//filter will reemit the data if cb(err,pass) pass is truthy

// reduce is more tricky
// maybe we want to group the reductions or emit progress updates occasionally
// the most basic reduce just emits one 'data' event after it has recieved 'end'


var through = require('through')
var Decoder = require('string_decoder').StringDecoder

module.exports = split

//TODO pass in a function to map across the lines.

function split (matcher, mapper) {
  var decoder = new Decoder()
  var soFar = ''
  if('function' === typeof matcher)
    mapper = matcher, matcher = null
  if (!matcher)
    matcher = /\r?\n/

  function emit(stream, piece) {
    if(mapper) {
      try {
        piece = mapper(piece)
      }
      catch (err) {
        return stream.emit('error', err)
      }
      if('undefined' !== typeof piece)
        stream.queue(piece)
    }
    else
      stream.queue(piece)
  }

  function next (stream, buffer) { 
    var pieces = (soFar + buffer).split(matcher)
    soFar = pieces.pop()

    for (var i = 0; i < pieces.length; i++) {
      var piece = pieces[i]
      emit(stream, piece)
    }
  }

  return through(function (b) {
    next(this, decoder.write(b))
  },
  function () {
    if(decoder.end) 
      next(this, decoder.end())
    if(soFar != null)
      emit(this, soFar)
    this.queue(null)
  })
}