// Copyright (C) 2026 Dcntrlized
//
// 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 <https://www.gnu.org/licenses/>.

package pki

import (
	"crypto/rand"
	"crypto/x509"
	"crypto/x509/pkix"
	"encoding/json"
	"encoding/pem"
	"fmt"
	"math/big"
	"os"
	"path/filepath"
	"strings"
	"time"
)

// RevokedCert is the persisted record of a revoked certificate (CRL source).
type RevokedCert struct {
	Serial    string    `json:"serial"`
	Service   string    `json:"service"`
	RevokedAt time.Time `json:"revoked_at"`
	Reason    string    `json:"reason,omitempty"`
}

// RevokedSerialsDir returns the directory holding revoked certificates.
func RevokedSerialsDir(dataRoot string) string {
	return filepath.Join(Dir(dataRoot), revokedDir)
}

// Revoke revokes a service's identity certificate: the leaf certificate is
// recorded as revoked and the current CRL is regenerated, then the service's
// identity files are removed. Peers stop accepting the cert within one CRL
// freshness window (security theme M3 §2).
func Revoke(dataRoot, serviceName string) error {
	ca, err := EnsureCA(dataRoot)
	if err != nil {
		return fmt.Errorf("load CA for revocation: %w", err)
	}

	meta, metaErr := readMeta(ServiceDir(dataRoot, serviceName))
	if metaErr == nil && meta.Serial != "" {
		serial := new(big.Int)
		if _, ok := serial.SetString(meta.Serial, 16); ok {
			if err := recordRevoked(ca, serviceName, serial, "uninstall"); err != nil {
				return err
			}
		}
	}

	// Remove the identity files so a revoked service cannot be restarted with
	// a valid certificate (it will get a fresh one on reinstall).
	if err := os.RemoveAll(ServiceDir(dataRoot, serviceName)); err != nil {
		return fmt.Errorf("remove identity for %s: %w", serviceName, err)
	}
	return nil
}

// recordRevoked persists a revoked certificate and regenerates the CRL.
func recordRevoked(ca *CA, serviceName string, serial *big.Int, reason string) error {
	dir := RevokedSerialsDir(ca.DataRoot)
	if err := os.MkdirAll(dir, 0o700); err != nil {
		return fmt.Errorf("create revoked dir: %w", err)
	}

	revoked := RevokedCert{
		Serial:    serial.Text(16),
		Service:   serviceName,
		RevokedAt: now(),
		Reason:    reason,
	}
	if err := writeFile(filepath.Join(dir, serial.Text(16)+".json"), marshalOrDie(revoked), 0o600); err != nil {
		return fmt.Errorf("record revoked cert: %w", err)
	}

	return regenerateCRL(ca)
}

// LoadRevokedSerials returns the serials currently revoked for this instance.
func LoadRevokedSerials(dataRoot string) ([]*big.Int, error) {
	dir := RevokedSerialsDir(dataRoot)
	entries, err := os.ReadDir(dir)
	if err != nil {
		if os.IsNotExist(err) {
			return nil, nil
		}
		return nil, err
	}
	var serials []*big.Int
	for _, e := range entries {
		if e.IsDir() || !strings.HasSuffix(e.Name(), ".json") {
			continue
		}
		name := strings.TrimSuffix(e.Name(), ".json")
		serial := new(big.Int)
		if _, ok := serial.SetString(name, 16); ok {
			serials = append(serials, serial)
		}
	}
	return serials, nil
}

// regenerateCRL writes crl.pem listing every currently-revoked serial. Peers
// re-fetch it with a short freshness window (security theme M3).
func regenerateCRL(ca *CA) error {
	serials, err := LoadRevokedSerials(ca.DataRoot)
	if err != nil {
		return fmt.Errorf("load revoked serials: %w", err)
	}

	template := &x509.RevocationList{
		Number:              mustSerial(),
		ThisUpdate:          now(),
		NextUpdate:          now().Add(24 * time.Hour),
		RevokedCertificates: revokedEntries(serials),
	}

	der, err := x509.CreateRevocationList(rand.Reader, template, ca.Cert, ca.PrivateKey)
	if err != nil {
		return fmt.Errorf("create CRL: %w", err)
	}
	if err := writeFile(CRLPath(ca.DataRoot), pem.EncodeToMemory(&pem.Block{Type: "X509 CRL", Bytes: der}), 0o644); err != nil {
		return fmt.Errorf("write CRL: %w", err)
	}
	return nil
}

func revokedEntries(serials []*big.Int) []pkix.RevokedCertificate {
	entries := make([]pkix.RevokedCertificate, 0, len(serials))
	for _, s := range serials {
		entries = append(entries, pkix.RevokedCertificate{
			SerialNumber:   s,
			RevocationTime: now(),
		})
	}
	return entries
}

func mustSerial() *big.Int {
	serial, err := randomSerial()
	if err != nil {
		panic(fmt.Sprintf("pki: failed to generate CRL number: %v", err))
	}
	return serial
}

func marshalOrDie(v any) []byte {
	data, err := json.MarshalIndent(v, "", "  ")
	if err != nil {
		panic(fmt.Sprintf("pki: failed to marshal: %v", err))
	}
	return data
}