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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.swarm = swarm
self.numPieces = numPieces
function initWire (wire) {
wire.on('have', function (index) {
self.pieces[index]++
})
wire.on('bitfield', self.recalculate.bind(self))
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
self.pieces = []
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
}
}
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