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/*
* Copyright (c) 2014, Yawning Angel <yawning at torproject dot org>
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions are met:
*
* * Redistributions of source code must retain the above copyright notice,
* this list of conditions and the following disclaimer.
*
* * Redistributions in binary form must reproduce the above copyright notice,
* this list of conditions and the following disclaimer in the documentation
* and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
* AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
* ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
* LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
* CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
* SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
* INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
* CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
* ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
* POSSIBILITY OF SUCH DAMAGE.
*/
package obfs4
import (
"fmt"
"math/rand"
"github.com/yawning/obfs4/csrand"
"github.com/yawning/obfs4/drbg"
)
const (
minBuckets = 1
maxBuckets = 100
)
// wDist is a weighted distribution.
type wDist struct {
minValue int
maxValue int
values []int
buckets []int64
totalWeight int64
rng *rand.Rand
}
// newWDist creates a weighted distribution of values ranging from min to max
// based on a HashDrbg initialized with seed.
func newWDist(seed *drbg.Seed, min, max int) (w *wDist) {
w = new(wDist)
w.minValue = min
w.maxValue = max
if max <= min {
panic(fmt.Sprintf("wDist.Reset(): min >= max (%d, %d)", min, max))
}
w.reset(seed)
return
}
// sample generates a random value according to the distribution.
func (w *wDist) sample() int {
retIdx := 0
var totalWeight int64
weight := csrand.Int63n(w.totalWeight)
for i, bucketWeight := range w.buckets {
totalWeight += bucketWeight
if weight <= totalWeight {
retIdx = i
break
}
}
return w.minValue + w.values[retIdx]
}
// reset generates a new distribution with the same min/max based on a new seed.
func (w *wDist) reset(seed *drbg.Seed) {
// Initialize the deterministic random number generator.
drbg := drbg.NewHashDrbg(seed)
w.rng = rand.New(drbg)
nBuckets := (w.maxValue + 1) - w.minValue
w.values = w.rng.Perm(nBuckets)
if nBuckets < minBuckets {
nBuckets = minBuckets
}
if nBuckets > maxBuckets {
nBuckets = maxBuckets
}
nBuckets = w.rng.Intn(nBuckets) + 1
w.totalWeight = 0
w.buckets = make([]int64, nBuckets)
for i, _ := range w.buckets {
prob := w.rng.Int63n(1000)
w.buckets[i] = prob
w.totalWeight += prob
}
w.buckets[len(w.buckets)-1] = w.totalWeight
}
/* vim :set ts=4 sw=4 sts=4 noet : */
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