audioaxis/updates.go

package main

import (
	"time"
	"sync"
)

const (
	addAction      = 0
	removeAction   = 1
	moveUpAction   = 2
	moveDownAction = 3
)
type Update struct {
	Song *Song
	Action uint
	Timestamp int64
	Next *Update
}
type Listener struct {
	L chan bool
	Next *Listener
}

var headUpdates map[int]*Update
var tailUpdates map[int]*Update
var listeners map[int]*Listener
var updateLock sync.Mutex

func init() {
	headUpdates = make(map[int]*Update)
	tailUpdates = make(map[int]*Update)
	listeners = make(map[int]*Listener)
}

func addUpdate(pid int, action uint, song *Song) {
	update := new(Update)
	update.Song = song
	update.Action = action
	update.Timestamp = time.Nanoseconds()

	updateLock.Lock()
	defer updateLock.Unlock()

	// write new update
	pup := tailUpdates[pid]
	if pup != nil {
		pup.Next = update
	} else {
		headUpdates[pid] = update
	}
	tailUpdates[pid] = update

	// expire old updates
	const expiryTime = 1e9 * 3 * 60 // 3 minutes
	pup = headUpdates[pid]
	for pup != nil && pup.Timestamp < update.Timestamp - expiryTime {
		pup = pup.Next
		headUpdates[pid] = pup
	}

	// notify listeners
	listener := listeners[pid]
	for listener != nil {
		listener.L <- true
		listener = listener.Next
	}
	listeners[pid] = nil
}

// assumes caller has updateLock
func checkUpdates(pid int, timestamp int64) *Update {
	pup, _ := headUpdates[pid]
	for pup != nil {
		if pup.Timestamp > timestamp {
			return pup
		}
		pup = pup.Next
	}
	return nil
}

func getUpdates(id string, timestamp int64) *Update {
	db := <-dbPool
	pid := getpid(db, id)
	dbPool <- db
	if pid == -1 {
		return nil
	}

	updateLock.Lock()
	pup := checkUpdates(pid, timestamp)
	if pup != nil {
		updateLock.Unlock()
		return pup
	}

	// didn't get updates
	listener := new(Listener)
	listener.L = make(chan bool)
	lhead := listeners[pid]
	if lhead != nil {
		listener.Next = lhead
	}
	listeners[pid] = listener
	updateLock.Unlock()
	<-listener.L

	updateLock.Lock()
	pup = checkUpdates(pid, timestamp)
	updateLock.Unlock()
	return pup
}