// Copyright 2019 Prometheus Team
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package webhook

import (
	"bytes"
	"context"
	"encoding/json"
	"fmt"
	"net/http"

	"github.com/go-kit/kit/log"
	"github.com/go-kit/kit/log/level"
	commoncfg "github.com/prometheus/common/config"
	"github.com/prometheus/common/version"

	"github.com/prometheus/alertmanager/config"
	"github.com/prometheus/alertmanager/notify"
	"github.com/prometheus/alertmanager/template"
	"github.com/prometheus/alertmanager/types"
)

var userAgentHeader = fmt.Sprintf("Alertmanager/%s", version.Version)

// Notifier implements a Notifier for generic webhooks.
type Notifier struct {
	conf   *config.WebhookConfig
	tmpl   *template.Template
	logger log.Logger
	client *http.Client
}

// New returns a new Webhook.
func New(conf *config.WebhookConfig, t *template.Template, l log.Logger) (*Notifier, error) {
	client, err := commoncfg.NewClientFromConfig(*conf.HTTPConfig, "webhook")
	if err != nil {
		return nil, err
	}
	return &Notifier{
		conf:   conf,
		tmpl:   t,
		logger: l,
		client: client,
	}, nil
}

// Message defines the JSON object send to webhook endpoints.
type Message struct {
	*template.Data

	// The protocol version.
	Version  string `json:"version"`
	GroupKey string `json:"groupKey"`
}

// Notify implements the Notifier interface.
func (n *Notifier) Notify(ctx context.Context, alerts ...*types.Alert) (bool, error) {
	data := notify.GetTemplateData(ctx, n.tmpl, alerts, n.logger)

	groupKey, err := notify.ExtractGroupKey(ctx)
	if err != nil {
		level.Error(n.logger).Log("err", err)
	}

	msg := &Message{
		Version:  "4",
		Data:     data,
		GroupKey: groupKey.String(),
	}

	var buf bytes.Buffer
	if err := json.NewEncoder(&buf).Encode(msg); err != nil {
		return false, err
	}

	req, err := http.NewRequest("POST", n.conf.URL.String(), &buf)
	if err != nil {
		return true, err
	}
	req.Header.Set("Content-Type", "application/json")
	req.Header.Set("User-Agent", userAgentHeader)

	resp, err := n.client.Do(req.WithContext(ctx))
	if err != nil {
		return true, err
	}
	notify.Drain(resp)

	return n.retry(resp.StatusCode)
}

func (n *Notifier) retry(statusCode int) (bool, error) {
	// Webhooks are assumed to respond with 2xx response codes on a successful
	// request and 5xx response codes are assumed to be recoverable.
	if statusCode/100 != 2 {
		return (statusCode/100 == 5), fmt.Errorf("unexpected status code %v from %s", statusCode, n.conf.URL)
	}

	return false, nil
}