commit 6482410063806e57d1ff0e58462162f17fa83e7a
Author: Tessa Nordgren <tessa@sudo.ca>
Date:   Thu Apr 11 16:21:27 2019 +0200

    initial working release

diff --git a/README.md b/README.md
new file mode 100644
index 0000000..65e4629
--- /dev/null
+++ b/README.md
@@ -0,0 +1,29 @@
+# gtq - the go task queue
+
+## what?
+a simple goroutine scheduler for longer-running goroutines, allowing you
+to put them into priority buckets, and get roughly fair queuing of which
+tasks will be processed next.
+
+## why?
+goroutines are awesome, and typically super fast. but what if you need to
+queue up a lot of longer running work in the background? and what if you
+want some control over which chunks of work will get executed next?
+
+gtq provides simple task queuing and scheduling for longer running goroutines.
+in this case, "long" means on the order of 1ms or longer, as many simple
+goroutines can run in a matter of ns.
+
+## how?
+We use simple heap based queues for each priority level, and track statistics
+in a rolling window for task execution times in each of those. Then we
+have a scheduler which looks at those statistics, in relation to the priority
+level for the queue, and decides which queue to pull from next and feed to task
+runners.
+
+Overhead for this process is typically < 1000ns, so it's not suitable for
+typical short lived goroutines, but those type of goroutines don't really,
+need scheduling anyway.
+
+## usage
+TODO
diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..8a5f97d
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,8 @@
+module github.com/nergdron/gtq
+
+go 1.12
+
+require (
+	gopkg.in/eapache/queue.v1 v1.1.0
+	gopkg.in/yaml.v2 v2.2.2
+)
diff --git a/go.sum b/go.sum
new file mode 100644
index 0000000..ce9e261
--- /dev/null
+++ b/go.sum
@@ -0,0 +1,5 @@
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
+gopkg.in/eapache/queue.v1 v1.1.0 h1:EldqoJEGtXYiVCMRo2C9mePO2UUGnYn2+qLmlQSqPdc=
+gopkg.in/eapache/queue.v1 v1.1.0/go.mod h1:wNtmx1/O7kZSR9zNT1TTOJ7GLpm3Vn7srzlfylFbQwU=
+gopkg.in/yaml.v2 v2.2.2 h1:ZCJp+EgiOT7lHqUV2J862kp8Qj64Jo6az82+3Td9dZw=
+gopkg.in/yaml.v2 v2.2.2/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
diff --git a/queue.go b/queue.go
new file mode 100644
index 0000000..d2d944d
--- /dev/null
+++ b/queue.go
@@ -0,0 +1,71 @@
+package gtq
+
+import (
+	"gopkg.in/eapache/queue.v1"
+	"sync"
+)
+
+// NewQueue initializes and returns an new Queue.
+func NewQueue() (q *Queue) {
+	return &Queue{
+		queue: queue.New(),
+		Done:  NewStat(0),
+	}
+}
+
+// Queue wraps eapache/queue with locking for thread safety. Also bundles in
+// Stats, used by schedulers to help pick tasks to execute.
+type Queue struct {
+	queue *queue.Queue
+	mutex sync.Mutex
+	// Stat which tracks task removal rate from the queue.
+	Done *Stat
+}
+
+// Add puts an element on the end of the queue.
+func (q *Queue) Add(task func()) {
+	q.mutex.Lock()
+	defer q.mutex.Unlock()
+	q.queue.Add(task)
+}
+
+// Get returns the element at index i in the queue.
+// This method accepts both positive and negative index values.
+// Index 0 refers to the first element, and index -1 refers to the last.
+func (q *Queue) Get(i int) (task func()) {
+	q.mutex.Lock()
+	defer q.mutex.Unlock()
+	if (q.queue.Length() < 1 || i > q.queue.Length()-1) || (i < -1) {
+		return nil
+	}
+	return q.queue.Get(i).(func())
+}
+
+// Length returns the number of elements currently stored in the queue.
+func (q *Queue) Length() int {
+	q.mutex.Lock()
+	defer q.mutex.Unlock()
+	return q.queue.Length()
+}
+
+// Peek returns the element at the head of the queue.
+func (q *Queue) Peek() (task func()) {
+	q.mutex.Lock()
+	defer q.mutex.Unlock()
+	if q.Length() < 1 {
+		return nil
+	}
+	return q.queue.Peek().(func())
+}
+
+// Remove removes and returns the element from the front of the queue.
