var pull = require('pull-stream')
var cat = require('pull-cat')
var loop = require('looper')
var multicb = require('multicb')
var crypto = require('crypto')
var skipFooter = require('pull-skip-footer')
var packidx = require('pull-git-packidx-parser')
var blockFilter = require('pull-block-filter')

function packHeader(numObjects) {
  var header = new Buffer(12)
  header.write('PACK')
  header.writeUInt32BE(2, 4)
  header.writeUInt32BE(numObjects, 8)
  return header
}

function forEachAsync(arr, fn, cb) {
  var i = 0
  loop(function (next) {
    if (i >= arr.length) return cb && cb()
    fn(arr[i++], function (err) {
      if (err) return cb && cb(err)
      next()
    })
  })
}

function compareByOffset(a, b) {
  return a.offset - b.offset
}

function dedupPacks(packs, cb) {
  var seen = {}
  var numObjects = 0
  forEachAsync(packs, function (pack, cb) {
    return pull(pack.readIdx, packidx(function (err, idx) {
      if (err) return cb(err)
      var blocks = []
      var lastBlock
      offset = 0
      var objs = idx.objects.sort(compareByOffset)
      for (var i = 0; i < objs.length; i++) {
        var obj = objs[i]
        var id = obj.oid.toString('hex')
        if (seen[id]) continue
        seen[id] = true
        numObjects++
        if (obj.offset > offset) {
          blocks.push(lastBlock = {skip: obj.offset - offset, length: 0})
          offset = obj.offset
        } else if (obj.offset < offset) {
          return cb(new Error('bad offset'))
        }
        var len = obj.next ? obj.next.offset - obj.offset : Infinity
        lastBlock.length += len
        offset += len
      }
      pack.read = pull(
        pack.read,
        skipFooter(20),
        blockFilter(pull.values(blocks))
      )
      cb()
    }))
  }, function (err) {
    cb(err, numObjects)
  })
}

function closePacks(packs, cb) {
  forEachAsync(packs, function (pack, cb) {
    pack.read(true, cb)
  }, cb)
}

module.exports = function concatPacks(packs) {
  /* packs: [{read: source, readIdx: source}] */
  if (packs.length === 1) return packs[0].read

  var checksum = crypto.createHash('sha1')
  var packI = 0
  var state = 'begin'

  return function next(end, cb) {
    switch (state) {
      case 'begin':
        if (end) return closePacks(cb)
        return dedupPacks(packs, function (err, numObjects) {
          if (err) return cb(err)
          var header = packHeader(numObjects)
          checksum.update(header)
          state = 'payload'
          cb(null, header)
        })

      case 'payload':
        if (end) return closePacks(cb)
        if (packI >= packs.length) {
          state = 'end' 
          return cb(null, checksum.digest())
        }
        var pack = packs[packI]
        return pack.read(null, function (err, data) {
          if (err === true) {
            packI++
            return next(null, cb)
          }
          if (err) return cb(err)
          checksum.update(data)
          cb(null, data)
        })

      case 'end':
        return cb(true)
    }
  }
}