// Copyright 2020 the Pinniped contributors. All Rights Reserved.
// SPDX-License-Identifier: Apache-2.0

// Package discovery provides a handler for the OIDC discovery endpoint.
package discovery

import (
	"encoding/json"
	"fmt"
	"net/http"
)

// Metadata holds all fields (that we care about) from the OpenID Provider Metadata section in the
// OpenID Connect Discovery specification:
// https://openid.net/specs/openid-connect-discovery-1_0.html#rfc.section.3.
type Metadata struct {
	Issuer string `json:"issuer"`

	AuthorizationEndpoint string `json:"authorization_endpoint"`
	TokenEndpoint         string `json:"token_endpoint"`
	JWKSURL               string `json:"jwks_url"`

	ResponseTypesSupported           []string `json:"response_types_supported"`
	SubjectTypesSupported            []string `json:"subject_types_supported"`
	IDTokenSigningAlgValuesSupported []string `json:"id_token_signing_alg_values_supported"`
}

// IssuerGetter holds onto an issuer which can be retrieved via its GetIssuer function. If there is
// no valid issuer, then nil will be returned.
//
// Implementations of this type should be thread-safe to support calls from multiple goroutines.
type IssuerGetter interface {
	GetIssuer() *string
}

// New returns an http.Handler that will use information from the provided IssuerGetter to serve an
// OIDC discovery endpoint.
func New(ig IssuerGetter) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		w.Header().Set("Content-Type", "application/json")

		if r.Method != http.MethodGet {
			http.Error(w, `{"error": "Method not allowed (try GET)"}`, http.StatusMethodNotAllowed)
			return
		}

		issuer := ig.GetIssuer()
		if issuer == nil {
			http.Error(w, `{"error": "OIDC discovery not available (unknown issuer)"}`, http.StatusServiceUnavailable)
			return
		}

		oidcConfig := Metadata{
			Issuer:                           *issuer,
			AuthorizationEndpoint:            fmt.Sprintf("%s/oauth2/v0/auth", *issuer),
			TokenEndpoint:                    fmt.Sprintf("%s/oauth2/v0/token", *issuer),
			JWKSURL:                          fmt.Sprintf("%s/oauth2/v0/keys", *issuer),
			ResponseTypesSupported:           []string{},
			SubjectTypesSupported:            []string{},
			IDTokenSigningAlgValuesSupported: []string{},
		}
		if err := json.NewEncoder(w).Encode(&oidcConfig); err != nil {
			http.Error(w, err.Error(), http.StatusInternalServerError)
		}
	})
}