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 []*wdCheck
	mu     sync.Mutex

	monitoring bool // is monitoring currently in progress

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

	timeout time.Duration

	running int
}

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

// New creates instance of Watchdog with
// provided checks.
func New(checks ...Check) *Watchdog {
	ch := make([]*wdCheck, len(checks))

	for i := range checks {
		ch[i] = &wdCheck{
			check: checks[i],
		}
	}

	w := &Watchdog{
		checks: ch,
	}

	return w
}

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

	out := make([]Check, len(w.checks))
	for i := range w.checks {
		out[i] = w.checks[i].check
	}

	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 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()

	for i := range checks {
		nc := &wdCheck{
			check: checks[i],
		}
		w.checks = append(w.checks, nc)

		if w.monitoring {
			w.startMonitoring(nc)
		}
	}
}

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

	remaining := make([]*wdCheck, 0, len(w.checks)-len(names))
	for _, c := range w.checks {
		if slices.Contains(names, c.check.Name) {
			if w.monitoring {
				w.stopMonitoring(c)
			}
			continue
		}

		remaining = append(remaining, c)
	}

	w.checks = remaining
}

// 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 i := range w.checks {
		w.startMonitoring(w.checks[i])
	}

	w.monitoring = true

	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 i := range w.checks {
		w.stopMonitoring(w.checks[i])
	}

	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() // 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))
	for i := range w.checks {
		cp[i] = w.checks[i].check
	}

	return cp
}

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

	state := CheckResult{}
	ticker := time.Tick(wdc.check.Interval)

	c := wdc.check

	// this method is called only with
	// w.mu locked
	w.running++

	go func() {
		defer func() {
			w.mu.Lock()
			defer w.mu.Unlock()

			w.running--
			if w.running == 0 {
				// last goroutine to exit will also
				// close the output chan
				close(w.events)
				w.monitoring = false
			}
		}()

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

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

			status, err := c.Check(ctx)

			<-w.limiter

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

			if s.Status != state.Status || s.Error != nil {
				w.events <- s
			}

			state = s

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

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

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

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

	count := len(ch)

	for _, e := range ch {
		sema <- struct{}{} // acquire
		go func() error {
			defer func() {
				<-sema
				done <- struct{}{}
			}()

			// relying on assumption 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)

			return nil
		}()
	}

	// wait for all to finish
	for range done {
		count--
		if count == 0 {
			close(done)
		}
	}

	return statuses
}