package logs

import (
	"bufio"
	"context"
	"errors"
	"fmt"
	"io"
	"log"
	"strings"
	"sync"
	"time"

	corev1 "k8s.io/api/core/v1"
	apierrors "k8s.io/apimachinery/pkg/api/errors"
	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
	"k8s.io/apimachinery/pkg/labels"
	"k8s.io/apimachinery/pkg/watch"
)

const nl = "\n"

type stream struct {
	podName   string
	namespace string
	done      chan struct{}
}

func newStream(podName, namespace string) stream {
	return stream{podName: podName, namespace: namespace, done: make(chan struct{})}
}

func (s stream) close() {
	close(s.done)
}

type streamError struct {
	err         error
	stream      stream
	recoverable bool
}

func (re *streamError) Error() string {
	return re.err.Error()
}

func newStreamError(err error, stream stream) *streamError {
	return &streamError{err: err, stream: stream}
}

func newRecoverableError(err error, stream stream) *streamError {
	return &streamError{err: err, stream: stream, recoverable: true}
}

// PodWatcher consumes and merges the logs for a specified set of pods.
type PodWatcher struct {
	client        KubernetesClient
	namespace     string
	container     string
	labelSelector labels.Selector
	spec          map[string]*corev1.Pod
	status        map[string]stream
	streamResults chan error
	dst           io.Writer
	closeChan     chan struct{}
	logger        *log.Logger
}

// NewPodWatcher initializes a new PodWatcher.
func NewPodWatcher(client KubernetesClient, namespace string, container string, labelSelector labels.Selector, dst io.Writer, logger *log.Logger) PodWatcherInterface {
	return &PodWatcher{
		client:        client,
		namespace:     namespace,
		container:     container,
		labelSelector: labelSelector,
		spec:          make(map[string]*corev1.Pod),
		status:        make(map[string]stream),
		streamResults: make(chan error),
		dst:           dst,
		closeChan:     make(chan struct{}),
		logger:        logger,
	}
}

// WatchPods blocks while it watches the pods which match the Selector of the
// provided deployment.
//
// Cancelling ctx will result in the event loop exiting immediately.
func (pw *PodWatcher) WatchPods(ctx context.Context) error {
	var wg sync.WaitGroup
	err := pw.watchPods(ctx, &wg)

	wg.Wait()
	pw.logger.Println("[PodWatcher] all goroutines exited, exiting")

	return err
}

const tickerInterval = time.Millisecond * 250

// Close terminates the PodWatcher.
func (pw *PodWatcher) Close() {
	close(pw.closeChan)
}

func (pw *PodWatcher) watchPods(ctx context.Context, wg *sync.WaitGroup) error {
	podsClient := pw.client.Typed.CoreV1().Pods(pw.namespace)

	// Returns a watcher which notifies of changes in the relevant pods.
	// We don't defer Stop() on the returned value because the sender is the
	// Kubernetes SDK, and that may introduce a send-on-closed-channel panic.
	watcher, err := podsClient.Watch(ctx, metav1.ListOptions{LabelSelector: pw.labelSelector.String()})
	if err != nil {
		return err
	}

	ticker := time.NewTicker(tickerInterval)
	defer ticker.Stop()

	logStream := make(chan string)
	streamErrors := make(chan error)

	resultChan := watcher.ResultChan()
	for {
		select {
		case <-ctx.Done():
			return ctx.Err()

		case <-pw.closeChan:
			for _, stream := range pw.status {
				stream.close()
			}
			return nil

		case <-ticker.C:
			// Iterate through the desired state (w.spec) and launch goroutines to
			// process the logs of any missing pods.
			for podName, pod := range pw.spec {
				if _, ok := pw.status[podName]; !ok {
					pw.logger.Printf("[PodWatcher] adding pod, name = %s", pod.Name)
					s := newStream(pod.Name, pod.Namespace)
					pw.status[pod.Name] = s

					wg.Add(1)
					go func() {
						if err := copyPodLogs(ctx, wg, pw.client, s, pw.container, logStream); err != nil {
							streamErrors <- err
						}
					}()
				}
			}
			// For any pods which no longer exist, remove the pod.
			for podName, stream := range pw.status {
				if _, ok := pw.spec[podName]; !ok {
					pw.removePod(stream)
				}
			}

		// logStream is never closed
		case l := <-logStream:
			if _, err := pw.dst.Write([]byte(l)); err != nil {
				return fmt.Errorf("error writing logs: %v", err)
			}

		// streamErrors is never closed
		case err := <-streamErrors:
			var streamErr *streamError
			if errors.As(err, &streamErr) && streamErr.recoverable {
				// if the error is recoverable, we just remove the pod from the status
				// map. It will be recreated and retried on the next iteration.
				pw.removePod(streamErr.stream)
			} else {
				return fmt.Errorf("error streaming logs: %w", streamErr)
			}

		case evt, ok := <-resultChan:
			if !ok {
				return nil
			}
			switch evt.Type {
			case watch.Added, watch.Modified:
				pod := evt.Object.(*corev1.Pod)
				if pod.Status.Phase == corev1.PodRunning {
					pw.spec[pod.Name] = pod
				}
			case watch.Deleted:
				pod := evt.Object.(*corev1.Pod)
				delete(pw.spec, pod.Name)
			}
		}
	}
}

func (pw *PodWatcher) removePod(stream stream) {
	pw.logger.Printf("[PodWatcher] removing pod, name = %s", stream.podName)
	stream.close()
	delete(pw.status, stream.podName)
}

func copyPodLogs(ctx context.Context, wg *sync.WaitGroup, client KubernetesClient, stream stream, container string, logStream chan string) error {
	defer wg.Done()

	req := client.Typed.CoreV1().Pods(stream.namespace).GetLogs(stream.podName, &corev1.PodLogOptions{Follow: true, Container: container})
	logs, err := req.Stream(ctx)

	// If one container is still being created, do not treat this as a fatal error.
	var statusErr *apierrors.StatusError
	if errors.As(err, &statusErr) && statusErr.Status().Reason == metav1.StatusReasonBadRequest && strings.Contains(statusErr.Error(), "ContainerCreating") {
		return newRecoverableError(err, stream)
	} else if err != nil {
		return newStreamError(err, stream)
	}

	// Closing the reader ensures that the goroutine below is not leaked.
	defer func() { _ = logs.Close() }()

	done := make(chan error, 1)
	go func() {
		scanner := bufio.NewScanner(logs)
		for scanner.Scan() {
			logStream <- "[" + stream.podName + "] " + scanner.Text() + nl
		}
		if err := scanner.Err(); err != nil {
			done <- newStreamError(fmt.Errorf("error scanning logs: %v", err), stream)
			return
		}
		done <- nil
	}()

	for {
		select {
		case err := <-done:
			return err
		case <-stream.done:
			return nil
		case <-ctx.Done():
			return ctx.Err()
		}
	}
}