package watchdog

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

	"golang.org/x/sync/errgroup"
)

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  []Check
	running *running
	mu      sync.Mutex
}

type running struct {
	out  chan CheckResult
	stop chan struct{}
}

// New accepts checks to run.
func New(checks ...Check) *Watchdog {
	w := &Watchdog{
		checks: checks,
	}

	return w
}

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

	out := make([]Check, len(w.checks))
	copy(out, w.checks)

	return out
}

// AddChecks adds checks to the group. This DOES NOT
// affect already runnning monitoring for group. Use Stop and
// then Start to restart monitoring when a new check is added.
// Check may have duplicate Name fields but note that RemoveChecks removes checks
// by their Name fields.
func (w *Watchdog) AddChecks(checks ...Check) {
	w.mu.Lock()
	defer w.mu.Unlock()

	w.checks = append(w.checks, checks...)
}

// RemoveChecks removes the named checks.
// This does not affect the already running monitoring for the group.
func (w *Watchdog) RemoveChecks(names ...string) {
	w.mu.Lock()
	defer w.mu.Unlock()

	remaining := make([]Check, 0, len(w.checks)-len(names))
	for _, e := range w.checks {
		if slices.Contains(names, e.Name) {
			continue
		}

		remaining = append(remaining, e)
	}

	w.checks = remaining
}

// Start starts monitoring.
// Subsequent calls to start just returns 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 ALL checks of
// the group to the channel.
// Subsequently only changes of status are pushed regardless of return error values of
// CheckFunc provided to Watchdog.
// Concurrency limits the number of checks that can run concurrently. 0 means no
// limit (all checks may run concurrently).
func (w *Watchdog) Start(ctx context.Context, concurrency int) (<-chan CheckResult, error) {
	w.mu.Lock()
	defer w.mu.Unlock()

	if w.running != nil {
		return w.running.out, nil
	}

	// start new session
	if len(w.checks) == 0 {
		return nil, ErrNotConfigured
	}

	cp := w.copyChecks()
	if concurrency == 0 {
		concurrency = len(cp)
	}

	w.runMonitoringForGroup(ctx, cp, concurrency)

	return w.running.out, nil
}

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

	if w.running == nil {
		return ErrNotRunning
	}

	close(w.running.stop)

	w.running = nil

	return nil
}

// RunImmediately runs configured checks for group concurrently and returns results.
// Concurrency limits number of checks that are allowed run concurrently. Setting
// concurrency to 0 means that all check of the group are 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() // release

	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, len(w.checks))
	copy(cp, w.checks)

	return cp
}

func (w *Watchdog) runMonitoringForGroup(ctx context.Context, checks []Check, concurrency int) {
	events := make(chan CheckResult, len(checks))
	stop := make(chan struct{})

	w.running = &running{out: events, stop: stop}

	grp := errgroup.Group{}
	sema := make(chan struct{}, concurrency)

	for _, c := range checks {
		grp.Go(func() error {
			state := CheckResult{}
			ticker := time.Tick(c.Interval)

			for {
				sema <- struct{}{}

				status, err := c.Check(ctx)

				<-sema

				s := CheckResult{
					Name:   c.Name,
					Status: status,
					Error:  err,
				}

				if s.Status != state.Status {
					events <- s
				}

				state = s

				select {
				case <-ticker:
					// continue looping
				case <-stop:
					return nil
				case <-ctx.Done():
					return ctx.Err()
				}
			}
		})
	}

	// separate goroutine to close the output chan
	// when everyone's dead
	go func() {
		grp.Wait()
		close(events)

		w.mu.Lock()
		defer w.mu.Unlock()
		if w.running != nil {
			w.running = nil
		}
	}()
}

func runChecksConcurrently(ctx context.Context, ch []Check, concurrency int) []CheckResult {
	statuses := make([]CheckResult, 0, len(ch))
	m := sync.Mutex{} // for append operations

	group := errgroup.Group{}

	sema := make(chan struct{}, concurrency) // semaphore to limit concurrency

	for _, e := range ch {
		sema <- struct{}{} // acquire

		group.Go(func() error {
			defer func() {
			}()

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

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

			m.Lock()
			defer m.Unlock()
			statuses = append(statuses, r)

			<-sema // release

			return nil
		})
	}

	group.Wait()

	return statuses
}