discovery.js

// https://github.com/bitcoin/bips/blob/master/bip-0044.mediawiki#account-discovery
module.exports = function discovery (chain, gapLimit, queryCb, done) {
  let gap = 0
  let checked = 0

  function cycle () {
    const batch = [chain.get()]
    checked++

    while (batch.length < gapLimit) {
      chain.next()
      batch.push(chain.get())

      checked++
    }

    queryCb(batch, function (err, queryResultSet) {
      if (Array.isArray(queryResultSet)) return done(new TypeError('Expected query set, not Array'))
      if (err) return done(err)

      // iterate batch, guarantees order agnostic of queryCb result ordering
      batch.forEach(function (address) {
        if (queryResultSet[address]) {
          gap = 0
        } else {
          gap += 1
        }
      })

      if (gap >= gapLimit) {
        const used = checked - gap

        return done(undefined, used, checked)
      } else {
        chain.next()
      }

      cycle()
    })
  }

  cycle()
}