Calculate actual transaction size

[maciejka]

No description provided.

[eigmax]

From my experience, we used to estimate the gas fee based on the tx base size and witness data, since we know all them in advance.

Given that:

Est fee = fee rate * (vsize + 10)

signature: 64-byte

vsize = vweight/4
vweight = tx_base_size * 3 + tx_total_size
tx_total_size ~= tx_base_size + signature + redeem_script + control_block
control_block_size (byte) = 1 + 32 + 32 × depth

   let mut tx = build_spending_tx(
        funding_tx.txid(),
        0,
        taproot_addr,
        funding_tx.output[0].value,
    );
    // base_size is tx size with witness stripped.   assume there are 3 signature. 
    let vsize = (tx.base_size() as u64 * 4 + (redeem_script.to_bytes().len() as u64 + 64 * 3 + 33))/4 + 10;
    println!("spending tx size: {}", vsize);
    tx.output[0].value = tx.output[0].value - fee_rate * vsize;

[eigmax]

fee rate:

let estimate_smart_fee_result = rpc_client.estimate_smart_fee(6, None).unwrap();
    let fee_rate = if let Some(fee_rate) = estimate_smart_fee_result.fee_rate {
        fee_rate
    } else {
        // If you're on regtest, this may not work well (always returns null), so fallback to a constant (e.g., 5–10 sat/vB).
        Amount::from_sat(5)
    };