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343 lines
6.9 KiB
343 lines
6.9 KiB
package clist
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/*
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The purpose of CList is to provide a goroutine-safe linked-list.
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This list can be traversed concurrently by any number of goroutines.
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However, removed CElements cannot be added back.
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NOTE: Not all methods of container/list are (yet) implemented.
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NOTE: Removed elements need to DetachPrev or DetachNext consistently
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to ensure garbage collection of removed elements.
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*/
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import (
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"sync"
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)
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/*
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CElement is an element of a linked-list
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Traversal from a CElement is goroutine-safe.
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We can't avoid using WaitGroups or for-loops given the documentation
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spec without re-implementing the primitives that already exist in
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golang/sync. Notice that WaitGroup allows many go-routines to be
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simultaneously released, which is what we want. Mutex doesn't do
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this. RWMutex does this, but it's clumsy to use in the way that a
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WaitGroup would be used -- and we'd end up having two RWMutex's for
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prev/next each, which is doubly confusing.
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sync.Cond would be sort-of useful, but we don't need a write-lock in
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the for-loop. Use sync.Cond when you need serial access to the
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"condition". In our case our condition is if `next != nil || removed`,
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and there's no reason to serialize that condition for goroutines
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waiting on NextWait() (since it's just a read operation).
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*/
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type CElement struct {
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mtx sync.RWMutex
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prev *CElement
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prevWg *sync.WaitGroup
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next *CElement
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nextWg *sync.WaitGroup
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removed bool
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Value interface{} // immutable
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}
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// Blocking implementation of Next().
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// May return nil iff CElement was tail and got removed.
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func (e *CElement) NextWait() *CElement {
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for {
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e.mtx.RLock()
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next := e.next
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nextWg := e.nextWg
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removed := e.removed
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e.mtx.RUnlock()
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if next != nil || removed {
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return next
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}
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nextWg.Wait()
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// e.next doesn't necessarily exist here.
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// That's why we need to continue a for-loop.
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}
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}
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// Blocking implementation of Prev().
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// May return nil iff CElement was head and got removed.
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func (e *CElement) PrevWait() *CElement {
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for {
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e.mtx.RLock()
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prev := e.prev
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prevWg := e.prevWg
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removed := e.removed
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e.mtx.RUnlock()
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if prev != nil || removed {
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return prev
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}
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prevWg.Wait()
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}
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}
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// Nonblocking, may return nil if at the end.
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func (e *CElement) Next() *CElement {
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e.mtx.RLock()
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defer e.mtx.RUnlock()
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return e.next
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}
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// Nonblocking, may return nil if at the end.
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func (e *CElement) Prev() *CElement {
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e.mtx.RLock()
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defer e.mtx.RUnlock()
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return e.prev
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}
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func (e *CElement) Removed() bool {
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e.mtx.RLock()
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defer e.mtx.RUnlock()
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return e.removed
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}
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func (e *CElement) DetachNext() {
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if !e.Removed() {
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panic("DetachNext() must be called after Remove(e)")
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}
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e.mtx.Lock()
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defer e.mtx.Unlock()
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e.next = nil
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}
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func (e *CElement) DetachPrev() {
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if !e.Removed() {
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panic("DetachPrev() must be called after Remove(e)")
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}
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e.mtx.Lock()
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defer e.mtx.Unlock()
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e.prev = nil
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}
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// NOTE: This function needs to be safe for
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// concurrent goroutines waiting on nextWg.
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func (e *CElement) SetNext(newNext *CElement) {
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e.mtx.Lock()
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defer e.mtx.Unlock()
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oldNext := e.next
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e.next = newNext
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if oldNext != nil && newNext == nil {
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// See https://golang.org/pkg/sync/:
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//
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// If a WaitGroup is reused to wait for several independent sets of
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// events, new Add calls must happen after all previous Wait calls have
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// returned.
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e.nextWg = waitGroup1() // WaitGroups are difficult to re-use.
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}
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if oldNext == nil && newNext != nil {
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e.nextWg.Done()
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}
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}
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// NOTE: This function needs to be safe for
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// concurrent goroutines waiting on prevWg
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func (e *CElement) SetPrev(newPrev *CElement) {
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e.mtx.Lock()
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defer e.mtx.Unlock()
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oldPrev := e.prev
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e.prev = newPrev
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if oldPrev != nil && newPrev == nil {
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e.prevWg = waitGroup1() // WaitGroups are difficult to re-use.
