/*
 * From: https://github.com/serverless/event-gateway/blob/master/event/event.go
 * Modified: Strip to handler requirements
 *
 * Copyright 2017 Serverless, Inc.
 *
 * 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 cloudevents

import (
	"encoding/json"
	"errors"
	"fmt"
	"io/ioutil"
	"mime"
	"net/http"
	"strings"
	"time"
	"unicode"

	"github.com/google/uuid"
	validator "gopkg.in/go-playground/validator.v9"
)

const (
	// TransformationVersion is indicative of the revision of how Event Gateway transforms a request into CloudEvents format.
	TransformationVersion = "0.1"

	// CloudEventsVersion currently supported by Event Gateway
	CloudEventsVersion = "0.1"
)

// Event is a default event structure. All data that passes through the Event Gateway
// is formatted to a format defined CloudEvents v0.1 spec.
type Event struct {
	EventType          string                 `json:"eventType" validate:"required"`
	EventTypeVersion   string                 `json:"eventTypeVersion,omitempty"`
	CloudEventsVersion string                 `json:"cloudEventsVersion" validate:"required"`
	Source             string                 `json:"source" validate:"uri,required"`
	EventID            string                 `json:"eventID" validate:"required"`
	EventTime          *time.Time             `json:"eventTime,omitempty"`
	SchemaURL          string                 `json:"schemaURL,omitempty"`
	Extensions         map[string]interface{} `json:"extensions,omitempty"`
	ContentType        string                 `json:"contentType,omitempty"`
	Data               interface{}            `json:"data"`
}

// New return new instance of Event.
func New(eventType string, mimeType string, payload interface{}) *Event {
	now := time.Now()

	event := &Event{
		EventType:          eventType,
		CloudEventsVersion: CloudEventsVersion,
		Source:             "https://micro.mu",
		EventID:            uuid.New().String(),
		EventTime:          &now,
		ContentType:        mimeType,
		Data:               payload,
		Extensions: map[string]interface{}{
			"eventgateway": map[string]interface{}{
				"transformed":            "true",
				"transformation-version": TransformationVersion,
			},
		},
	}

	event.Data = normalizePayload(event.Data, event.ContentType)
	return event
}

// FromRequest takes an HTTP request and returns an Event along with path. Most of the implementation
// is based on https://github.com/cloudevents/spec/blob/master/http-transport-binding.md.
// This function also supports legacy mode where event type is sent in Event header.
func FromRequest(r *http.Request) (*Event, error) {
	contentType := r.Header.Get("Content-Type")
	mimeType, _, err := mime.ParseMediaType(contentType)
	if err != nil {
		if err.Error() != "mime: no media type" {
			return nil, err
		}
		mimeType = "application/octet-stream"
	}
	// Read request body
	body := []byte{}
	if r.Body != nil {
		body, err = ioutil.ReadAll(r.Body)
		if err != nil {
			return nil, err
		}
	}

	var event *Event
	if mimeType == mimeCloudEventsJSON { // CloudEvents Structured Content Mode
		return parseAsCloudEvent(mimeType, body)
	} else if isCloudEventsBinaryContentMode(r.Header) { // CloudEvents Binary Content Mode
		return parseAsCloudEventBinary(r.Header, body)
	} else if isLegacyMode(r.Header) {
		if mimeType == mimeJSON { // CloudEvent in Legacy Mode
			event, err = parseAsCloudEvent(mimeType, body)
			if err != nil {
				return New(string(r.Header.Get("event")), mimeType, body), nil
			}
			return event, err
		}

		return New(string(r.Header.Get("event")), mimeType, body), nil
	}

	return New("http.request", mimeJSON, newHTTPRequestData(r, body)), nil
}

// Validate Event struct
func (e *Event) Validate() error {
	validate := validator.New()
	err := validate.Struct(e)
	if err != nil {
		return fmt.Errorf("CloudEvent not valid: %v", err)
	}
	return nil
}

func isLegacyMode(headers http.Header) bool {
	if headers.Get("Event") != "" {
		return true
	}

