package logs

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

	corev1 "k8s.io/api/core/v1"
	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
	"k8s.io/apimachinery/pkg/labels"
	"k8s.io/apimachinery/pkg/watch"
	"k8s.io/client-go/kubernetes"
)

// KubernetesClient wraps a Kubernetes clientset.
type KubernetesClient struct {
	kubernetes.Interface
}

// concurrentWriter implements io.Writer.
type concurrentWriter struct {
	w  io.Writer
	mu sync.Mutex
}

// NewConcurrentWriter returns an io.Writer which can be safely written to from
// multiple goroutines without further synchronization.
func NewConcurrentWriter(w io.Writer) io.Writer {
	return &concurrentWriter{w: w}
}

// Write implements io.Writer.
func (cw *concurrentWriter) Write(p []byte) (int, error) {
	cw.mu.Lock()
	defer cw.mu.Unlock()

	return cw.w.Write(p)
}

const nl = "\n"

type streamError struct {
	err         error
	podName     string
	recoverable bool
}

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

// Watcher watches a deployment and tails the logs for its currently active
// pods.
type Watcher struct {
	deployName    string
	container     string
	clientset     KubernetesClient
	spec          map[string]*corev1.Pod
	status        map[string]bool
	streamResults chan error
	dst           io.Writer
}

// NewWatcher creates a new Watcher.
func NewWatcher(deployName string, container string, clientset KubernetesClient, dst io.Writer) *Watcher {
	return &Watcher{
		deployName:    deployName,
		container:     container,
		clientset:     clientset,
		spec:          make(map[string]*corev1.Pod),
		status:        make(map[string]bool),
		streamResults: make(chan error),
		dst:           dst,
	}
}

// Watch watches a deployment.
func (w *Watcher) Watch(ctx context.Context) error {
	dst := NewConcurrentWriter(w.dst)

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

	deploymentsClient := w.clientset.AppsV1().Deployments(corev1.NamespaceDefault)
	var opts metav1.GetOptions
	deployment, err := deploymentsClient.Get(ctx, w.deployName, opts)
	if err != nil {
		return err
	}

	podsClient := w.clientset.CoreV1().Pods(corev1.NamespaceDefault)
	labelsMap, err := metav1.LabelSelectorAsMap(deployment.Spec.Selector)
	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()

	// streamErrors is never closed.
	streamErrors := make(chan error)

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

		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 w.spec {
				pod := pod
				if _, ok := w.status[podName]; !ok {
					log.Printf("adding pod, name = %s", pod.Name)
					w.status[pod.Name] = true
					go func() {
						if err := copyPodLogs(ctx, w.clientset, pod, w.container, 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 w.status {
				if _, ok := w.spec[podName]; !ok {
					w.removePod(podName)
				}
			}

		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.
				w.removePod(streamErr.podName)
			} else {
				return streamErr
			}

		case evt := <-watcher.ResultChan():
			switch evt.Type {
			case watch.Added, watch.Modified:
				pod := evt.Object.(*corev1.Pod)
				log.Printf("event rcvd, type = %s, pod name = %s, phase = %s", evt.Type, pod.Name, pod.Status.Phase)
				if pod.Status.Phase == corev1.PodRunning {
					w.spec[pod.Name] = pod
				}
			case watch.Deleted:
				pod := evt.Object.(*corev1.Pod)
				delete(w.spec, pod.Name)
				log.Printf("event rcvd, type = DELETED, pod name = %s", pod.Name)
			}
		}
	}
}

func copyPodLogs(ctx context.Context, clientset KubernetesClient, p *corev1.Pod, container string, dst io.Writer) error {
	podLogOpts := corev1.PodLogOptions{
		Follow:    true,
		Container: container,
	}
	req := clientset.CoreV1().Pods(p.Namespace).GetLogs(p.Name, &podLogOpts)
	logs, err := req.Stream(ctx)

	// If one pod or container is in a non-running state, we don't want to quit.
	// Checking the response string avoids the risk of a race condition but
	// obviously feels a bit brittle too.
	if err != nil && strings.Contains(err.Error(), "is waiting to start") {
		return &streamError{err: err, podName: p.Name, recoverable: true}
	} else if err != nil {
		return &streamError{err: err, podName: p.Name}
	}

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

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

	if err := scanner.Err(); err != nil {
		return &streamError{err: fmt.Errorf("error scanning: %v", err), podName: p.Name}
	}

	return nil
}

func (w *Watcher) removePod(podName string) {
	log.Printf("removing pod, name = %s", podName)
	delete(w.status, podName)
}