package balancer

import (
	"context"
	"net/http"
	"sync"
	"time"
)

const (
	defaultHealthInterval = 10 * time.Second
	defaultHealthPath     = "/health"
	defaultHealthTimeout  = 5 * time.Second
)

// HealthOption configures the HealthChecker.
type HealthOption func(*HealthChecker)

// WithHealthInterval sets the interval between health check probes.
// Default is 10 seconds.
func WithHealthInterval(d time.Duration) HealthOption {
	return func(h *HealthChecker) {
		h.interval = d
	}
}

// WithHealthPath sets the HTTP path to probe for health checks.
// Default is "/health".
func WithHealthPath(path string) HealthOption {
	return func(h *HealthChecker) {
		h.path = path
	}
}

// WithHealthTimeout sets the timeout for each health check request.
// Default is 5 seconds.
func WithHealthTimeout(d time.Duration) HealthOption {
	return func(h *HealthChecker) {
		h.timeout = d
	}
}

// HealthChecker periodically probes endpoints to determine their health status.
type HealthChecker struct {
	interval time.Duration
	path     string
	timeout  time.Duration
	client   *http.Client

	mu      sync.RWMutex
	status  map[string]bool
	cancel  context.CancelFunc
	stopped chan struct{}
}

func newHealthChecker(opts ...HealthOption) *HealthChecker {
	h := &HealthChecker{
		interval: defaultHealthInterval,
		path:     defaultHealthPath,
		timeout:  defaultHealthTimeout,
		status:   make(map[string]bool),
	}
	for _, opt := range opts {
		opt(h)
	}
	h.client = &http.Client{
		Timeout: h.timeout,
	}
	return h
}

// Start begins the background health checking loop for the given endpoints.
// All endpoints are initially considered healthy.
func (h *HealthChecker) Start(endpoints []Endpoint) {
	h.mu.Lock()
	for _, ep := range endpoints {
		h.status[ep.URL] = true
	}
	h.mu.Unlock()

	ctx, cancel := context.WithCancel(context.Background())
	h.cancel = cancel
	h.stopped = make(chan struct{})

	go h.loop(ctx, endpoints)
}

// Stop terminates the background health checking goroutine and waits for
// it to finish.
func (h *HealthChecker) Stop() {
	if h.cancel != nil {
		h.cancel()
		<-h.stopped
	}
}

// IsHealthy reports whether the given endpoint is currently healthy.
func (h *HealthChecker) IsHealthy(ep Endpoint) bool {
	h.mu.RLock()
	defer h.mu.RUnlock()

	healthy, ok := h.status[ep.URL]
	if !ok {
		return false
	}
	return healthy
}

// Healthy returns the subset of endpoints that are currently healthy.
func (h *HealthChecker) Healthy(endpoints []Endpoint) []Endpoint {
	h.mu.RLock()
	defer h.mu.RUnlock()

	var result []Endpoint
	for _, ep := range endpoints {
		if h.status[ep.URL] {
			result = append(result, ep)
		}
	}
	return result
}

func (h *HealthChecker) loop(ctx context.Context, endpoints []Endpoint) {
	defer close(h.stopped)

	ticker := time.NewTicker(h.interval)
	defer ticker.Stop()

	for {
		select {
		case <-ctx.Done():
			return
		case <-ticker.C:
			h.probe(ctx, endpoints)
		}
	}
}

func (h *HealthChecker) probe(ctx context.Context, endpoints []Endpoint) {
	for _, ep := range endpoints {
		healthy := h.check(ctx, ep)

		h.mu.Lock()
		h.status[ep.URL] = healthy
		h.mu.Unlock()
	}
}

func (h *HealthChecker) check(ctx context.Context, ep Endpoint) bool {
	req, err := http.NewRequestWithContext(ctx, http.MethodGet, ep.URL+h.path, nil)
	if err != nil {
		return false
	}

	resp, err := h.client.Do(req)
	if err != nil {
		return false
	}
	resp.Body.Close()

	return resp.StatusCode >= 200 && resp.StatusCode < 300
}