read/write stream

gistfile1.js

var fs = require('fs');

var readable = fs.createReadStream('large.txt', {encoding: 'utf8'});
var writable = fs.createWriteStream('out.txt', {encoding: 'utf8'});

var count;
var left = '';
var reader = new Reader;
readable.on('data', function(chunk) {
  reader.set(chunk);
  if (count == null) {
    count = reader.read();
  }
  var line;
  while (line = reader.read()) {
    if (line.length < 9) {
      break;
    }
    var pair = line.split(' ');
    var out = rate(pair[0], pair[1]);
    writable.write(out.H + 'H' + out.B + 'B\n');
  }
});

function Reader() {
  this.chunk = '';
  this.needle = 0;
}
Reader.prototype.set = function(chunk) {
  this.chunk = this.chunk.slice(this.needle) + chunk;
  this.needle = 0;
};
Reader.prototype.read = function() {
  var curr = this.chunk.indexOf('\n', this.needle);
  var rv = this.chunk.slice(this.needle, curr);
  this.needle = curr + 1;
  return rv;
};

function rate(secret, answer) {
  return answer.split('').reduce(function(rv, num, j) {
    if (num == secret[j]) {
      ++rv.H;
    } else if (~secret.indexOf(num)) {
      ++rv.B;
    }
    return rv;
  }, {H: 0, B: 0});
}