package pubsub

import (
	"fmt"
	"sync"
)

type Step int

const (
	NotStarted Step = iota
	Publishing
	Done
)

type subscriberState struct {
	step     Step
	m        sync.Mutex
	subChans []chan<- string
}

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(id string) (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(id string) <-chan string
}

type progressTracker struct {
	subscribers map[string]*subscriberState
	m           sync.Mutex
}

func NewProgressTracker() ProgressTracker {
	return &progressTracker{
		subscribers: map[string]*subscriberState{},
	}
}

func (pt *progressTracker) Publish(id string) (chan<- string, error) {
	var state *subscriberState
	func() {
		pt.m.Lock()
		defer pt.m.Unlock()
		state = pt.subscribers[id]
		if state == nil {
			state = &subscriberState{step: NotStarted}
			pt.subscribers[id] = state
		}
	}()

	err := func() error {
		state.m.Lock()
		defer state.m.Unlock()
		if state.step != NotStarted {
			return InvalidState{state.step}
		}
		state.step = Publishing
		return nil
	}()
	if err != nil {
		return nil, err
	}

	prodChan := make(chan string, 100)
	go func() {
		var update string
		prodChanOpen := true
		for {
			for prodChanOpen {
				update, prodChanOpen = <-prodChan
				if !prodChanOpen {
					break
				}
				select {
				case update, prodChanOpen = <-prodChan:
				default:
					break
				}
			}
			var scs []chan<- string
			func() {
				state.m.Lock()
				defer state.m.Unlock()
				scs = state.subChans
				if !prodChanOpen {
					for _, subChan := range scs {
						close(subChan)
					}
					state.step = Done
				}
			}()
			if !prodChanOpen {
				return
			}
			for _, subChan := range scs {
				select {
				case subChan <- update:
				default:
				}
			}
		}
	}()
	return prodChan, nil
}

func (pt *progressTracker) Subscribe(id string) <-chan string {
	c := make(chan string, 1)
	var state *subscriberState
	func() {
		pt.m.Lock()
		defer pt.m.Unlock()
		state = pt.subscribers[id]
		if state == nil {
			pt.subscribers[id] = &subscriberState{
				step:     NotStarted,
				subChans: []chan<- string{c},
			}
		}
	}()
	if state == nil {
		return c
	}
	state.m.Lock()
	defer state.m.Unlock()
	if state.step == Done {
		return nil
	}
	state.subChans = append(state.subChans, c)
	return c
}