package oidc

import (
	"context"
	"crypto/sha256"
	"encoding/hex"
	"errors"
	"fmt"
	"strings"
	"sync"

	gooidc "github.com/coreos/go-oidc/v3/oidc"
	"golang.org/x/oauth2"
)

// Claims sind die aus dem ID-Token extrahierten Felder, die der Login-
// Flow braucht. Bewusst minimal — Rolle kommt NIE aus dem Token.
type Claims struct {
	Subject       string
	Email         string
	EmailVerified bool
	Nonce         string
}

// Authenticator ist der testbare Seam: Aufbau der Auth-URL und der
// Code-Exchange inkl. ID-Token-Verification + Claim-Extraktion. Der
// Handler hängt nur hieran, sodass Tests einen Fake injizieren können.
type Authenticator interface {
	// AuthCodeURL baut die Redirect-URL zum IdP (state + nonce + PKCE-Challenge).
	AuthCodeURL(ctx context.Context, redirectURI, state, nonce, pkceVerifier string) (string, error)
	// Exchange tauscht den Code (PKCE), verifiziert das ID-Token und gibt
	// die Claims zurück. Prüft Issuer/Audience/Signatur/Expiry.
	Exchange(ctx context.Context, redirectURI, code, pkceVerifier string) (*Claims, error)
}

// Client implementiert Authenticator gegen einen echten OIDC-Provider.
// Provider+Verifier werden lazy aufgebaut und gecached; bei geänderten
// Settings (Fingerprint) neu aufgebaut.
type Client struct {
	repo *Repo

	mu       sync.Mutex
	cacheKey string
	provider *gooidc.Provider
	verifier *gooidc.IDTokenVerifier
}

func NewClient(repo *Repo) *Client { return &Client{repo: repo} }

// loaded baut (oder reused) Provider+Verifier aus den aktuellen Settings.
// Cache-Key = Fingerprint(issuer, client_id, scopes); Rebuild bei Änderung.
func (c *Client) loaded(ctx context.Context) (*gooidc.Provider, *gooidc.IDTokenVerifier, error) {
	s, err := c.repo.Get(ctx)
	if err != nil {
		return nil, nil, err
	}
	if !s.Enabled {
		return nil, nil, ErrDisabled
	}
	if strings.TrimSpace(s.IssuerURL) == "" || strings.TrimSpace(s.ClientID) == "" {
		return nil, nil, fmt.Errorf("oidc: issuer_url and client_id required")
	}
	key := fingerprint(s.IssuerURL, s.ClientID, s.Scopes)
	c.mu.Lock()
	defer c.mu.Unlock()
	if c.provider == nil || c.cacheKey != key {
		prov, err := gooidc.NewProvider(ctx, s.IssuerURL)
		if err != nil {
			return nil, nil, fmt.Errorf("oidc: discovery: %w", err)
		}
		c.provider = prov
		c.verifier = prov.Verifier(&gooidc.Config{ClientID: s.ClientID})
		c.cacheKey = key
	}
	return c.provider, c.verifier, nil
}

func (c *Client) oauthConfig(prov *gooidc.Provider, clientID, secret, redirectURI, scopes string) oauth2.Config {
	return oauth2.Config{
		ClientID:     clientID,
		ClientSecret: secret,
		Endpoint:     prov.Endpoint(),
		RedirectURL:  redirectURI,
		Scopes:       splitScopes(scopes),
	}
}

// AuthCodeURL implementiert Authenticator.
func (c *Client) AuthCodeURL(ctx context.Context, redirectURI, state, nonce, pkceVerifier string) (string, error) {
	s, err := c.repo.Get(ctx)
	if err != nil {
		return "", err
	}
	prov, _, err := c.loaded(ctx)
	if err != nil {
		return "", err
	}
	secret, _ := c.repo.ClientSecret(ctx)
	cfg := c.oauthConfig(prov, s.ClientID, secret, redirectURI, s.Scopes)
	return cfg.AuthCodeURL(state,
		gooidc.Nonce(nonce),
		oauth2.S256ChallengeOption(pkceVerifier),
	), nil
}

// Exchange implementiert Authenticator.
func (c *Client) Exchange(ctx context.Context, redirectURI, code, pkceVerifier string) (*Claims, error) {
	s, err := c.repo.Get(ctx)
	if err != nil {
		return nil, err
	}
	prov, verifier, err := c.loaded(ctx)
	if err != nil {
		return nil, err
	}
	secret, _ := c.repo.ClientSecret(ctx)
	cfg := c.oauthConfig(prov, s.ClientID, secret, redirectURI, s.Scopes)

	tok, err := cfg.Exchange(ctx, code, oauth2.VerifierOption(pkceVerifier))
	if err != nil {
		return nil, fmt.Errorf("oidc: code exchange: %w", err)
	}
	rawID, ok := tok.Extra("id_token").(string)
	if !ok || rawID == "" {
		return nil, errors.New("oidc: no id_token in response")
	}
	idToken, err := verifier.Verify(ctx, rawID)
	if err != nil {
		return nil, fmt.Errorf("oidc: id_token verify: %w", err)
	}
	return extractClaims(idToken, s.EmailClaim)
}

// extractClaims liest E-Mail (via konfigurierbarem Claim), email_verified,
// sub und nonce aus dem verifizierten ID-Token.
func extractClaims(idToken *gooidc.IDToken, emailClaim string) (*Claims, error) {
	var raw map[string]any
	if err := idToken.Claims(&raw); err != nil {
		return nil, fmt.Errorf("oidc: decode claims: %w", err)
	}
	if emailClaim == "" {
		emailClaim = "email"
	}
	out := &Claims{Subject: idToken.Subject}
	if v, ok := raw[emailClaim].(string); ok {
		out.Email = strings.TrimSpace(strings.ToLower(v))
	}
	// email_verified kann bool oder "true"/"false" sein.
	switch ev := raw["email_verified"].(type) {
	case bool:
		out.EmailVerified = ev
	case string:
		out.EmailVerified = ev == "true"
	}
	if n, ok := raw["nonce"].(string); ok {
		out.Nonce = n
	}
	return out, nil
}

func splitScopes(s string) []string {
	out := []string{}
	for _, p := range strings.Fields(s) {
		if p != "" {
			out = append(out, p)
		}
	}
	if len(out) == 0 {
		out = []string{gooidc.ScopeOpenID, "email", "profile"}
	}
	return out
}

func fingerprint(parts ...string) string {
	h := sha256.New()
	for _, p := range parts {
		h.Write([]byte(p))
		h.Write([]byte{0})
	}
	return hex.EncodeToString(h.Sum(nil))
}