package watchdog

import (
	"context"
	"errors"
	"slices"
	"sync"
	"time"
)

var (
	ErrNotConfigured = errors.New("no checks configured")
	ErrNotRunning    = errors.New("watchdog is not running")
)

// Watchdog keeps checks to run either periodically
// or on demand.
type Watchdog struct {
	checks map[string]*wdCheck
	mu     sync.Mutex

	events  chan CheckResult // output channel
	limiter chan struct{}    // TODO: use proper limiter here

	timeout time.Duration // timeout for checks to complete

	monitoring bool // is monitoring currently in progress
	running    int  // number of active checks monitored
}

type wdCheck struct {
	check Check
	stop  chan struct{}
}

// New creates instance of Watchdog with
// provided checks.
func New(checks ...Check) *Watchdog {
	w := Watchdog{
		checks: make(map[string]*wdCheck),
	}

	for _, c := range checks {
		nc := &wdCheck{
			check: c,
		}

		w.checks[c.Name] = nc
	}

	return &w
}

func (w *Watchdog) ListChecks() []Check {
	w.mu.Lock()
	defer w.mu.Unlock()

	out := w.copyChecks()

	return out
}

// SetTimeout sets timeout for all checks that
// get started with Start method. Changing this
// value does not affect running checks.
// Watchdog does not enforce this timeout, it
// just passes context.WithTimemout to check functions.
// If this method is not called the default timeout
// of 10 seconds is used.
func (w *Watchdog) SetTimeout(d time.Duration) {
	w.mu.Lock()
	defer w.mu.Unlock()

	w.timeout = d
}

// AddChecks adds checks to the group.
// If monitoring is in progress then monitoring it started for the newly added
// check as well.
// Check may have not have duplicate Name fields. New check with the same
// hame overwrites the previous one.
func (w *Watchdog) AddChecks(checks ...Check) {
	w.mu.Lock()
	defer w.mu.Unlock()

	if w.checks == nil {
		w.checks = make(map[string]*wdCheck)
	}

	for _, c := range checks {
		nc := &wdCheck{
			check: c,
		}

		old, haveOld := w.checks[c.Name]

		w.checks[c.Name] = nc

		if w.monitoring {
			w.startMonitoring(nc)

			if haveOld {
				w.stopMonitoring(old)
			}
		}
	}
}

// RemoveChecks removes the named checks.
func (w *Watchdog) RemoveChecks(names ...string) {
	w.mu.Lock()
	defer w.mu.Unlock()

	for _, name := range names {
		c, ok := w.checks[name]
		if !ok {
			continue
		}

		if w.monitoring {
			w.stopMonitoring(c)
		}

		delete(w.checks, name)
	}
}

// Start starts monitoring.
// Subsequent calls to start return the SAME channel. If you need
// to have more that one reader from the channel - fan out on your side.
// On start Watchdog runs all provided check and pushes current status of checks of
// to the channel.
// Subsequently if check func returns different status or if it returns an error the
// result is pushed to the channel.
// Concurrency argument limits the number of checks that can run concurrently. 0 means no
// limit (all checks may run concurrently).
func (w *Watchdog) Start(concurrency int) (<-chan CheckResult, error) {
	w.mu.Lock()
	defer w.mu.Unlock()

	if len(w.checks) == 0 {
		return nil, ErrNotConfigured
	}

	if w.monitoring {
		return w.events, nil
	}

	if concurrency == 0 {
		concurrency = len(w.checks)
	}

	if w.timeout == 0 {
		w.timeout = DefaultTimeout
	}

	w.events = make(chan CheckResult, concurrency)
	w.limiter = make(chan struct{}, concurrency)

	for _, c := range w.checks {
		w.startMonitoring(c)
	}

	return w.events, nil
}

// Stop stops execution of checks.
// Subsequent calls return ErrNotRunning.
func (w *Watchdog) Stop() error {
	w.mu.Lock()
	defer w.mu.Unlock()

	if !w.monitoring {
		return ErrNotRunning
	}

	for _, c := range w.checks {
		w.stopMonitoring(c)
	}

	return nil
}

// RunImmediately runs configured checks concurrently and returns results.
// Setting concurrency to 0 means that all check of the group are allowed to run simultaneously.
// Otherwise at most concurrency checks will be allowed to run simultaneously.
func (w *Watchdog) RunImmediately(ctx context.Context, concurrency int) ([]CheckResult, error) {
	w.mu.Lock()

	if len(w.checks) == 0 {
		w.mu.Unlock()
		return nil, ErrNotConfigured
	}

	cp := w.copyChecks()
	w.mu.Unlock()

	if concurrency == 0 {
		concurrency = len(cp)
	}

	statuses := runChecksConcurrently(ctx, cp, concurrency)

	slices.SortFunc(statuses, func(a, b CheckResult) int {
		if a.Name < b.Name {
			return -1
		} else if a.Name > b.Name {
			return 1
		}
		return 0
	})

	return statuses, nil
}

func (w *Watchdog) copyChecks() []Check {
	cp := make([]Check, 0, len(w.checks))
	for _, v := range w.checks {
		cp = append(cp, v.check)
	}

	return cp
}

func (w *Watchdog) startMonitoring(wdc *wdCheck) {
	wdc.stop = make(chan struct{})
	c := wdc.check

	if !w.monitoring {
		w.monitoring = true
	}

	w.running++

	go func() {
		var curr error = nil

		ticker := time.Tick(wdc.check.Interval)

		for {
			w.limiter <- struct{}{}

			ctx, cancel := context.WithTimeout(context.Background(), w.timeout)
			defer cancel()

			err := c.Check(ctx)

			<-w.limiter

			r := CheckResult{
				Name:  c.Name,
				Error: err,
			}

			// if status changed or we've got an error
			// then report this
			if !errors.Is(r.Error, curr) {
				w.events <- r
			}

			curr = r.Error

			select {
			case <-ticker:
				// continue looping
			case <-wdc.stop:
				// stopping this specific check
				return
			}
		}
	}()
}

func (w *Watchdog) stopMonitoring(wdc *wdCheck) {
	close(wdc.stop)
	w.running--

	if w.running == 0 {
		w.monitoring = false
		close(w.events)
	}
}

func runChecksConcurrently(ctx context.Context, ch []Check, concurrency int) []CheckResult {
	sema := make(chan struct{}, concurrency) // semaphore to limit concurrency
	done := make(chan CheckResult, len(ch))

	wg := new(sync.WaitGroup)
	wg.Add(len(ch))
	for _, e := range ch {
		go func() {
			sema <- struct{}{} // acquire
			defer func() {
				<-sema // release
				wg.Done()
			}()

			// relying on assumption that CheckFunc obeys context
			// cancellation
			err := e.Check(ctx)

			r := CheckResult{
				Name:  e.Name,
				Error: err,
			}

			done <- r
		}()
	}

	go func() {
		wg.Wait()
		close(done)
	}()

	results := make([]CheckResult, 0, len(ch))

	// collect results
	for r := range done {
		results = append(results, r)
	}

	return results
}