// Package oidc is the small slice of OpenID Connect Nurse needs: the authorization code flow
// with PKCE against one provider (Authentik), using only the standard library.
//
// The ID token is taken straight from the provider's token endpoint over TLS, which is the case
// OpenID Connect Core section 3.1.3.7 allows to skip checking its signature. Its issuer, audience,
// expiry and nonce are still checked.
package oidc

import (
	"context"
	"crypto/rand"
	"crypto/sha256"
	"crypto/subtle"
	"encoding/base64"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"net/http"
	"net/url"
	"strings"
	"sync"
	"time"

	"git.veillax.com/Veillax/nurse/internal/config"
)

// ErrUnavailable means the provider could not be reached or answered nonsense, as opposed to
// rejecting the sign-in.
var ErrUnavailable = errors.New("the sign-in provider is not reachable")

type Client struct {
	issuer, id, secret, redirect string
	http                         *http.Client

	mu      sync.Mutex
	disco   *Discovery
	fetched time.Time
}

// Discovery is the part of the provider's metadata that is used.
type Discovery struct {
	Issuer                string `json:"issuer"`
	AuthorizationEndpoint string `json:"authorization_endpoint"`
	TokenEndpoint         string `json:"token_endpoint"`
}

type Identity struct {
	Sub      string
	Email    string
	Name     string
	Username string
}

func New(issuer, clientID, clientSecret, redirect string) *Client {
	return &Client{
		issuer: strings.TrimRight(issuer, "/"), id: clientID, secret: clientSecret, redirect: redirect,
		http: &http.Client{
			Timeout:       10 * time.Second,
			CheckRedirect: func(*http.Request, []*http.Request) error { return http.ErrUseLastResponse },
		},
	}
}

// Random returns n random bytes as URL-safe text.
func Random(n int) string {
	b := make([]byte, n)
	rand.Read(b)
	return base64.RawURLEncoding.EncodeToString(b)
}

// Challenge is the PKCE S256 challenge for a verifier.
func Challenge(verifier string) string {
	sum := sha256.Sum256([]byte(verifier))
	return base64.RawURLEncoding.EncodeToString(sum[:])
}

func secureEnough(raw string) bool {
	u, err := url.Parse(raw)
	if err != nil || u.Host == "" {
		return false
	}
	return u.Scheme == "https" || (u.Scheme == "http" && config.IsLocal(u))
}

// Discover fetches (and for an hour remembers) the provider's metadata.
func (c *Client) Discover(ctx context.Context) (*Discovery, error) {
	c.mu.Lock()
	defer c.mu.Unlock()
	if c.disco != nil && time.Since(c.fetched) < time.Hour {
		return c.disco, nil
	}
	req, _ := http.NewRequestWithContext(ctx, "GET", c.issuer+"/.well-known/openid-configuration", nil)
	resp, err := c.http.Do(req)
	if err != nil {
		return nil, fmt.Errorf("%w: %v", ErrUnavailable, err)
	}
	defer resp.Body.Close()
	if resp.StatusCode != http.StatusOK {
		return nil, fmt.Errorf("%w: %s answered %s (is the issuer address right?)", ErrUnavailable, req.URL, resp.Status)
	}
	var d Discovery
	if err := json.NewDecoder(io.LimitReader(resp.Body, 1<<20)).Decode(&d); err != nil {
		return nil, fmt.Errorf("%w: unreadable metadata: %v", ErrUnavailable, err)
	}
	if strings.TrimRight(d.Issuer, "/") != c.issuer {
		return nil, fmt.Errorf("%w: the provider says its issuer is %q, not %q", ErrUnavailable, d.Issuer, c.issuer)
	}
	if !secureEnough(d.AuthorizationEndpoint) || !secureEnough(d.TokenEndpoint) {
		return nil, fmt.Errorf("%w: the provider's endpoints are missing or not https", ErrUnavailable)
	}
	c.disco, c.fetched = &d, time.Now()
	return c.disco, nil
}

