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// Copyright 2016 The go-ethereum Authors
// This file is part of the go-ethereum library.
//
// The go-ethereum library is free software: you can redistribute it and/or modify
// it under the terms of the GNU Lesser General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// The go-ethereum library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU Lesser General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public License
// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
// Package les implements the Light Ethereum Subprotocol.
package les
import (
"math/rand"
)
// wrsItem interface should be implemented by any entries that are to be selected from
// a weightedRandomSelect set. Note that recalculating monotonously decreasing item
// weights on-demand (without constantly calling update) is allowed
type wrsItem interface {
Weight() int64
}
// weightedRandomSelect is capable of weighted random selection from a set of items
type weightedRandomSelect struct {
root *wrsNode
idx map[wrsItem]int
}
// newWeightedRandomSelect returns a new weightedRandomSelect structure
func newWeightedRandomSelect() *weightedRandomSelect {
return &weightedRandomSelect{root: &wrsNode{maxItems: wrsBranches}, idx: make(map[wrsItem]int)}
}
// update updates an item's weight, adds it if it was non-existent or removes it if
// the new weight is zero. Note that explicitly updating decreasing weights is not necessary.
func (w *weightedRandomSelect) update(item wrsItem) {
w.setWeight(item, item.Weight())
}
// remove removes an item from the set
func (w *weightedRandomSelect) remove(item wrsItem) {
w.setWeight(item, 0)
}
// setWeight sets an item's weight to a specific value (removes it if zero)
func (w *weightedRandomSelect) setWeight(item wrsItem, weight int64) {
idx, ok := w.idx[item]
if ok {
w.root.setWeight(idx, weight)
if weight == 0 {
delete(w.idx, item)
}
} else {
if weight != 0 {
if w.root.itemCnt == w.root.maxItems {
// add a new level
newRoot := &wrsNode{sumWeight: w.root.sumWeight, itemCnt: w.root.itemCnt, level: w.root.level + 1, maxItems: w.root.maxItems * wrsBranches}
newRoot.items[0] = w.root
newRoot.weights[0] = w.root.sumWeight
w.root = newRoot
}
w.idx[item] = w.root.insert(item, weight)
}
}
}
// choose randomly selects an item from the set, with a chance proportional to its
// current weight. If the weight of the chosen element has been decreased since the
// last stored value, returns it with a newWeight/oldWeight chance, otherwise just
// updates its weight and selects another one
func (w *weightedRandomSelect) choose() wrsItem {
for {
if w.root.sumWeight == 0 {
return nil
}
val := rand.Int63n(w.root.sumWeight)
choice, lastWeight := w.root.choose(val)
weight := choice.Weight()
if weight != lastWeight {
w.setWeight(choice, weight)
}
if weight >= lastWeight || rand.Int63n(lastWeight) < weight {
return choice
}
}
}
const wrsBranches = 8 // max number of branches in the wrsNode tree
// wrsNode is a node of a tree structure that can store wrsItems or further wrsNodes.
type wrsNode struct {
items [wrsBranches]interface{}
weights [wrsBranches]int64
sumWeight int64
level, itemCnt, maxItems int
}
// insert recursively inserts a new item to the tree and returns the item index
func (n *wrsNode) insert(item wrsItem, weight int64) int {
branch := 0
for n.items[branch] != nil && (n.level == 0 || n.items[branch].(*wrsNode).itemCnt == n.items[branch].(*wrsNode).maxItems) {
branch++
if branch == wrsBranches {
panic(nil)
}
}
n.itemCnt++
n.sumWeight += weight
n.weights[branch] += weight
if n.level == 0 {
n.items[branch] = item
return branch
} else {
var subNode *wrsNode
if n.items[branch] == nil {
subNode = &wrsNode{maxItems: n.maxItems / wrsBranches, level: n.level - 1}
n.items[branch] = subNode
} else {
subNode = n.items[branch].(*wrsNode)
}
subIdx := subNode.insert(item, weight)
return subNode.maxItems*branch + subIdx
}
}
// setWeight updates the weight of a certain item (which should exist) and returns
// the change of the last weight value stored in the tree
func (n *wrsNode) setWeight(idx int, weight int64) int64 {
if n.level == 0 {
oldWeight := n.weights[idx]
n.weights[idx] = weight
diff := weight - oldWeight
n.sumWeight += diff
if weight == 0 {
n.items[idx] = nil
n.itemCnt--
}
return diff
}
branchItems := n.maxItems / wrsBranches
branch := idx / branchItems
diff := n.items[branch].(*wrsNode).setWeight(idx-branch*branchItems, weight)
n.weights[branch] += diff
n.sumWeight += diff
if weight == 0 {
n.itemCnt--
}
return diff
}
// choose recursively selects an item from the tree and returns it along with its weight
func (n *wrsNode) choose(val int64) (wrsItem, int64) {
for i, w := range n.weights {
if val < w {
if n.level == 0 {
return n.items[i].(wrsItem), n.weights[i]
} else {
return n.items[i].(*wrsNode).choose(val)
}
} else {
val -= w
}
}
panic(nil)
}
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