// Licensed to Elasticsearch B.V. under one or more contributor
// license agreements. See the NOTICE file distributed with
// this work for additional information regarding copyright
// ownership. Elasticsearch B.V. licenses this file to you 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 eventlog

import (
	"fmt"
	"sort"
	"strings"

	"github.com/elastic/beats/v7/libbeat/common"
)

// ConfigCommon is the common configuration data used to instantiate a new
// EventLog. Each implementation is free to support additional configuration
// options.
type ConfigCommon struct {
	API      string `config:"api"`       // Name of the API to use. Optional.
	Name     string `config:"name"`      // Name of the event log or channel or file.
	ID       string `config:"id"`        // Identifier for the event log.
	XMLQuery string `config:"xml_query"` // Custom query XML. Must not be used with the keys from eventlog.query.
}

type validator interface {
	Validate() error
}

func readConfig(c *common.Config, config interface{}) error {
	if err := c.Unpack(config); err != nil {
		return fmt.Errorf("failed unpacking config. %v", err)
	}

	if v, ok := config.(validator); ok {
		if err := v.Validate(); err != nil {
			return err
		}
	}

	return nil
}

// Producer produces a new event log instance for reading event log records.
type producer func(*common.Config) (EventLog, error)

// Channels lists the available channels (event logs).
type channels func() ([]string, error)

// eventLogInfo is the registration info associated with an event log API.
type eventLogInfo struct {
	apiName  string
	priority int
	producer producer
	channels func() ([]string, error)
}

// eventLogs is a map of priorities to eventLogInfo. The lower numbers have
// higher priorities.
var eventLogs = make(map[int]eventLogInfo)

// Register registers an EventLog API. Only the APIs that are available for the
// runtime OS should be registered. Each API must have a unique priority.
func Register(apiName string, priority int, producer producer, channels channels) {
	info, exists := eventLogs[priority]
	if exists {
		panic(fmt.Sprintf("%s API is already registered with priority %d. "+
			"Cannot register %s", info.apiName, info.priority, apiName))
	}

	eventLogs[priority] = eventLogInfo{
		apiName:  apiName,
		priority: priority,
		producer: producer,
		channels: channels,
	}
}

// New creates and returns a new EventLog instance based on the given config
// and the registered EventLog producers.
func New(options *common.Config) (EventLog, error) {
	if len(eventLogs) == 0 {
		return nil, fmt.Errorf("No event log API is available on this system")
	}

	var config ConfigCommon
	if err := readConfig(options, &config); err != nil {
		return nil, err
	}

	// A specific API is being requested (usually done for testing).
	if config.API != "" {
		for _, v := range eventLogs {
			debugf("Checking %s", v.apiName)
			if strings.EqualFold(v.apiName, config.API) {
				debugf("Using %s API for event log %s", v.apiName, config.Name)
				e, err := v.producer(options)
				return e, err
			}
		}

		return nil, fmt.Errorf("%s API is not available", config.API)
	}

	// Use the API with the highest priority.
	keys := make([]int, 0, len(eventLogs))
	for key := range eventLogs {
		keys = append(keys, key)
	}
	sort.Ints(keys)

	eventLog := eventLogs[keys[0]]
	debugf("Using highest priority API, %s, for event log %s",
		eventLog.apiName, config.Name)
	e, err := eventLog.producer(options)
	return e, err
}
