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Analyzing_Transaction_Fee.md

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Observing the fee trends using Rust

Fees in bitcoin is a hot topic now and will continue to grow in future given the fact that fees will be the largest revenue factor for a miner than a block reward.

Problem statement

Given a block find the transaction with the largest fee.

struct TxId(String);
// Return type is Option<_>  because some blocks have a single transaction
// which is the coinbase transaction
fn high_fee_transaction(block: Block) -> Option<TxId> {
    unimplemented!("TODO")
}

The Block struct has txdata field which is just a Vec<Transaction>, so if it's length is 0 then we know that we have to return None as it carries no fee.

Computing the Fee of a Transaction

In order to compute the fee of a transaction we need to do the following.

  1. Find the total value of the inputs
  2. Find the total value of the outputs
  3. The difference between them is the fee.

Computing the Total value of outputs

This step is much easier because the TxOut type has the value field which gives the Amount of that output. So the following would suffice,

let total_output_value: u64 = tx.output.iter().map(|txout| txout.value.to_sat()).sum();

Read here to look at how to use the Amount type.

Computing the Total value of inputs

This is a bit involved as the TxIn type has reference to the previous OutPoint which has the txid and vout of the previous transaction, vout refers to the index of the output.

So to indentify it's value we have to make request with bitcoinrpc to fetch TxOut data of that previous transaction.

let total_input_value: u64 = tx.input.iter().map(|txin| {
    let previous_outpoint = txin.previous_output;
    let previous_transaction: GetRawTransactionResult = client
        // We don't know the block_hash of the txin
        .get_raw_transaction_info(&previous_outpoint.txid, None)
        .expect("The bitcoin node does not support transaction indexing or the given block has invalid data");
    let previous_output: &GetRawTransactionResultVout =
        &previous_transaction.vout[previous_outpoint.vout as usize];
    previous_output.value.to_sat() // input value of this txin
}).sum();

putting it all together the entire code of this tutorial can be found here