diff --git a/lib/rarity-map.js b/lib/rarity-map.js
index 430f62f8..76307344 100644
--- a/lib/rarity-map.js
+++ b/lib/rarity-map.js
@@ -1,121 +1,108 @@
-module.exports = RarityMap
 
 /**
  * Mapping of torrent pieces to their respective availability in the torrent swarm. Used
  * by the torrent manager for implementing the rarest piece first selection strategy.
  */
-function RarityMap (torrent) {
-  var self = this
-
-  self._torrent = torrent
-  self._numPieces = torrent.pieces.length
-  self._pieces = []
+class RarityMap {
+  constructor (torrent) {
+    this._torrent = torrent
+    this._numPieces = torrent.pieces.length
+    this._pieces = new Array(this._numPieces)
+
+    this._onWire = wire => {
+      this.recalculate()
+      this._initWire(wire)
+    }
+    this._onWireHave = index => {
+      this._pieces[index] += 1
+    }
+    this._onWireBitfield = () => {
+      this.recalculate()
+    }
 
-  self._onWire = function (wire) {
-    self.recalculate()
-    self._initWire(wire)
-  }
-  self._onWireHave = function (index) {
-    self._pieces[index] += 1
-  }
-  self._onWireBitfield = function () {
-    self.recalculate()
+    this._torrent.wires.forEach(wire => {
+      this._initWire(wire)
+    })
+    this._torrent.on('wire', this._onWire)
+    this.recalculate()
   }
 
-  self._torrent.wires.forEach(function (wire) {
-    self._initWire(wire)
-  })
-  self._torrent.on('wire', self._onWire)
-  self.recalculate()
-}
-
-/**
- * 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) {
-  if (!pieceFilterFunc) pieceFilterFunc = trueFn
-
-  var candidates = []
-  var min = Infinity
-
-  for (var i = 0; i < this._numPieces; ++i) {
-    if (!pieceFilterFunc(i)) continue
+  /**
+   * 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
+   */
+  getRarestPiece (pieceFilterFunc) {
+    let candidates = []
+    let min = Infinity
+
+    for (let i = 0; i < this._numPieces; ++i) {
+      if (pieceFilterFunc && !pieceFilterFunc(i)) continue
+
+      const availability = this._pieces[i]
+      if (availability === min) {
+        candidates.push(i)
+      } else if (availability < min) {
+        candidates = [ i ]
+        min = availability
+      }
+    }
 
-    var availability = this._pieces[i]
-    if (availability === min) {
-      candidates.push(i)
-    } else if (availability < min) {
-      candidates = [ i ]
-      min = availability
+    if (candidates.length) {
+      // if there are multiple pieces with the same availability, choose one randomly
+      return candidates[Math.random() * candidates.length | 0]
+    } else {
+      return -1
     }
   }
 
-  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
+  destroy () {
+    this._torrent.removeListener('wire', this._onWire)
+    this._torrent.wires.forEach(wire => {
+      this._cleanupWireEvents(wire)
+    })
+    this._torrent = null
+    this._pieces = null
+
+    this._onWire = null
+    this._onWireHave = null
+    this._onWireBitfield = null
   }
-}
 
-RarityMap.prototype.destroy = function () {
-  var self = this
-  self._torrent.removeListener('wire', self._onWire)
-  self._torrent.wires.forEach(function (wire) {
-    self._cleanupWireEvents(wire)
-  })
-  self._torrent = null
-  self._pieces = null
-
-  self._onWire = null
-  self._onWireHave = null
-  self._onWireBitfield = null
-}
-
-RarityMap.prototype._initWire = function (wire) {
-  var self = this
-
-  wire._onClose = function () {
-    self._cleanupWireEvents(wire)
-    for (var i = 0; i < this._numPieces; ++i) {
-      self._pieces[i] -= wire.peerPieces.get(i)
+  _initWire (wire) {
+    wire._onClose = () => {
+      this._cleanupWireEvents(wire)
+      for (let i = 0; i < this._numPieces; ++i) {
+        this._pieces[i] -= wire.peerPieces.get(i)
+      }
     }
-  }
-
-  wire.on('have', self._onWireHave)
-  wire.on('bitfield', self._onWireBitfield)
-  wire.once('close', wire._onClose)
-}
 
-/**
- * Recalculates piece availability across all peers in the torrent.
- */
-RarityMap.prototype.recalculate = function () {
-  var i
-  for (i = 0; i < this._numPieces; ++i) {
-    this._pieces[i] = 0
+    wire.on('have', this._onWireHave)
+    wire.on('bitfield', this._onWireBitfield)
+    wire.once('close', wire._onClose)
   }
 
-  var numWires = this._torrent.wires.length
-  for (i = 0; i < numWires; ++i) {
-    var wire = this._torrent.wires[i]
-    for (var j = 0; j < this._numPieces; ++j) {
-      this._pieces[j] += wire.peerPieces.get(j)
+  /**
+   * Recalculates piece availability across all peers in the torrent.
+   */
+  recalculate () {
+    this._pieces.fill(0)
+
+    for (const wire of this._torrent.wires) {
+      for (let i = 0; i < this._numPieces; ++i) {
+        this._pieces[i] += wire.peerPieces.get(i)
+      }
     }
   }
-}
 
-RarityMap.prototype._cleanupWireEvents = function (wire) {
-  wire.removeListener('have', this._onWireHave)
-  wire.removeListener('bitfield', this._onWireBitfield)
-  if (wire._onClose) wire.removeListener('close', wire._onClose)
-  wire._onClose = null
+  _cleanupWireEvents (wire) {
+    wire.removeListener('have', this._onWireHave)
+    wire.removeListener('bitfield', this._onWireBitfield)
+    if (wire._onClose) wire.removeListener('close', wire._onClose)
+    wire._onClose = null
+  }
 }
 
-function trueFn () {
-  return true
-}
+module.exports = RarityMap