utils.js

'use strict';

const assert = require('assert');
const bcoin = require('bcoin');
const MTX = bcoin.mtx;
const Script = bcoin.script;
const Coin = bcoin.coin;

const getFeeForInput = function getFeeForInput(coin, address, keyring, rate) {
  const fundingTarget = 100000000; // 1 BTC (arbitrary for purposes of this function)
  const testMTX = new MTX();

  // we're not actually going to use this tx for anything other than calculate what fee should be

  addInput(coin, 0, testMTX, keyring);

  return testMTX.getMinFee(null, rate);
}

const addInput = function addInput(coin, inputIndex, mtx, keyring, hashType) {
  const sampleCoin = coin instanceof Coin ? coin : Coin.fromJSON(coin);
  if(!hashType) hashType = Script.hashType.ANYONECANPAY | Script.hashType.ALL;

  mtx.addCoin(sampleCoin);
  mtx.scriptInput(inputIndex, sampleCoin, keyring);
  mtx.signInput(inputIndex, sampleCoin, keyring, hashType);
  assert(mtx.isSigned(), 'Input was not signed properly');
}

const splitCoinbase = async function splitCoinbase(funderKeyring, coin, targetAmount, txRate) {
  // loop through each coinbase coin to split
  let coins = [];

  const mtx = new MTX();

  assert(coin.value > targetAmount, 'coin value is not enough!');

  // creating a transaction that will have an output equal to what we want to fund
  mtx.addOutput({
    address: funderKeyring.getAddress(),
    value: targetAmount
  });

  // shift off the coinbase coin to use to fund the splitting transaction
  // the fund method will automatically split the remaining funds to the change address
  // Note that in a real application these splitting transactions will also
  // have to be broadcast to the network
  await mtx.fund([coin], {
    rate: txRate,
    // send change back to an address belonging to the funder
    changeAddress: funderKeyring.getAddress()
  }).then(() => {
    // sign the mtx to finalize split
    mtx.sign(funderKeyring);
    assert(mtx.verify());

    const tx = mtx.toTX();
    assert(tx.verify(mtx.view));

    const outputs = tx.outputs;

    // get coins from tx
    outputs.forEach((outputs, index) => {
      coins.push(Coin.fromTX(tx, index, -1));
    });
  })
  .catch(e => console.log('There was an error: ', e));

  return coins;
};

module.exports = {
  getFeeForInput,
  addInput,
  splitCoinbase
}