package auth

import (
	"context"
	"log/slog"
	"net/http"
	"slices"
	"strings"
	"sync"
	"time"

	"github.com/coreos/go-oidc/v3/oidc"
)

// BearerOIDC verifies `Authorization: Bearer <jwt>` against an OIDC issuer's
// JWKS. Discovery is lazy and retried per request so startup never depends on
// the issuer being reachable.
type BearerOIDC struct {
	issuer    string
	audiences []string

	mu       sync.Mutex
	verifier *oidc.IDTokenVerifier
}

// NewBearerOIDC returns nil when issuer is empty, meaning auth is disabled.
func NewBearerOIDC(issuer string, audiences []string) *BearerOIDC {
	if issuer == "" {
		return nil
	}
	return &BearerOIDC{issuer: issuer, audiences: audiences}
}

func (b *BearerOIDC) getVerifier(ctx context.Context) (*oidc.IDTokenVerifier, error) {
	b.mu.Lock()
	defer b.mu.Unlock()
	if b.verifier != nil {
		return b.verifier, nil
	}
	ctx, cancel := context.WithTimeout(ctx, 10*time.Second)
	defer cancel()
	// The provider keeps this context for JWKS refreshes; it must outlive the request.
	p, err := oidc.NewProvider(oidc.ClientContext(context.WithoutCancel(ctx), http.DefaultClient), b.issuer)
	if err != nil {
		return nil, err
	}
	b.verifier = p.Verifier(&oidc.Config{SkipClientIDCheck: true})
	return b.verifier, nil
}

// Require gates every request through next on a valid token. A nil receiver
// passes everything through.
func (b *BearerOIDC) Require(next http.Handler) http.Handler {
	if b == nil {
		return next
	}
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		raw, ok := strings.CutPrefix(r.Header.Get("Authorization"), "Bearer ")
		if !ok || raw == "" {
			unauthorized(w, "missing bearer token")
			return
		}
		v, err := b.getVerifier(r.Context())
		if err != nil {
			slog.Error("oidc discovery failed", "issuer", b.issuer, "error", err)
			w.Header().Set("Content-Type", "application/json")
			w.WriteHeader(http.StatusServiceUnavailable)
			_, _ = w.Write([]byte(`{"error":"token issuer unavailable"}`))
			return
		}
		tok, err := v.Verify(r.Context(), raw)
		if err != nil {
			slog.Info("bearer token rejected", "error", err)
			unauthorized(w, "invalid token")
			return
		}
		if len(b.audiences) > 0 && !slices.ContainsFunc(tok.Audience, func(a string) bool { return slices.Contains(b.audiences, a) }) {
			slog.Info("bearer token rejected", "error", "audience not allowed", "aud", tok.Audience)
			unauthorized(w, "invalid token")
			return
		}
		slog.Info("bearer token accepted", "sub", tok.Subject, "method", r.Method, "path", r.URL.Path)
		next.ServeHTTP(w, r)
	})
}

// RequireWrites gates only mutating methods.
func (b *BearerOIDC) RequireWrites(next http.Handler) http.Handler {
	gated := b.Require(next)
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		switch r.Method {
		case http.MethodGet, http.MethodHead, http.MethodOptions:
			next.ServeHTTP(w, r)
		default:
			gated.ServeHTTP(w, r)
		}
	})
}

func unauthorized(w http.ResponseWriter, msg string) {
	w.Header().Set("Content-Type", "application/json")
	w.Header().Set("WWW-Authenticate", "Bearer")
	w.WriteHeader(http.StatusUnauthorized)
	_, _ = w.Write([]byte(`{"error":"` + msg + `"}`))
}