// Copyright (C) 2022 Marius Schellenberger

package fs

import (
	"context"
	"errors"
	"sync"

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

type Preload struct {
	Name    string
	cancel  func()
	Status  int
	Running bool
}

type Preloads []*Preload

func (Preloads) Less(i, j *Preload) bool { return i.Name < j.Name }

type PreloadHandler struct {
	mu    sync.RWMutex
	log   logr.Logger
	fs    *FS
	pm    map[string]*Preload
	sched chan *Preload
	fin   chan string
	pre   int
	max   int
}

func NewPreloadHandler(ctx context.Context, fs *FS, max int, log logr.Logger) (ph *PreloadHandler) {
	if max < -1 {
		max = -1
	}
	ph = &PreloadHandler{
		log:   log,
		fs:    fs,
		pm:    make(map[string]*Preload),
		sched: make(chan *Preload, 1),
		fin:   make(chan string, 1),
		max:   max,
	}
	go ph.preload(ctx)
	go ph.preloadFinish(ctx)
	return
}

func (ph *PreloadHandler) CancelPreload(name string) {
	ph.mu.Lock()
	defer ph.mu.Unlock()
	if p, ok := ph.pm[name]; ok && p.Running {
		p.cancel()
	}
}

func (ph *PreloadHandler) Preloads() (ps Preloads) {
	ph.mu.RLock()
	defer ph.mu.RUnlock()
	for _, p := range ph.pm {
		ps = append(ps, &Preload{
			Name:    p.Name,
			Running: p.Running,
			Status:  ph.fs.CacheStatus(p.Name),
		})
	}
	slices.SortFunc(ps, ps.Less)
	return
}

func (ph *PreloadHandler) Preload(name string) {
	ph.mu.Lock()
	defer ph.mu.Unlock()

	p, ok := ph.pm[name]
	if !ok {
		p = &Preload{Name: name}
		ph.pm[name] = p
		if ph.max == -1 || ph.pre < ph.max {
			ph.sched <- p
		}
		return
	}

	if p.Running {
		ph.log.WithValues("file", name).Error(errors.New("preload is running"), "error starting preload")
	}
	return
}

func (ph *PreloadHandler) nextPreload() {
	if ph.pre == ph.max {
		return
	}
	for _, p := range ph.pm {
		if p.Running {
			continue
		}
		ph.sched <- p
		return
	}
}

func (ph *PreloadHandler) preload(ctx context.Context) {
	var p *Preload
	for {
		select {
		case <-ctx.Done():
			return
		case p = <-ph.sched:
		}
		name := p.Name
		file, err := ph.fs.Open(name)
		if err != nil {
			ph.log.Error(err, "error staring next preload", "file", name)
			return
		}
		f, ok := file.(*File)
		if !ok {
			continue
		}
		ctx, cancel := context.WithCancel(context.Background())
		ph.mu.Lock()
		ph.pre++
		p.Running = true
		p.cancel = cancel
		ph.mu.Unlock()
		go f.Preload(ctx, func() {
			ph.fin <- name
		})
	}
}

func (ph *PreloadHandler) preloadFinish(ctx context.Context) {
	for {
		select {
		case <-ctx.Done():
			ph.close()
			return
		case name := <-ph.fin:
			ph.mu.Lock()
			delete(ph.pm, name)
			ph.pre--
			ph.nextPreload()
			ph.mu.Unlock()
		}
	}
}

func (ph *PreloadHandler) close() {
	for _, p := range ph.pm {
		if p.Running {
			p.cancel()
		}
	}
	for {
		select {
		case <-ph.sched:
		case <-ph.fin:
		default:
			return
		}
	}
}