package gemini

import (
	"crypto"
	"crypto/ed25519"
	"crypto/rand"
	"crypto/tls"
	"crypto/x509"
	"math/big"
	"net"
	"path"
	"path/filepath"
	"strings"
	"time"
)

// CertificateStore maps hostnames to certificates.
// The zero value of CertificateStore is an empty store ready to use.
type CertificateStore struct {
	store map[string]tls.Certificate
}

// Add adds a certificate for the given scope to the store.
// It tries to parse the certificate if it is not already parsed.
func (c *CertificateStore) Add(scope string, cert tls.Certificate) {
	if c.store == nil {
		c.store = map[string]tls.Certificate{}
	}
	// Parse certificate if not already parsed
	if cert.Leaf == nil {
		parsed, err := x509.ParseCertificate(cert.Certificate[0])
		if err == nil {
			cert.Leaf = parsed
		}
	}
	c.store[scope] = cert
}

// Lookup returns the certificate for the given hostname.
func (c *CertificateStore) Lookup(hostname string) (*tls.Certificate, error) {
	cert, ok := c.store[hostname]
	if !ok {
		return nil, ErrCertificateUnknown
	}
	// Ensure that the certificate is not expired
	if cert.Leaf != nil && cert.Leaf.NotAfter.Before(time.Now()) {
		return &cert, ErrCertificateExpired
	}
	return &cert, nil
}

// lookup returns the certificate for the given hostname + path.
func (c *CertificateStore) lookup(scope string) (*tls.Certificate, error) {
	for {
		cert, err := c.Lookup(scope)
		switch err {
		case ErrCertificateExpired, nil:
			return cert, err
		}
		scope = path.Dir(scope)
		if scope == "." {
			break
		}
	}
	return nil, ErrCertificateUnknown
}

// Load loads certificates from the given path.
// The path should lead to a directory containing certificates and private keys
// in the form hostname.crt and hostname.key.
// For example, the hostname "localhost" would have the corresponding files
// localhost.crt (certificate) and localhost.key (private key).
func (c *CertificateStore) Load(path string) error {
	matches, err := filepath.Glob(filepath.Join(path, "*.crt"))
	if err != nil {
		return err
	}
	for _, crtPath := range matches {
		keyPath := strings.TrimSuffix(crtPath, ".crt") + ".key"
		cert, err := tls.LoadX509KeyPair(crtPath, keyPath)
		if err != nil {
			continue
		}
		hostname := strings.TrimSuffix(filepath.Base(crtPath), ".crt")
		c.Add(hostname, cert)
	}
	return nil
}

// CertificateOptions configures how a certificate is created.
type CertificateOptions struct {
	IPAddresses []net.IP
	DNSNames    []string
	Duration    time.Duration
}

// CreateCertificate creates a new TLS certificate.
func CreateCertificate(options CertificateOptions) (tls.Certificate, error) {
	crt, priv, err := newX509KeyPair(options)
	if err != nil {
		return tls.Certificate{}, err
	}
	var cert tls.Certificate
	cert.Leaf = crt
	cert.Certificate = append(cert.Certificate, crt.Raw)
	cert.PrivateKey = priv
	return cert, nil
}

// newX509KeyPair creates and returns a new certificate and private key.
func newX509KeyPair(options CertificateOptions) (*x509.Certificate, crypto.PrivateKey, error) {
	// Generate an ED25519 private key
	_, priv, err := ed25519.GenerateKey(rand.Reader)
	if err != nil {
		return nil, nil, err
	}
	public := priv.Public()

	// ED25519 keys should have the DigitalSignature KeyUsage bits set
	// in the x509.Certificate template
	keyUsage := x509.KeyUsageDigitalSignature

	serialNumberLimit := new(big.Int).Lsh(big.NewInt(1), 128)
	serialNumber, err := rand.Int(rand.Reader, serialNumberLimit)
	if err != nil {
		return nil, nil, err
	}

	notBefore := time.Now()
	notAfter := notBefore.Add(options.Duration)

	template := x509.Certificate{
		SerialNumber:          serialNumber,
		NotBefore:             notBefore,
		NotAfter:              notAfter,
		KeyUsage:              keyUsage,
		ExtKeyUsage:           []x509.ExtKeyUsage{x509.ExtKeyUsageServerAuth},
		BasicConstraintsValid: true,
		IPAddresses:           options.IPAddresses,
		DNSNames:              options.DNSNames,
	}

	crt, err := x509.CreateCertificate(rand.Reader, &template, &template, public, priv)
	if err != nil {
		return nil, nil, err
	}
	cert, err := x509.ParseCertificate(crt)
	if err != nil {
		return nil, nil, err
	}
	return cert, priv, nil
}