package tls

import (
	"crypto/ecdsa"
	"crypto/elliptic"
	"crypto/rand"
	"crypto/tls"
	"crypto/x509"
	"crypto/x509/pkix"
	"encoding/pem"
	"fmt"
	"math/big"
	"net"
	"os"
	"time"
)

// generateSelfSigned creates a self-signed certificate for the given IP,
// saves it to disk, and returns the tls.Certificate.
func generateSelfSigned(lanIP, certFile, keyFile string) (tls.Certificate, error) {
	key, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
	if err != nil {
		return tls.Certificate{}, fmt.Errorf("generate key: %w", err)
	}

	serialNumber, err := rand.Int(rand.Reader, new(big.Int).Lsh(big.NewInt(1), 128))
	if err != nil {
		return tls.Certificate{}, fmt.Errorf("generate serial: %w", err)
	}

	template := x509.Certificate{
		SerialNumber: serialNumber,
		Subject: pkix.Name{
			Organization: []string{"VoicePaste"},
			CommonName:   "VoicePaste Local",
		},
		NotBefore:             time.Now(),
		NotAfter:              time.Now().Add(365 * 24 * time.Hour), // 1 year
		KeyUsage:              x509.KeyUsageDigitalSignature | x509.KeyUsageKeyEncipherment,
		ExtKeyUsage:           []x509.ExtKeyUsage{x509.ExtKeyUsageServerAuth},
		BasicConstraintsValid: true,
		IPAddresses:           []net.IP{net.ParseIP("127.0.0.1"), net.ParseIP(lanIP)},
		DNSNames:              []string{"localhost"},
	}

	certDER, err := x509.CreateCertificate(rand.Reader, &template, &template, &key.PublicKey, key)
	if err != nil {
		return tls.Certificate{}, fmt.Errorf("create certificate: %w", err)
	}

	// Save cert PEM
	certOut, err := os.Create(certFile)
	if err != nil {
		return tls.Certificate{}, fmt.Errorf("create cert file: %w", err)
	}
	defer certOut.Close()
	pem.Encode(certOut, &pem.Block{Type: "CERTIFICATE", Bytes: certDER})

	// Save key PEM
	keyDER, err := x509.MarshalECPrivateKey(key)
	if err != nil {
		return tls.Certificate{}, fmt.Errorf("marshal key: %w", err)
	}
	keyOut, err := os.OpenFile(keyFile, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0600)
	if err != nil {
		return tls.Certificate{}, fmt.Errorf("create key file: %w", err)
	}
	defer keyOut.Close()
	pem.Encode(keyOut, &pem.Block{Type: "EC PRIVATE KEY", Bytes: keyDER})

	return tls.LoadX509KeyPair(certFile, keyFile)
}