	return false
}

func isCloudEventsBinaryContentMode(headers http.Header) bool {
	if headers.Get("CE-EventType") != "" &&
		headers.Get("CE-CloudEventsVersion") != "" &&
		headers.Get("CE-Source") != "" &&
		headers.Get("CE-EventID") != "" {
		return true
	}

	return false
}

func parseAsCloudEventBinary(headers http.Header, payload interface{}) (*Event, error) {
	event := &Event{
		EventType:          headers.Get("CE-EventType"),
		EventTypeVersion:   headers.Get("CE-EventTypeVersion"),
		CloudEventsVersion: headers.Get("CE-CloudEventsVersion"),
		Source:             headers.Get("CE-Source"),
		EventID:            headers.Get("CE-EventID"),
		ContentType:        headers.Get("Content-Type"),
		Data:               payload,
	}

	err := event.Validate()
	if err != nil {
		return nil, err
	}

	if headers.Get("CE-EventTime") != "" {
		val, err := time.Parse(time.RFC3339, headers.Get("CE-EventTime"))
		if err != nil {
			return nil, err
		}
		event.EventTime = &val
	}

	if val := headers.Get("CE-SchemaURL"); len(val) > 0 {
		event.SchemaURL = val
	}

	event.Extensions = map[string]interface{}{}
	for key, val := range flatten(headers) {
		if strings.HasPrefix(key, "Ce-X-") {
			key = strings.TrimLeft(key, "Ce-X-")
			// Make first character lowercase
			runes := []rune(key)
			runes[0] = unicode.ToLower(runes[0])
			event.Extensions[string(runes)] = val
		}
	}

	event.Data = normalizePayload(event.Data, event.ContentType)
	return event, nil
}

func flatten(h http.Header) map[string]string {
	headers := map[string]string{}
	for key, header := range h {
		headers[key] = header[0]
		if len(header) > 1 {
			headers[key] = strings.Join(header, ", ")
		}
	}
	return headers
}

func parseAsCloudEvent(mime string, payload interface{}) (*Event, error) {
	body, ok := payload.([]byte)
	if ok {
		event := &Event{}
		err := json.Unmarshal(body, event)
		if err != nil {
			return nil, err
		}

		err = event.Validate()
		if err != nil {
			return nil, err
		}

		event.Data = normalizePayload(event.Data, event.ContentType)
		return event, nil
	}

	return nil, errors.New("couldn't cast to []byte")
}

const (
	mimeJSON            = "application/json"
	mimeFormMultipart   = "multipart/form-data"
	mimeFormURLEncoded  = "application/x-www-form-urlencoded"
	mimeCloudEventsJSON = "application/cloudevents+json"
)

// normalizePayload takes anything, checks if it's []byte array and depending on provided mime
// type converts it to either string or map[string]interface to avoid having base64 string after
// JSON marshaling.
func normalizePayload(payload interface{}, mime string) interface{} {
	if bytePayload, ok := payload.([]byte); ok && len(bytePayload) > 0 {
		switch {
		case mime == mimeJSON || strings.HasSuffix(mime, "+json"):
			var result map[string]interface{}
			err := json.Unmarshal(bytePayload, &result)
			if err != nil {
				return payload
			}
			return result
		case strings.HasPrefix(mime, mimeFormMultipart), mime == mimeFormURLEncoded:
			return string(bytePayload)
		}
	}

	return payload
}

// HTTPRequestData is a event schema used for sending events to HTTP subscriptions.
type HTTPRequestData struct {
	Headers map[string]string   `json:"headers"`
	Query   map[string][]string `json:"query"`
	Body    interface{}         `json:"body"`
	Host    string              `json:"host"`
	Path    string              `json:"path"`
	Method  string              `json:"method"`
	Params  map[string]string   `json:"params"`
}

// NewHTTPRequestData returns a new instance of HTTPRequestData
func newHTTPRequestData(r *http.Request, eventData interface{}) *HTTPRequestData {
	req := &HTTPRequestData{
		Headers: flatten(r.Header),
		Query:   r.URL.Query(),
		Body:    eventData,
		Host:    r.Host,
		Path:    r.URL.Path,
		Method:  r.Method,
	}

	req.Body = normalizePayload(req.Body, r.Header.Get("content-type"))
	return req
}