+func (q *Queue) Remove() (task func()) {
+	q.mutex.Lock()
+	defer q.mutex.Unlock()
+	if q.queue.Length() < 1 {
+		return nil
+	}
+	task = q.queue.Remove().(func())
+	q.Done.Add(1)
+	return task
+}
diff --git a/queue_test.go b/queue_test.go
new file mode 100644
index 0000000..63dc120
--- /dev/null
+++ b/queue_test.go
@@ -0,0 +1,87 @@
+package gtq
+
+import (
+	"runtime"
+	"sync"
+	"testing"
+	"time"
+)
+
+var testQ = NewQueue()
+
+func testAdd(n int) {
+	for i := 0; i < n; i++ {
+		testQ.Add(func() {
+			time.Sleep(1)
+		})
+	}
+}
+
+func testRemove(wg *sync.WaitGroup, removed chan int) {
+	defer wg.Done()
+	var i int
+	for testQ.Remove() != nil {
+		i++
+	}
+	removed <- i
+}
+
+func TestParallelQueueAdd(t *testing.T) {
+	max := runtime.GOMAXPROCS(0)
+	var wg sync.WaitGroup
+	for n := 0; n < max; n++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			testAdd(1000000)
+		}()
+	}
+	wg.Wait()
+	t.Log("Queue size is", testQ.Length())
+	targetSize := max * 1000000
+	if testQ.Length() != targetSize {
+		t.Error("Queue size should be", targetSize)
+	}
+}
+
+func TestParallelQueueRemove(t *testing.T) {
+	target := testQ.Length()
+	var wg sync.WaitGroup
+	max := runtime.GOMAXPROCS(0)
+	removed := make(chan int, max)
+	for n := 0; n < max; n++ {
+		wg.Add(1)
+		go testRemove(&wg, removed)
+	}
+	wg.Wait()
+	close(removed)
+	var total int
+	for count := range removed {
+		total += count
+	}
+	if total != target {
+		t.Error("removed", total, "but expected", target)
+	}
+	t.Log("removed", total)
+}
+
+func BenchmarkParallelQueue(b *testing.B) {
+	testQ = NewQueue()
+	b.ResetTimer()
+	max := runtime.GOMAXPROCS(0)
+	var wg1, wg2 sync.WaitGroup
+	removed := make(chan int, max)
+	for i := 0; i < max; i++ {
+		wg1.Add(1)
+		wg2.Add(1)
+		go func() {
+			testAdd(b.N / (max * 2))
+			wg1.Done()
+		}()
+		go func() {
+			testRemove(&wg2, removed)
+		}()
+	}
+	wg1.Wait()
+	wg2.Wait()
+}
diff --git a/scheduler.go b/scheduler.go
new file mode 100644
index 0000000..d8d2a58
--- /dev/null
+++ b/scheduler.go
@@ -0,0 +1,39 @@
+package gtq
+
+import (
+	"sort"
+)
+
+// Scheduler is an internal goroutine which examines the Queues and
+// choses which tasks to run next. Returns true if tasks were scheduled,
+// or false if there's nothing left to schedule.
+type Scheduler func(tq *TaskQueue) bool
+
+// SimpleScheduler is the simplest possible implementation, which just takes
+// tasks off the highest priority queue.
+func SimpleScheduler(tq *TaskQueue) bool {
+	pc := tq.PriorityCounts()
+	// sort priorities
+	prios := make([]uint, 0, len(pc))
+	for k := range pc {
+		prios = append(prios, k)
+	}
+	sort.Sort(UIntSlice(prios))
+
+	var queued uint
+	for _, prio := range prios {
+		q, ok := tq.queues.Load(prio)
+		if !ok {
+			continue
+		}
+		for q.(*Queue).Length() > 0 {
+			task := q.(*Queue).Remove()
+			tq.nextTask <- task
+			queued++
+		}
+	}
+	if queued >= (tq.numJobs * 2) {
+		return true
+	}
+	return false
+}
diff --git a/stats.go b/stats.go
new file mode 100644
index 0000000..988bd0a
--- /dev/null
+++ b/stats.go
@@ -0,0 +1,118 @@
+package gtq
+
+import (
+	"gopkg.in/yaml.v2"
+	"sync"
+	"time"
+)
+
+// Counter is an incrementing statistical value, with a Start time for rate
+// calculation.
+type Counter struct {
+	Start time.Time
+	Value uint64
+}
+
+// Stat for a queue priority level, grouped into timeslice buckets.
+type Stat struct {
+	timeSlices   []Counter
+	currentSlice int
+	window       time.Duration
+	mutex        sync.Mutex
+}
+
+// NewStat initializes and returns a new Stat. window determines
+// the size of the rolling statistics timeframe that is captured,
+// which in turn determines how finely grained stats collection and task
+// scheduling can be. window defaults to 1 second.
+func NewStat(window time.Duration) (s *Stat) {
+	s = &Stat{window: window}
+	if s.window == 0 {
+		s.window = time.Second
+	}
+	// default to 10 timeslice buckets, maybe make configurable later.
