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 streamError struct {
	err         error
	podName     string
	recoverable bool
}

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

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

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

// PodWatcher consumes and merges the logs for a specified set of pods.
type PodWatcher struct {
	clientset     KubernetesClient
	container     string
	labelSelector *metav1.LabelSelector
	spec          map[string]*corev1.Pod
	status        map[string]bool
	streamResults chan error
	dst           io.Writer
	closeChan     chan struct{}
}

// NewPodWatcher initializes a new PodWatcher.
func NewPodWatcher(clientset KubernetesClient, container string, labelSelector *metav1.LabelSelector, dst io.Writer) PodWatcherInterface {
	return &PodWatcher{
		clientset:     clientset,
		container:     container,
		labelSelector: labelSelector,
		spec:          make(map[string]*corev1.Pod),
		status:        make(map[string]bool),
		streamResults: make(chan error),
		dst:           dst,
	}
}

// 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()
	log.Println("[PodWatcher] all goroutines exited, exiting")

	return err
}

// Close terminates the watcher, waiting for all logs to be consumed before
// exiting.
func (pw *PodWatcher) Close() {
	pw.closeChan <- struct{}{}
}

func (pw *PodWatcher) watchPods(ctx context.Context, wg *sync.WaitGroup) error {
	podsClient := pw.clientset.CoreV1().Pods(corev1.NamespaceDefault)
	labelsMap, err := metav1.LabelSelectorAsMap(pw.labelSelector)
	if err != nil {
		return err
	}
	watcher, err := podsClient.Watch(ctx, metav1.ListOptions{LabelSelector: labels.SelectorFromSet(labelsMap).String()})
	if err != nil {
		return err
	}
	defer watcher.Stop()

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

	// streamErrors is never closed.
	streamErrors := make(chan *streamError)
	resultChan := watcher.ResultChan()

	for {
		select {
		case <-ctx.Done():
			return ctx.Err()

		case <-pw.closeChan:
			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 {
				pod := pod
				if _, ok := pw.status[podName]; !ok {
					log.Printf("[PodWatcher] adding pod, name = %s", pod.Name)
					pw.status[pod.Name] = true
					wg.Add(1)
					go func() {
						if err := copyPodLogs(ctx, wg, pw.clientset, pod, pw.container, pw.dst); err != nil {
							streamErrors <- err
						}
					}()
				}
			}
			// For any pods which no longer exist, remove the pod.
			// TODO: check this is needed when a pod's labels change to no longer
			// match the deployment's selector.
			for podName := range pw.status {
				if _, ok := pw.spec[podName]; !ok {
					pw.removePod(podName)
				}
			}

		case streamErr := <-streamErrors:
			if 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.podName)
			} else {
				return fmt.Errorf("error streaming logs: %w", streamErr)
			}

		case evt, ok := <-resultChan:
			if !ok {
				resultChan = nil
				continue
			}
			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(podName string) {
	log.Printf("[PodWatcher] removing pod, name = %s", podName)
	delete(pw.status, podName)
}

func copyPodLogs(ctx context.Context, wg *sync.WaitGroup, clientset KubernetesClient, pod *corev1.Pod, container string, dst io.Writer) *streamError {
	defer wg.Done()

	podLogOpts := corev1.PodLogOptions{
		Follow:    true,
		Container: container,
	}
	req := clientset.CoreV1().Pods(pod.Namespace).GetLogs(pod.Name, &podLogOpts)
	logs, err := req.Stream(ctx)

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

	defer func() { _ = logs.Close() }()

	scanner := bufio.NewScanner(logs)
	for scanner.Scan() {
		if _, err = dst.Write([]byte("[" + pod.Name + "] " + scanner.Text() + nl)); err != nil {
			return newStreamError(fmt.Errorf("error writing logs: %v", err), pod.Name)
		}
	}
	if err := scanner.Err(); err != nil {
		return newStreamError(fmt.Errorf("error scanning logs: %v", err), pod.Name)
	}
	return nil
}