package pubsub

import (
	"fmt"
	"sync"
	"time"
)

//go:generate stringer -type=Step
type Step int

const (
	NotStarted Step = iota
	Publishing
	Done
)

type InvalidState struct{ Step Step }

func (i InvalidState) Error() string {
	return fmt.Sprintf("Invalid state: %v", i.Step)
}

// One thread publishes progress, one or more threads subscribes to watch the progress
// Subscribers may not get all updates. They will get the latest status when waiting on the channel
type ProgressTracker interface {
	// Only one publisher sends update. Should close when done
	// Error if there is/was existing publisher
	Publish() (chan<- string, error)

	// Can subscribe even if there no publisher yet
	// If already done, nil channel is returned
	// channel will be closed when done
	Subscribe() <-chan string
}

type subChan struct {
	c               chan<- string
	lastUpdateIndex int
}

type progressTracker struct {
	step Step
	m    sync.Mutex
	sc   []subChan
}

func NewProgressTracker() ProgressTracker {
	return &progressTracker{
		step: NotStarted,
	}
}

func (pt *progressTracker) Publish() (chan<- string, error) {
	err := func() error {
		pt.m.Lock()
		defer pt.m.Unlock()
		if pt.step != NotStarted {
			return InvalidState{pt.step}
		}
		pt.step = Publishing
		return nil
	}()

	if err != nil {
		return nil, err
	}

	prodChan := make(chan string, 100)
	go func() {
		var update string
		prodChanOpen := true
		ticker := time.NewTicker(100 * time.Millisecond)
		defer ticker.Stop()
		lastUpdateIndex := 0
		for range ticker.C {
		LoopReader:
			for prodChanOpen {
				select {
				case update, prodChanOpen = <-prodChan:
					lastUpdateIndex++
				default:
					break LoopReader
				}
			}
			var scs []subChan
			func() {
				pt.m.Lock()
				defer pt.m.Unlock()
				scs = pt.sc
				if !prodChanOpen {
					pt.step = Done
				}
			}()
			if !prodChanOpen {
				for _, subChan := range scs {
					close(subChan.c)
				}
				return
			}
			for _, subChan := range scs {
				if subChan.lastUpdateIndex != lastUpdateIndex {
					select {
					case subChan.c <- update:
						subChan.lastUpdateIndex = lastUpdateIndex
					default:
					}
				}
			}
		}
	}()
	return prodChan, nil
}

func (pt *progressTracker) Subscribe() <-chan string {
	c := make(chan string, 1)
	sc := subChan{c: c}
	pt.m.Lock()
	defer pt.m.Unlock()
	if pt.step == Done {
		return nil
	}
	pt.sc = append(pt.sc, sc)
	return c
}