// Copyright (c) Alex Ellis 2017. All rights reserved.
// Licensed under the MIT license. See LICENSE file in the project root for full license information.

package handlers

import (
	"context"
	"encoding/json"
	"io/ioutil"
	"log"
	"net/http"

	"strconv"

	"fmt"

	"github.com/alexellis/faas/gateway/requests"
	"github.com/docker/docker/api/types"
	"github.com/docker/docker/client"
)

// DefaultMaxReplicas is the amount of replicas a service will auto-scale up to.
const DefaultMaxReplicas = 20

// MakeAlertHandler handles alerts from Prometheus Alertmanager
func MakeAlertHandler(c *client.Client) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		log.Println("Alert received.")

		body, readErr := ioutil.ReadAll(r.Body)

		log.Println(string(body))

		if readErr != nil {
			w.WriteHeader(http.StatusBadRequest)
			w.Write([]byte("Unable to read alert."))

			log.Println(readErr)
			return
		}

		var req requests.PrometheusAlert
		err := json.Unmarshal(body, &req)
		if err != nil {
			w.WriteHeader(http.StatusBadRequest)
			w.Write([]byte("Unable to parse alert, bad format."))
			log.Println(err)
			return
		}

		errors := handleAlerts(&req, c)
		if len(errors) > 0 {
			log.Println(errors)
			w.WriteHeader(http.StatusInternalServerError)

			var errorOutput string
			for d, err := range errors {
				errorOutput += fmt.Sprintf("[%d] %s\n", d, err)
			}
			w.Write([]byte(errorOutput))
		} else {
			w.WriteHeader(http.StatusOK)
		}
	}
}

func handleAlerts(req *requests.PrometheusAlert, c *client.Client) []error {
	var errors []error
	for _, alert := range req.Alerts {
		if err := scaleService(alert, c); err != nil {
			log.Println(err)
			errors = append(errors, err)
		}
	}

	return errors
}

func scaleService(alert requests.PrometheusInnerAlert, c *client.Client) error {
	var err error
	serviceName := alert.Labels.FunctionName

	if len(serviceName) > 0 {
		opts := types.ServiceInspectOptions{
			InsertDefaults: true,
		}

		service, _, inspectErr := c.ServiceInspectWithRaw(context.Background(), serviceName, opts)
		if inspectErr == nil {

			currentReplicas := *service.Spec.Mode.Replicated.Replicas
			status := alert.Status

			replicaLabel := service.Spec.TaskTemplate.ContainerSpec.Labels["com.faas.max_replicas"]
			maxReplicas := DefaultMaxReplicas
			if len(replicaLabel) > 0 {
				maxReplicas, err = strconv.Atoi(replicaLabel)
				if err != nil {
					log.Printf("Bad replica count: %s, should be uint.\n", replicaLabel)
				}
			}
			newReplicas := CalculateReplicas(status, currentReplicas, uint64(maxReplicas))

			log.Printf("[Scale] function=%s %d => %d.\n", serviceName, currentReplicas, newReplicas)
			if newReplicas == currentReplicas {
				return nil
			}

			service.Spec.Mode.Replicated.Replicas = &newReplicas
			updateOpts := types.ServiceUpdateOptions{}
			updateOpts.RegistryAuthFrom = types.RegistryAuthFromSpec

			_, updateErr := c.ServiceUpdate(context.Background(), service.ID, service.Version, service.Spec, updateOpts)
			if updateErr != nil {
				err = updateErr
			}

		} else {
			err = inspectErr
		}
	}
	return err
}

// CalculateReplicas decides what replica count to set depending on current/desired amount
func CalculateReplicas(status string, currentReplicas uint64, maxReplicas uint64) uint64 {
	newReplicas := currentReplicas
	const step = 5

	if status == "firing" {
		if currentReplicas == 1 {
			// First jump is from 1 to "step" i.e. 1->5
			newReplicas = step
		} else {
			if currentReplicas+step > maxReplicas {
				newReplicas = maxReplicas
			} else {
				newReplicas = currentReplicas + step
			}
		}
	} else { // Resolved event.
		newReplicas = 1
	}
	return newReplicas
}