precedence of the transaction. Undershooting the goal payment fee is problematic, since some fee processors require a minimal payment fee to just accept a fee and falling under the default minRelayTxFeeRate can forestall a transaction from relaying altogether.
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