/*
   GoToSocial
   Copyright (C) 2021-2022 GoToSocial Authors admin@gotosocial.org

   This program is free software: you can redistribute it and/or modify
   it under the terms of the GNU Affero General Public License as published by
   the Free Software Foundation, either version 3 of the License, or
   (at your option) any later version.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU Affero General Public License for more details.

   You should have received a copy of the GNU Affero General Public License
   along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/

package api

import (
	"errors"
	"fmt"

	"github.com/gin-gonic/gin"
)

// Offer represents an offered mime-type.
type Offer string

const (
	AppJSON           Offer = `application/json`                                                     // AppJSON is the mime type for 'application/json'.
	AppActivityJSON   Offer = `application/activity+json`                                            // AppActivityJSON is the mime type for 'application/activity+json'.
	AppActivityLDJSON Offer = `application/ld+json; profile="https://www.w3.org/ns/activitystreams"` // AppActivityLDJSON is the mime type for 'application/ld+json; profile="https://www.w3.org/ns/activitystreams"'
	TextHTML          Offer = `text/html`                                                            // TextHTML is the mime type for 'text/html'.
)

// ActivityPubAcceptHeaders represents the Accept headers mentioned here:
// https://www.w3.org/TR/activitypub/#retrieving-objects
var ActivityPubAcceptHeaders = []Offer{
	AppActivityJSON,
	AppActivityLDJSON,
}

// JSONAcceptHeaders is a slice of offers that just contains application/json types.
var JSONAcceptHeaders = []Offer{
	AppJSON,
}

// HTMLAcceptHeaders is a slice of offers that just contains text/html types.
var HTMLAcceptHeaders = []Offer{
	TextHTML,
}

// NegotiateAccept takes the *gin.Context from an incoming request, and a
// slice of Offers, and performs content negotiation for the given request
// with the given content-type offers. It will return a string representation
// of the first suitable content-type, or an error if something goes wrong or
// a suitable content-type cannot be matched.
//
// For example, if the request in the *gin.Context has Accept headers of value
// [application/json, text/html], and the provided offers are of value
// [application/json, application/xml], then the returned string will be
// 'application/json', which indicates the content-type that should be returned.
//
// If the length of offers is 0, then an error will be returned, so this function
// should only be called in places where format negotiation is actually needed.
//
// If there are no Accept headers in the request, then the first offer will be returned,
// under the assumption that it's better to serve *something* than error out completely.
//
// Callers can use the offer slices exported in this package as shortcuts for
// often-used Accept types.
//
// See https://developer.mozilla.org/en-US/docs/Web/HTTP/Content_negotiation#server-driven_content_negotiation
func NegotiateAccept(c *gin.Context, offers ...Offer) (string, error) {
	if len(offers) == 0 {
		return "", errors.New("no format offered")
	}

	strings := []string{}
	for _, o := range offers {
		strings = append(strings, string(o))
	}

	accepts := c.Request.Header.Values("Accept")
	if len(accepts) == 0 {
		// there's no accept header set, just return the first offer
		return strings[0], nil
	}

	format := c.NegotiateFormat(strings...)
	if format == "" {
		return "", fmt.Errorf("no format can be offered for requested Accept header(s) %s; this endpoint offers %s", accepts, offers)
	}

	return format, nil
}