// Copyright (C) 2022 Marius Schellenberger

package fs

import (
	"context"
	"sync"

	"github.com/go-logr/logr"
	"golang.org/x/exp/slices"
)

type queue []*Preload

func (queue) LessReverse(i, j *Preload) bool { return i.Prio > j.Prio }

func (q *queue) Sort() {
	slices.SortFunc(*q, q.LessReverse)
}

func (q *queue) Add(ph *PreloadHandler, name string, prio int) {
	for _, p := range *q {
		if p.Name == name {
			p.Prio += prio
			if p.Prio < 0 {
				p.Prio = 0
			}
			q.Sort()
			return
		}
	}
	*q = append(*q, &Preload{Name: name, ph: ph})
	q.Sort()
}

func (q *queue) Remove(name string) {
	for i, p := range *q {
		if p.Name == name {
			p.stop()
			*q = slices.Delete(*q, i, i+1)
			q.Sort()
			return
		}
	}
}

type Preload struct {
	ph      *PreloadHandler
	Name    string
	cancel  func()
	Prio    int
	Status  int
	Running bool
}

type Preloads []Preload

type PreloadHandler struct {
	mu  sync.RWMutex
	log logr.Logger
	fs  *FS
	q   queue
	fin chan string
	max int
}

func NewPreloadHandler(ctx context.Context, fs *FS, max int, log logr.Logger) (ph *PreloadHandler) {
	if max <= 0 {
		max = 1
	}
	ph = &PreloadHandler{
		log: log,
		fs:  fs,
		q:   make(queue, 0),
		fin: make(chan string, 1),
		max: max,
	}
	go ph.preloadFinish(ctx)
	return
}

func (ph *PreloadHandler) RemovePreload(name string) {
	ph.mu.RLock()
	defer ph.mu.RUnlock()
	ph.q.Remove(name)
	ph.schedule()
}

func (ph *PreloadHandler) Preloads() (ps Preloads) {
	ph.mu.RLock()
	defer ph.mu.RUnlock()
	ps = make(Preloads, len(ph.q))
	for i, v := range ph.q {
		ps[i] = *v
		ps[i].Status = ph.fs.CacheStatus(v.Name)
	}
	return
}

func (ph *PreloadHandler) Preload(name string, prio int) {
	ph.mu.Lock()
	defer ph.mu.Unlock()
	ph.q.Add(ph, name, prio)
	ph.schedule()
}

func (ph *PreloadHandler) schedule() {
	for i := 0; i < len(ph.q); i++ {
		if i < ph.max {
			ph.q[i].start()
		} else {
			ph.q[i].stop()
		}
	}
}

func (p *Preload) start() {
	if p.Running {
		return
	}
	name := p.Name
	file, err := p.ph.fs.Open(name)
	if err != nil {
		p.ph.log.Error(err, "error staring next preload", "file", name)
		return
	}
	f, ok := file.(*File)
	if !ok {
		return
	}
	ctx, cancel := context.WithCancel(context.Background())
	p.Running = true
	p.cancel = cancel
	go f.Preload(ctx, func() {
		p.ph.fin <- name
	})
}

func (p *Preload) stop() {
	if p.Running {
		p.cancel()
		p.Running = false
	}
}

func (ph *PreloadHandler) preloadFinish(ctx context.Context) {
	for {
		select {
		case <-ctx.Done():
			ph.close()
			return
		case name := <-ph.fin:
			ph.mu.Lock()
			ph.q.Remove(name)
			ph.schedule()
			ph.mu.Unlock()
		}
	}
}

func (ph *PreloadHandler) close() {
	for _, p := range ph.q {
		p.stop()
	}
	for {
		select {
		case <-ph.fin:
		default:
			return
		}
	}
}