Computing witness lengths of a TxIn

[yancyribbens]

To use the constructor for InputWeightPrediction, the input_script_len and witness_element_lengths is needed. Given a TxIn and accessing the Witness field, is it possible to compute the witness_element_lengths?

For example:

    #[test]
    fn tx_in_weight_predict() {
        let tx_in = TxIn::default();
        let input_script_len = tx_in.script_sig.len();
        let witness_element_lengths = tx_in.witness.witness_element_lengths; // how to compute?
        let i = InputWeightPrediction::new(input_script_len, witness_element_lengths);
    }

The Witness struct has content and witness_elements (which are private) but not witness_element_lengths from what I can tell..

[sanket1729]

As discussed in rust-minsicript issue, you need a descriptor to do this. rust-bitcoin/rust-miniscript#675 (comment)

[apoelstra]

If the witness is already populated then Witness::size gives you this.

[yancyribbens]

If the witness is already populated then Witness::size gives you this.

@apoelstra thanks, although my confusion is that The Witness size method returns a usize, while PredictIntputWeight constructor requires something that can be iterable. I mean size() could just be shoved into a vec but that doesn't feel like the intent.

[sanket1729]

In most/all cases we never know the exact witness before using PredictInputWeight, thus descriptors. I am somewhat confused with this flow. There are several constants P2PKH_MAX and more that help more most common use cases. For other use-cases involving script hash, you need some information about the descriptor and the spend path that you plan to use.

[yancyribbens]

In most/all cases we never know the exact witness before using PredictInputWeight, thus descriptors. I am somewhat confused with this flow. There are several constants P2PKH_MAX and more that help more most common use cases. For other use-cases involving script hash, you need some information about the descriptor and the spend path that you plan to use.

@sanket1729 hmm ok. I'll take a look at how to incorporate descriptors. Thanks!