block-iterator.js

var fs = require('fs')


var pull = require('pull-stream')

var Iterator =
pull.Source(function (next, close) {
  var i = 0
  close = close || function (cb) { cb() }
  return function (end, cb) {
    if(end)
      close ? close(function (err) {
        cb(err || true)
      }) : cb && cb(end)
    else
      next(i++, function (err, data) {
        if(err || data == null) {
          close(function (err) {
            cb(err || true)
          })
        } else
          cb(null, data) //means end in an iterator.
      })
  }
})

function calcBlock(stat, opts, i) {
  opts = opts || {}
  if(opts.offset)
    i = i + opts.offset

  var blocks = Math.floor(stat.size / stat.blksize)
  var j = opts.reverse ? blocks - i : i
  var start = j * stat.blksize
  var limit = opts.limit ? opts.limit - 1 : blocks
  if(i > Math.min(blocks, limit)) return false
  return {
    length: (
      (j + 1) * stat.blksize > stat.size
      ? stat.size - j * stat.blksize
      : stat.blksize
    ),
    position: start,
    size    : stat.size,
    i       : j,
    end     : i > blocks
  }
}

var blockIterator = module.exports = function (stat, opts) {
  opts = opts || {}
  var blocks = Math.floor(stat.size / stat.blksize)

  return Iterator(function (i, cb) {
    var range = calcBlock(stat, opts, i)

    if(!range) return cb(null, null)
    //this is not good memory management!
    var block = new Buffer(range.length)
    fs.read(stat.fd, block, 0, range.length, range.position, function (err) {
      cb(err, block.toString())
    })
  })
}

module.exports.calcBlock = calcBlock