accumulative.js

var utils = require('./utils')
var ext = require('./bn-extensions')

// add inputs until we reach or surpass the target value (or deplete)
// worst-case: O(n)
module.exports = function accumulative (utxos, outputs, feeRate) {
  if (!utils.uintOrNull(feeRate)) return {}
  var bytesAccum = utils.transactionBytes([], outputs)

  var inAccum = ext.BN_ZERO
  var inputs = []
  var outAccum = utils.sumOrNaN(outputs)

  for (var i = 0; i < utxos.length; ++i) {
    var utxo = utxos[i]
    var utxoBytes = utils.inputBytes(utxo)
    var utxoFee = ext.mul(feeRate, utxoBytes)
    var utxoValue = utils.uintOrNull(utxo.value)

    // skip detrimental input
    if (ext.gt(utxoFee, utxoValue)) {
      if (i === utxos.length - 1) {
        return { fee: ext.mul(feeRate, ext.add(bytesAccum, utxoBytes)) }
      }
      continue
    }

    bytesAccum = ext.add(bytesAccum, utxoBytes)
    inAccum = ext.add(inAccum, utxoValue)
    inputs.push(utxo)

    var fee = ext.mul(feeRate, bytesAccum)

    // go again?
    if (ext.lt(inAccum, ext.add(outAccum, fee))) continue

    return utils.finalize(inputs, outputs, feeRate)
  }

  return { fee: ext.mul(feeRate, bytesAccum) }
}