+	s.timeSlices = make([]Counter, 10)
+	return s
+}
+
+// Add increments the specified counter by the amount in val. window specifies
+func (s *Stat) Add(val uint64) {
+	s.mutex.Lock()
+	defer s.mutex.Unlock()
+	// roll over to next timeslice if slice duration has been exceeded.
+	sliceLength := s.window / time.Duration(len(s.timeSlices))
+	now := time.Now()
+	elapsed := now.Sub(s.timeSlices[s.currentSlice].Start)
+	if elapsed > sliceLength {
+		s.currentSlice++
+		s.currentSlice = s.currentSlice % len(s.timeSlices)
+		s.timeSlices[s.currentSlice].Start = now
+		s.timeSlices[s.currentSlice].Value = val
+	}
+	s.timeSlices[s.currentSlice].Value += val
+}
+
+// Total returns the current rolling total for this Stat.
+func (s *Stat) Total() (t uint64) {
+	for _, slice := range s.timeSlices {
+		t += slice.Value
+	}
+	return t
+}
+
+// Start returns the start time for this statistics window.
+func (s *Stat) Start() (start time.Time) {
+	for i := s.currentSlice; i < 10; i++ {
+		if s.timeSlices[i%10].Start.After(time.Time{}) {
+			return s.timeSlices[i%10].Start
+		}
+	}
+	return start
+}
+
+// Duration returns the size of the current statistics window in this Stat.
+// This will fluctuate between 90-100% of the window size set when the state
+// was created, as internal buckets roll over.
+func (s *Stat) Duration() time.Duration {
+	return time.Now().Sub(s.Start())
+}
+
+// Rate returns the flow rate of this counter, which is the current rolling
+// value / the current statistics duration.
+func (s *Stat) Rate() (r float64) {
+	return float64(s.Total()) / (float64(s.Duration()) / float64(time.Second))
+}
+
+// QueueStats is a report of formatted Stat data about a specific priority
+// queue captured at a point in time.
+type QueueStats struct {
+	Start time.Time
+	time.Duration
+	Count uint64
+	Rate  float64
+}
+
+func (qs *QueueStats) String() string {
+	b, _ := yaml.Marshal(qs)
+	return string(b)
+}
+
+// Stats returns the current time usage statistics for all active task
+// priority levels.
+func (tq *TaskQueue) Stats() (stats map[uint]*QueueStats) {
+	stats = make(map[uint]*QueueStats)
+	tq.mutex.Lock()
+	defer tq.mutex.Unlock()
+	tq.queues.Range(func(prio, q interface{}) bool {
+		stat := q.(*Queue).Done
+		qs := &QueueStats{
+			Start:    stat.Start(),
+			Duration: stat.Duration(),
+			Count:    stat.Total(),
+			Rate:     stat.Rate(),
+		}
+		stats[prio.(uint)] = qs
+		return true
+	})
+	return stats
+}
diff --git a/stats_test.go b/stats_test.go
new file mode 100644
index 0000000..94869af
--- /dev/null
+++ b/stats_test.go
@@ -0,0 +1,22 @@
+package gtq
+
+import (
+	"math/rand"
+	"testing"
+	"time"
+)
+
+var testStat = NewStat(0)
+
+func TestStat(t *testing.T) {
+	total := int64(10000)
+	perOp := int64(time.Second) / total
+	for i := int64(0); i < total; i++ {
+		testStat.Add(1)
+		// random sleep to introduce some flow rate variation to capture.
+		time.Sleep(time.Duration(rand.Float64() * float64(perOp)))
+	}
+	t.Log("completed", testStat.Total(), "ops in", testStat.Duration(),
+		"for a total rate of", int64(testStat.Rate()), "ops/sec",
+	)
+}
diff --git a/taskqueue.go b/taskqueue.go
new file mode 100644
index 0000000..c7f1a1e
--- /dev/null
+++ b/taskqueue.go
@@ -0,0 +1,101 @@
+package gtq
+
+import (
+	"runtime"
+	"sync"
+	"time"
+)
+
+// TaskQueue manages multiple Queues with different priorities.
+// Task clients request tasks to run from this, and it decides which Task
+// to give the client, based on a roughly even time division based on the
+// priority numbers. Items from priority 2 will be given twice as much
+// processing time as items from priorty 1, for instance.
+type TaskQueue struct {
+	queues sync.Map
+	// task runners read off this channel, scheduler pushes onto it.
+	nextTask chan func()
+	// we need locking when cleaning up unused priorities.
+	mutex   sync.Mutex
+	running bool
+	numJobs uint
+}
+
+// NewTaskQueue returns an initialized TaskQueue.
+// numJobs specifies how many task workers you want running simultaneously, if
+// 0, defaults to runtime.GOMAXPROCS(0).
