Official Go implementation of the Ethereum protocol
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go-ethereum/common/prque/lazyqueue.go

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// Copyright 2019 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 prque
import (
"container/heap"
"time"
"github.com/ethereum/go-ethereum/common/mclock"
"golang.org/x/exp/constraints"
)
// LazyQueue is a priority queue data structure where priorities can change over
// time and are only evaluated on demand.
// Two callbacks are required:
// - priority evaluates the actual priority of an item
// - maxPriority gives an upper estimate for the priority in any moment between
// now and the given absolute time
//
// If the upper estimate is exceeded then Update should be called for that item.
// A global Refresh function should also be called periodically.
type LazyQueue[P constraints.Ordered, V any] struct {
clock mclock.Clock
// Items are stored in one of two internal queues ordered by estimated max
// priority until the next and the next-after-next refresh. Update and Refresh
// always places items in queue[1].
queue [2]*sstack[P, V]
popQueue *sstack[P, V]
period time.Duration
maxUntil mclock.AbsTime
indexOffset int
setIndex SetIndexCallback[V]
priority PriorityCallback[P, V]
maxPriority MaxPriorityCallback[P, V]
lastRefresh1, lastRefresh2 mclock.AbsTime
}
type (
PriorityCallback[P constraints.Ordered, V any] func(data V) P // actual priority callback
MaxPriorityCallback[P constraints.Ordered, V any] func(data V, until mclock.AbsTime) P // estimated maximum priority callback
)
// NewLazyQueue creates a new lazy queue
func NewLazyQueue[P constraints.Ordered, V any](setIndex SetIndexCallback[V], priority PriorityCallback[P, V], maxPriority MaxPriorityCallback[P, V], clock mclock.Clock, refreshPeriod time.Duration) *LazyQueue[P, V] {
q := &LazyQueue[P, V]{
popQueue: newSstack[P, V](nil),
setIndex: setIndex,
priority: priority,
maxPriority: maxPriority,
clock: clock,
period: refreshPeriod,
lastRefresh1: clock.Now(),
lastRefresh2: clock.Now(),
}
q.Reset()
q.refresh(clock.Now())
return q
}
// Reset clears the contents of the queue
func (q *LazyQueue[P, V]) Reset() {
q.queue[0] = newSstack[P, V](q.setIndex0)
q.queue[1] = newSstack[P, V](q.setIndex1)
}
// Refresh performs queue re-evaluation if necessary
func (q *LazyQueue[P, V]) Refresh() {
now := q.clock.Now()
for time.Duration(now-q.lastRefresh2) >= q.period*2 {
q.refresh(now)
q.lastRefresh2 = q.lastRefresh1
q.lastRefresh1 = now
}
}
// refresh re-evaluates items in the older queue and swaps the two queues
func (q *LazyQueue[P, V]) refresh(now mclock.AbsTime) {
q.maxUntil = now.Add(q.period)
for q.queue[0].Len() != 0 {
q.Push(heap.Pop(q.queue[0]).(*item[P, V]).value)
}
q.queue[0], q.queue[1] = q.queue[1], q.queue[0]
q.indexOffset = 1 - q.indexOffset
q.maxUntil = q.maxUntil.Add(q.period)
}
// Push adds an item to the queue
func (q *LazyQueue[P, V]) Push(data V) {
heap.Push(q.queue[1], &item[P, V]{data, q.maxPriority(data, q.maxUntil)})
}
// Update updates the upper priority estimate for the item with the given queue index
func (q *LazyQueue[P, V]) Update(index int) {
q.Push(q.Remove(index))
}
// Pop removes and returns the item with the greatest actual priority
func (q *LazyQueue[P, V]) Pop() (V, P) {
var (
resData V
resPri P
)
q.MultiPop(func(data V, priority P) bool {
resData = data
resPri = priority
return false
})
return resData, resPri
}
// peekIndex returns the index of the internal queue where the item with the
// highest estimated priority is or -1 if both are empty
func (q *LazyQueue[P, V]) peekIndex() int {
if q.queue[0].Len() != 0 {
if q.queue[1].Len() != 0 && q.queue[1].blocks[0][0].priority > q.queue[0].blocks[0][0].priority {
return 1
}
return 0
}
if q.queue[1].Len() != 0 {
return 1
}
return -1
}
// MultiPop pops multiple items from the queue and is more efficient than calling
// Pop multiple times. Popped items are passed to the callback. MultiPop returns
// when the callback returns false or there are no more items to pop.
func (q *LazyQueue[P, V]) MultiPop(callback func(data V, priority P) bool) {
nextIndex := q.peekIndex()
for nextIndex != -1 {
data := heap.Pop(q.queue[nextIndex]).(*item[P, V]).value
heap.Push(q.popQueue, &item[P, V]{data, q.priority(data)})
nextIndex = q.peekIndex()
for q.popQueue.Len() != 0 && (nextIndex == -1 || q.queue[nextIndex].blocks[0][0].priority < q.popQueue.blocks[0][0].priority) {
i := heap.Pop(q.popQueue).(*item[P, V])
if !callback(i.value, i.priority) {
for q.popQueue.Len() != 0 {
q.Push(heap.Pop(q.popQueue).(*item[P, V]).value)
}
return
}
nextIndex = q.peekIndex() // re-check because callback is allowed to push items back
}
}
}
// PopItem pops the item from the queue only, dropping the associated priority value.
func (q *LazyQueue[P, V]) PopItem() V {
i, _ := q.Pop()
return i
}
// Remove removes the item with the given index.
func (q *LazyQueue[P, V]) Remove(index int) V {
return heap.Remove(q.queue[index&1^q.indexOffset], index>>1).(*item[P, V]).value
}
// Empty checks whether the priority queue is empty.
func (q *LazyQueue[P, V]) Empty() bool {
return q.queue[0].Len() == 0 && q.queue[1].Len() == 0
}
// Size returns the number of items in the priority queue.
func (q *LazyQueue[P, V]) Size() int {
return q.queue[0].Len() + q.queue[1].Len()
}
// setIndex0 translates internal queue item index to the virtual index space of LazyQueue
func (q *LazyQueue[P, V]) setIndex0(data V, index int) {
if index == -1 {
q.setIndex(data, -1)
} else {
q.setIndex(data, index+index)
}
}
// setIndex1 translates internal queue item index to the virtual index space of LazyQueue
func (q *LazyQueue[P, V]) setIndex1(data V, index int) {
q.setIndex(data, index+index+1)
}