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}
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if oldPrev == nil && newPrev != nil {
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e.prevWg.Done()
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}
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}
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func (e *CElement) SetRemoved() {
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e.mtx.Lock()
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defer e.mtx.Unlock()
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e.removed = true
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// This wakes up anyone waiting in either direction.
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if e.prev == nil {
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e.prevWg.Done()
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}
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if e.next == nil {
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e.nextWg.Done()
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}
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}
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//--------------------------------------------------------------------------------
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// CList represents a linked list.
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// The zero value for CList is an empty list ready to use.
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// Operations are goroutine-safe.
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type CList struct {
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mtx sync.RWMutex
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wg *sync.WaitGroup
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head *CElement // first element
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tail *CElement // last element
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len int // list length
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}
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func (l *CList) Init() *CList {
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l.mtx.Lock()
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defer l.mtx.Unlock()
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l.wg = waitGroup1()
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l.head = nil
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l.tail = nil
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l.len = 0
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return l
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}
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func New() *CList { return new(CList).Init() }
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func (l *CList) Len() int {
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l.mtx.RLock()
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defer l.mtx.RUnlock()
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return l.len
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}
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func (l *CList) Front() *CElement {
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l.mtx.RLock()
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defer l.mtx.RUnlock()
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return l.head
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}
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func (l *CList) FrontWait() *CElement {
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// Loop until the head is non-nil else wait and try again
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for {
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l.mtx.RLock()
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head := l.head
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wg := l.wg
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l.mtx.RUnlock()
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if head != nil {
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return head
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}
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wg.Wait()
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// NOTE: If you think l.head exists here, think harder.
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}
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}
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func (l *CList) Back() *CElement {
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l.mtx.RLock()
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defer l.mtx.RUnlock()
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return l.tail
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}
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func (l *CList) BackWait() *CElement {
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for {
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l.mtx.RLock()
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tail := l.tail
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wg := l.wg
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l.mtx.RUnlock()
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if tail != nil {
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return tail
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}
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wg.Wait()
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// l.tail doesn't necessarily exist here.
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// That's why we need to continue a for-loop.
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}
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}
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func (l *CList) PushBack(v interface{}) *CElement {
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l.mtx.Lock()
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defer l.mtx.Unlock()
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// Construct a new element
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e := &CElement{
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prev: nil,
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prevWg: waitGroup1(),
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next: nil,
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nextWg: waitGroup1(),
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removed: false,
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Value: v,
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}
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// Release waiters on FrontWait/BackWait maybe
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if l.len == 0 {
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l.wg.Done()
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}
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l.len += 1
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// Modify the tail
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if l.tail == nil {
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l.head = e
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l.tail = e
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} else {
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e.SetPrev(l.tail) // We must init e first.
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l.tail.SetNext(e) // This will make e accessible.
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l.tail = e // Update the list.
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}
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return e
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}
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// CONTRACT: Caller must call e.DetachPrev() and/or e.DetachNext() to avoid memory leaks.
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// NOTE: As per the contract of CList, removed elements cannot be added back.
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func (l *CList) Remove(e *CElement) interface{} {
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l.mtx.Lock()
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defer l.mtx.Unlock()
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prev := e.Prev()
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next := e.Next()
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if l.head == nil || l.tail == nil {
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panic("Remove(e) on empty CList")
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}
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if prev == nil && l.head != e {
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panic("Remove(e) with false head")
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}
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if next == nil && l.tail != e {
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panic("Remove(e) with false tail")
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}
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// If we're removing the only item, make CList FrontWait/BackWait wait.
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if l.len == 1 {
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l.wg = waitGroup1() // WaitGroups are difficult to re-use.
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}
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// Update l.len
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l.len -= 1
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// Connect next/prev and set head/tail
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if prev == nil {
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l.head = next
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} else {
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prev.SetNext(next)
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}
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if next == nil {
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l.tail = prev
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} else {
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next.SetPrev(prev)
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}
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// Set .Done() on e, otherwise waiters will wait forever.
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e.SetRemoved()
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return e.Value
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}
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func waitGroup1() (wg *sync.WaitGroup) {
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wg = &sync.WaitGroup{}
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wg.Add(1)
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return
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}
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