Suggestions for extending fee calculation

[ikripaka]

Describe the feature

This issue refers to an extension of the existing fee calculation. This is directly in the context of the function declared in simplicity-dex here.

Fee calculation in lwk_wollet during transaction finalization is declared here from line 1165 up to 1220.
The calculation of fees in simplicity-dex is almost the same, so the idea is to improve it and move it to a more general crate.

Improvements that can be made:

• Use discount_weight as defined here.
• Replace the manual LBTC testnet token definition with wollet.policy_asset() (see reference here).
• Move the logic out of the cli-client module into an outer crate to enable reuse in simplicityhl-core.

[KyrylR]

Use discount_weight as defined here.

It can be only applied for the confidential transactions, so it works for the issue-asset, but the discount does not work for merge tx

[KyrylR]

Replace the manual LBTC testnet token definition with wollet.policy_asset() (see reference here).

Yes, it is definitely an improvement, let's do this

[KyrylR]

Move the logic out of the cli-client module into an outer crate to enable reuse in simplicityhl-core.

How do you envision this?

[ikripaka]

Use discount_weight as defined here.

This applies only to confidential transactions. It works for asset issuance, but the discount does not apply to merge transactions.

Let's create a general finalization function responsible for inserting fees.
We also handle confidential outputs following token creation (e.g., in creation_option and funding_option, the first two outputs are confidential). Currently, it seems we are forced to repeat transaction creation both with and without PLACEHOLDER_FEE.

In my opinion, we need to:

• Remove the insertion of fee outputs in the contracts SDK where L-BTC is used. This implies separating the UTXOs used for stablecoins from those used for the policy asset (fees). Consequently, the PSET returned by the contract SDK would be "unfilled" and must be finalized by inserting fees. We can enforce this workflow using generics (similar to how the bitcoin crate handles Address with NetworkUnchecked and NetworkChecked).
• Move verify_tx_amt_proofs to the finalization step where fees are inserted (though this might be non-ideal, as we would catch contract-related errors too late).

For example, the flow could be transformed like this:

let (funding_pst_unchecked, option_branch): (PartiallySignedTransactionUnchecked, OptionBranch) = contracts::sdk::build_option_funding(
    &blinding_keypair.public_key(),
    option_token_utxo.clone(),
    grantor_token_utxo.clone(),
    collateral_utxo.clone(),
    funding_utxo.as_ref(),
    &args,
    *total_collateral
)?;

// Add fees and apply `verify_tx_amt_proofs`
// At this step, we also need to use `blind_last`, but I haven't finalized the approach yet.
let funding_pst_checked: PartiallySignedTransaction = funding_pst_unchecked.finalize_pset(fee_utxo.as_ref())?;

// Proceed with other steps as before

[KyrylR]

Use discount_weight as defined here.

Yep, if we can do that