// AuthURL is where to send the browser to sign in.
func (c *Client) AuthURL(ctx context.Context, state, nonce, challenge string) (string, error) {
	d, err := c.Discover(ctx)
	if err != nil {
		return "", err
	}
	u, _ := url.Parse(d.AuthorizationEndpoint)
	q := u.Query()
	q.Set("response_type", "code")
	q.Set("client_id", c.id)
	q.Set("redirect_uri", c.redirect)
	q.Set("scope", "openid profile email")
	q.Set("state", state)
	q.Set("nonce", nonce)
	q.Set("code_challenge", challenge)
	q.Set("code_challenge_method", "S256")
	u.RawQuery = q.Encode()
	return u.String(), nil
}

type audience []string

func (a *audience) UnmarshalJSON(b []byte) error {
	var one string
	if json.Unmarshal(b, &one) == nil {
		*a = audience{one}
		return nil
	}
	var many []string
	if err := json.Unmarshal(b, &many); err != nil {
		return err
	}
	*a = many
	return nil
}

type claims struct {
	Iss      string   `json:"iss"`
	Sub      string   `json:"sub"`
	Aud      audience `json:"aud"`
	Azp      string   `json:"azp"`
	Exp      float64  `json:"exp"`
	Nonce    string   `json:"nonce"`
	Email    string   `json:"email"`
	Name     string   `json:"name"`
	Username string   `json:"preferred_username"`
}

// Exchange trades the code from the redirect for who signed in.
func (c *Client) Exchange(ctx context.Context, code, verifier, nonce string) (Identity, error) {
	d, err := c.Discover(ctx)
	if err != nil {
		return Identity{}, err
	}
	form := url.Values{
		"grant_type": {"authorization_code"}, "code": {code}, "redirect_uri": {c.redirect},
		"client_id": {c.id}, "client_secret": {c.secret}, "code_verifier": {verifier},
	}
	req, _ := http.NewRequestWithContext(ctx, "POST", d.TokenEndpoint, strings.NewReader(form.Encode()))
	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
	req.Header.Set("Accept", "application/json")
	resp, err := c.http.Do(req)
	if err != nil {
		return Identity{}, fmt.Errorf("%w: %v", ErrUnavailable, err)
	}
	defer resp.Body.Close()
	var tok struct {
		IDToken string `json:"id_token"`
		Error   string `json:"error"`
		Desc    string `json:"error_description"`
	}
	json.NewDecoder(io.LimitReader(resp.Body, 1<<20)).Decode(&tok)
	if resp.StatusCode != http.StatusOK || tok.Error != "" {
		if resp.StatusCode >= 500 {
			return Identity{}, fmt.Errorf("%w: token endpoint answered %s", ErrUnavailable, resp.Status)
		}
		return Identity{}, fmt.Errorf("the provider refused the code: %s %s (%s)", tok.Error, tok.Desc, resp.Status)
	}
	return c.verify(tok.IDToken, nonce, time.Now())
}

func (c *Client) verify(idToken, nonce string, now time.Time) (Identity, error) {
	parts := strings.Split(idToken, ".")
	if len(parts) != 3 {
		return Identity{}, errors.New("the provider sent no ID token (is the openid scope enabled?)")
	}
	raw, err := base64.RawURLEncoding.DecodeString(strings.TrimRight(parts[1], "="))
	if err != nil {
		return Identity{}, errors.New("the ID token is not readable")
	}
	var cl claims
	if err := json.Unmarshal(raw, &cl); err != nil {
		return Identity{}, errors.New("the ID token is not readable")
	}
	switch {
	case strings.TrimRight(cl.Iss, "/") != c.issuer:
		return Identity{}, fmt.Errorf("the ID token is from %q, not %q", cl.Iss, c.issuer)
	case !contains(cl.Aud, c.id):
		return Identity{}, errors.New("the ID token is for a different application")
	case len(cl.Aud) > 1 && cl.Azp != c.id:
		return Identity{}, errors.New("the ID token was issued to a different application")
	case cl.Exp == 0 || now.Unix() > int64(cl.Exp)+60:
		return Identity{}, errors.New("the ID token has expired")
	case nonce == "" || subtle.ConstantTimeCompare([]byte(cl.Nonce), []byte(nonce)) != 1:
		return Identity{}, errors.New("the ID token does not match this sign-in attempt")
	case cl.Sub == "":
		return Identity{}, errors.New("the ID token has no subject")
	}
	return Identity{Sub: cl.Sub, Email: cl.Email, Name: cl.Name, Username: cl.Username}, nil
}

func contains(list []string, s string) bool {
	for _, x := range list {
		if x == s {
			return true
		}
	}
	return false
}