module.exports = RarityMap

/**
 * Mapping of torrent pieces to their respective availability in the swarm. Used by
 * the torrent manager for implementing the rarest piece first selection strategy.
 *
 * @param {Swarm}  swarm bittorrent-swarm to track availability
 * @param {number} numPieces number of pieces in the torrent
 */
function RarityMap (swarm, numPieces) {
  var self = this

  self.pieces = []
  self.swarm = swarm
  self.numPieces = numPieces

  function initWire (wire) {
    wire.on('have', function (index) {
      self.pieces[index] += 1
    })
    wire.on('bitfield', function () {
      self.recalculate()
    })
    wire.on('close', function () {
      for (var i = 0; i < self.numPieces; ++i) {
        self.pieces[i] -= wire.peerPieces.get(i)
      }
    })
  }

  self.swarm.wires.forEach(initWire)
  self.swarm.on('wire', function (wire) {
    self.recalculate()
    initWire(wire)
  })

  self.recalculate()
}

/**
 * Recalculates piece availability across all peers in the swarm.
 */
RarityMap.prototype.recalculate = function () {
  var self = this

  for (var i = 0; i < self.numPieces; ++i) {
    self.pieces[i] = 0
  }

  self.swarm.wires.forEach(function (wire) {
    for (var i = 0; i < self.numPieces; ++i) {
      self.pieces[i] += wire.peerPieces.get(i)
    }
  })
}

/**
 * Get the index of the rarest piece. Optionally, pass a filter function to exclude
 * certain pieces (for instance, those that we already have).
 *
 * @param {function} pieceFilterFunc
 * @return {number} index of rarest piece, or -1
 */
RarityMap.prototype.getRarestPiece = function (pieceFilterFunc) {
  var self = this
  var candidates = []
  var min = Infinity
  pieceFilterFunc = pieceFilterFunc || function () { return true }

  for (var i = 0; i < self.numPieces; ++i) {
    if (!pieceFilterFunc(i)) continue

    var availability = self.pieces[i]
    if (availability === min) {
      candidates.push(i)
    } else if (availability < min) {
      candidates = [ i ]
      min = availability
    }
  }

  if (candidates.length > 0) {
    // if there are multiple pieces with the same availability, choose one randomly
    return candidates[Math.random() * candidates.length | 0]
  } else {
    return -1
  }
}