+func NewTaskQueue(numJobs uint) (tq *TaskQueue) {
+	if numJobs == 0 {
+		numJobs = uint(runtime.GOMAXPROCS(0))
+	}
+	tq = &TaskQueue{
+		// make sure we can buffer up at least one extra job per worker.
+		nextTask: make(chan func(), numJobs*2),
+		numJobs:  numJobs,
+	}
+	return tq
+}
+
+// Add a task to the TaskQueue with a given priority.
+func (tq *TaskQueue) Add(task func(), priority uint) {
+	q, _ := tq.queues.LoadOrStore(priority, NewQueue())
+	q.(*Queue).Add(task)
+}
+
+// Length returns the number of tasks at all priorities.
+func (tq *TaskQueue) Length() (length uint) {
+	tq.mutex.Lock()
+	defer tq.mutex.Unlock()
+	tq.queues.Range(func(_, q interface{}) bool {
+		length += uint(q.(*Queue).Length())
+		return true
+	})
+	return length
+}
+
+// PriorityCounts returns a map where the key is an active priority level,
+// and the value is the number of tasks left at that priority.
+func (tq *TaskQueue) PriorityCounts() (pc map[uint]int) {
+	pc = make(map[uint]int)
+	tq.mutex.Lock()
+	defer tq.mutex.Unlock()
+	tq.queues.Range(func(prio, q interface{}) bool {
+		pc[prio.(uint)] = q.(*Queue).Length()
+		return true
+	})
+	return pc
+}
+
+// Start begins processing tasks.
+func (tq *TaskQueue) Start(scheduler Scheduler) {
+	tq.mutex.Lock()
+	tq.running = true
+	tq.mutex.Unlock()
+	// start workers!
+	var j uint
+	for j = 0; j < tq.numJobs; j++ {
+		go func() {
+			for job := range tq.nextTask {
+				job()
+			}
+		}()
+	}
+	if scheduler == nil {
+		scheduler = SimpleScheduler
+	}
+	go func() {
+		for tq.running {
+			workQueued := scheduler(tq)
+			if !workQueued {
+				time.Sleep(1 * time.Millisecond)
+			}
+		}
+	}()
+}
+
+// Stop pauses processing of tasks. TaskQueue state is retained.
+func (tq *TaskQueue) Stop() {
+	tq.mutex.Lock()
+	defer tq.mutex.Unlock()
+	tq.running = false
+}
diff --git a/taskqueue_test.go b/taskqueue_test.go
new file mode 100644
index 0000000..dd0df52
--- /dev/null
+++ b/taskqueue_test.go
@@ -0,0 +1,51 @@
+package gtq
+
+import (
+	"runtime"
+	"sync"
+	"testing"
+	"time"
+)
+
+var numTestPrios = 10
+var tq *TaskQueue
+
+func testTQAdd(n int) {
+	workers := runtime.GOMAXPROCS(0)
+	var wg sync.WaitGroup
+	for i := 0; i < workers; i++ {
+		wg.Add(1)
+		go func() {
+			for i := 0; i < n; i++ {
+				// simplest possible task
+				tq.Add(func() {
+					time.Sleep(0)
+				}, uint(i%numTestPrios))
+			}
+			wg.Done()
+		}()
+	}
+	wg.Wait()
+}
+
+func TestTaskQueueAdd(t *testing.T) {
+	tq = NewTaskQueue(0)
+	testTQAdd(1000000)
+	expected := 1000000 * runtime.GOMAXPROCS(0)
+	added := tq.Length()
+	if uint(expected) != added {
+		t.Error("expected to add", expected, "but added", added)
+	} else {
+		t.Log("added", added)
+	}
+}
+
+func BenchmarkTaskQueue(b *testing.B) {
+	tq = NewTaskQueue(0)
+	testTQAdd(b.N / (runtime.GOMAXPROCS(0)))
+	tq.Start(nil)
+	for tq.Length() > 0 {
+		time.Sleep(10 * time.Millisecond)
+	}
+	tq.Stop()
+}
diff --git a/uintslice.go b/uintslice.go
new file mode 100644
index 0000000..95f87ff
--- /dev/null
+++ b/uintslice.go
@@ -0,0 +1,25 @@
+package gtq
+
+// UIntSlice just subtypes []uint to fulfill sort.Interface, as demonstrated
+// for other types in the sort package.
+type UIntSlice []uint
+
+// Len helps fulfill sort.Interface.
+func (list UIntSlice) Len() int {
+	return len(list)
+}
+
+// Less helps fulfill sort.Interface.
+func (list UIntSlice) Less(i, j int) bool {
+	if list[i] <= list[j] {
+		return true
+	}
+	return false
+}
+
+// Swap helps fulfill sort.Interface
+func (list UIntSlice) Swap(i, j int) {
+	tmp := list[i]
+	list[i] = list[j]
+	list[j] = tmp
+}