package metacryptclient

import (
	"bytes"
	"context"
	"crypto"
	"crypto/ecdsa"
	"crypto/elliptic"
	"crypto/rand"
	"crypto/rsa"
	"crypto/tls"
	"crypto/x509"
	"crypto/x509/pkix"
	"encoding/json"
	"encoding/pem"
	"fmt"
	"io"
	"net/http"
	"os"
	"time"
)

// loginMCIAS authenticates against the MCIAS server and returns a bearer token.
func loginMCIAS(ctx context.Context, client *http.Client, mciasURL, username, password, totpCode string) (token string, exp time.Time, err error) {
	body, _ := json.Marshal(map[string]string{
		"username":  username,
		"password":  password,
		"totp_code": totpCode,
	})
	req, err := http.NewRequestWithContext(ctx, http.MethodPost, mciasURL+"/v1/auth/login", bytes.NewReader(body))
	if err != nil {
		return "", time.Time{}, err
	}
	req.Header.Set("Content-Type", "application/json")

	resp, err := client.Do(req)
	if err != nil {
		return "", time.Time{}, fmt.Errorf("MCIAS login request: %w", err)
	}
	defer resp.Body.Close()

	if resp.StatusCode != http.StatusOK {
		return "", time.Time{}, fmt.Errorf("MCIAS login: unexpected status %d", resp.StatusCode)
	}

	var result struct {
		Token     string `json:"token"`
		ExpiresAt string `json:"expires_at"`
	}
	if err := json.NewDecoder(resp.Body).Decode(&result); err != nil {
		return "", time.Time{}, fmt.Errorf("MCIAS login: decode response: %w", err)
	}
	if result.Token == "" {
		return "", time.Time{}, fmt.Errorf("MCIAS login: empty token in response")
	}

	var expTime time.Time
	if result.ExpiresAt != "" {
		expTime, _ = time.Parse(time.RFC3339, result.ExpiresAt)
	}
	return result.Token, expTime, nil
}

// fetchEAB calls POST /v1/acme/{mount}/eab on Metacrypt and returns the EAB kid and HMAC key.
func fetchEAB(ctx context.Context, client *http.Client, metacryptURL, mount, token string) (kid string, hmacKey []byte, err error) {
	url := metacryptURL + "/v1/acme/" + mount + "/eab"
	req, err := http.NewRequestWithContext(ctx, http.MethodPost, url, http.NoBody)
	if err != nil {
		return "", nil, err
	}
	req.Header.Set("Authorization", "Bearer "+token)
	req.Header.Set("Content-Type", "application/json")

	resp, err := client.Do(req)
	if err != nil {
		return "", nil, fmt.Errorf("fetch EAB: %w", err)
	}
	defer resp.Body.Close()

	if resp.StatusCode != http.StatusCreated {
		body, _ := io.ReadAll(resp.Body)
		return "", nil, fmt.Errorf("fetch EAB: unexpected status %d: %s", resp.StatusCode, body)
	}

	var result struct {
		KID     string `json:"kid"`
		HMACKey []byte `json:"hmac_key"`
	}
	if err := json.NewDecoder(resp.Body).Decode(&result); err != nil {
		return "", nil, fmt.Errorf("fetch EAB: decode response: %w", err)
	}
	return result.KID, result.HMACKey, nil
}

// buildHTTPClient creates an HTTP client that optionally trusts a custom CA.
func buildHTTPClient(caCertPath string) *http.Client {
	tlsCfg := &tls.Config{MinVersion: tls.VersionTLS13}

	if caCertPath != "" {
		pool := x509.NewCertPool()
		if data, err := os.ReadFile(caCertPath); err == nil {
			pool.AppendCertsFromPEM(data)
			tlsCfg.RootCAs = pool
		}
	}

	return &http.Client{
		Transport: &http.Transport{TLSClientConfig: tlsCfg},
		Timeout:   30 * time.Second,
	}
}

// generateKey generates a certificate key of the requested type.
// Supported types: "EC256" (default), "EC384", "RSA2048", "RSA4096".
func generateKey(keyType string) (crypto.Signer, error) {
	switch keyType {
	case "", "EC256":
		return ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
	case "EC384":
		return ecdsa.GenerateKey(elliptic.P384(), rand.Reader)
	case "RSA2048":
		return rsa.GenerateKey(rand.Reader, 2048)
	case "RSA4096":
		return rsa.GenerateKey(rand.Reader, 4096)
	default:
		return nil, fmt.Errorf("unsupported key type: %s", keyType)
	}
}

// buildCSR creates a DER-encoded certificate signing request.
func buildCSR(key crypto.Signer, domains []string) ([]byte, error) {
	if len(domains) == 0 {
		return nil, fmt.Errorf("at least one domain required")
	}
	template := &x509.CertificateRequest{
		Subject:  pkix.Name{CommonName: domains[0]},
		DNSNames: domains,
	}
	return x509.CreateCertificateRequest(rand.Reader, template, key)
}

// encodeCertChainPEM encodes a chain of DER-encoded certificates to PEM.
func encodeCertChainPEM(chain [][]byte) []byte {
	var buf bytes.Buffer
	for _, der := range chain {
		pem.Encode(&buf, &pem.Block{Type: "CERTIFICATE", Bytes: der})
	}
	return buf.Bytes()
}

// encodeKeyPEM encodes a private key to PEM (PKCS8 format).
func encodeKeyPEM(key crypto.Signer) ([]byte, error) {
	der, err := x509.MarshalPKCS8PrivateKey(key)
	if err != nil {
		return nil, err
	}
	return pem.EncodeToMemory(&pem.Block{Type: "PRIVATE KEY", Bytes: der}), nil
}

// parseCertExpiry parses the expiry time from a DER-encoded certificate.
func parseCertExpiry(der []byte) (time.Time, error) {
	cert, err := x509.ParseCertificate(der)
	if err != nil {
		return time.Time{}, err
	}
	return cert.NotAfter, nil
}