github.com/uber/kraken@v0.1.4/lib/torrent/scheduler/dispatch/piecerequest/default_policy.go (about)

     1  // Copyright (c) 2016-2019 Uber Technologies, Inc.
     2  //
     3  // Licensed under the Apache License, Version 2.0 (the "License");
     4  // you may not use this file except in compliance with the License.
     5  // You may obtain a copy of the License at
     6  //
     7  //     http://www.apache.org/licenses/LICENSE-2.0
     8  //
     9  // Unless required by applicable law or agreed to in writing, software
    10  // distributed under the License is distributed on an "AS IS" BASIS,
    11  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    12  // See the License for the specific language governing permissions and
    13  // limitations under the License.
    14  package piecerequest
    15  
    16  import (
    17  	"math/rand"
    18  
    19  	"github.com/uber/kraken/utils/syncutil"
    20  
    21  	"github.com/willf/bitset"
    22  )
    23  
    24  // DefaultPolicy randomly selects pieces to request.
    25  const DefaultPolicy = "default"
    26  
    27  type defaultPolicy struct{}
    28  
    29  func newDefaultPolicy() *defaultPolicy {
    30  	return &defaultPolicy{}
    31  }
    32  
    33  func (p *defaultPolicy) selectPieces(
    34  	limit int,
    35  	valid func(int) bool,
    36  	candidates *bitset.BitSet,
    37  	numPeersByPiece syncutil.Counters) ([]int, error) {
    38  
    39  	pieces := make([]int, 0, limit)
    40  	if limit == 0 {
    41  		return pieces, nil
    42  	}
    43  
    44  	// Reservoir sampling.
    45  	var k int
    46  	for i, e := candidates.NextSet(0); e; i, e = candidates.NextSet(i + 1) {
    47  		if !valid(int(i)) {
    48  			continue
    49  		}
    50  
    51  		// Fill the 'reservoir' until full.
    52  		if len(pieces) < limit {
    53  			pieces = append(pieces, int(i))
    54  
    55  			// Replace elements in the 'reservoir' with decreasing probability.
    56  		} else {
    57  			j := rand.Intn(k)
    58  			if j < limit {
    59  				pieces[j] = int(i)
    60  			}
    61  		}
    62  		k++
    63  	}
    64  
    65  	return pieces, nil
    66  }