TxHadler.java

import java.util.ArrayList;
import java.util.HashSet;

public class TxHandler {

    /**
     * Creates a public ledger whose current UTXOPool (collection of unspent transaction outputs) is
     * {@code utxoPool}. This should make a copy of utxoPool by using the UTXOPool(UTXOPool uPool)
     * constructor.
     */
    private UTXOPool utxoPool;
    public TxHandler(UTXOPool utxoPool) {
        this.utxoPool = new UTXOPool(utxoPool);
    }

    /**
     * @return true if:
     * (1) all outputs claimed by {@code tx} are in the current UTXO pool,
     * (2) the signatures on each input of {@code tx} are valid,
     * (3) no UTXO is claimed multiple times by {@code tx},
     * (4) all of {@code tx}s output values are non-negative, and
     * (5) the sum of {@code tx}s input values is greater than or equal to the sum of its output
     *     values; and false otherwise.
     */
    public boolean isValidTx(Transaction tx) {
        HashSet<UTXO> claimedUTXO = new HashSet<>();
        double inTotal = 0.d;
        for (int i = 0; i < tx.numInputs(); i++) {
            Transaction.Input in = tx.getInput(i);
            UTXO ut = new UTXO(in.prevTxHash, in.outputIndex);

            // (1)
            if (!utxoPool.contains(ut)) return false;

            // (2)
            Transaction.Output out = utxoPool.getTxOutput(ut);
            if (!Crypto.verifySignature(out.address, tx.getRawDataToSign(i), in.signature)) return false;

            // (3)
            if (claimedUTXO.contains(ut)) return false;
            claimedUTXO.add(ut);

            // for (5)
            inTotal += out.value;
        }

        for (Transaction.Output out : tx.getOutputs()) {
            // (4)
            if (out.value < 0.d) return false;

            // (5)
            inTotal -= out.value;
            if (inTotal < 0.d) return false; // when total output value exceeds total input
        }

        return true;
    }

    /**
     * Handles each epoch by receiving an unordered array of proposed transactions, checking each
     * transaction for correctness, returning a mutually valid array of accepted transactions, and
     * updating the current UTXO pool as appropriate.
     */
    public Transaction[] handleTxs(Transaction[] possibleTxs) {
        ArrayList<Transaction> acceptedTxs = new ArrayList<>();
        for (Transaction tx : possibleTxs) {
            if (!isValidTx(tx)) continue;
            acceptedTxs.add(tx);
            // remove spent from prev tx
            for (Transaction.Input in : tx.getInputs()) {
                UTXO ut = new UTXO(in.prevTxHash, in.outputIndex);
                utxoPool.removeUTXO(ut);
            }
            // add coins created by this tx
            for (int i = 0; i < tx.numOutputs(); i++) {
                UTXO ut = new UTXO(tx.getHash(), i);
                utxoPool.addUTXO(ut, tx.getOutput(i));
            }
        }
        return acceptedTxs.toArray(new Transaction[acceptedTxs.size()]);
    }

    public UTXOPool getUTXOPool() {
        return utxoPool;
    }

}