// Package ftp provides the FTP side of SnapGo's FTP/SFTP destination.
//
// Design rationale:
//   - Plain FTP needs a real protocol client for PASV/EPSV handling; hand-
//     rolling those details would be fragile across common servers.
//   - SFTP is delegated to the sibling SSH adapter so authentication, agent,
//     key discovery, and known_hosts behaviour stay consistent with SCP.
//   - Both implementations upload to a temporary name and rename only after
//     the payload is complete, preventing a failed transfer from exposing a
//     partially written screenshot at the final path.
package ftp

import (
	"bytes"
	"context"
	"fmt"
	"io"
	"log/slog"
	"net"
	"path"
	"strings"
	"sync"
	"time"

	ftpclient "github.com/jlaffaye/ftp"

	"github.com/mmmy/snapgo/internal/domain"
	sshpkg "github.com/mmmy/snapgo/internal/infrastructure/ssh"
)

// RemoteUploader is the common capability exposed to app.go for FTP and
// SFTP. The application service only needs Upload; TestConnection remains an
// infrastructure concern used by the Settings probe RPC.
type RemoteUploader interface {
	Upload(ctx context.Context, remoteRelPath string, data []byte) error
	TestConnection(ctx context.Context) error
}

// NewUploader validates cfg and returns the adapter for its selected protocol.
func NewUploader(cfg domain.FTPConfig) (RemoteUploader, error) {
	normalized, err := normalizeConfig(cfg)
	if err != nil {
		return nil, err
	}
	if normalized.Protocol == domain.FTPProtocolSFTP {
		return sshpkg.NewSFTPUploader(normalized), nil
	}
	return &plainUploader{cfg: normalized}, nil
}

type plainUploader struct {
	cfg domain.FTPConfig
}

func ftpLog() *slog.Logger { return slog.Default().With("component", "ftp") }

func normalizeConfig(cfg domain.FTPConfig) (domain.FTPConfig, error) {
	cfg.Protocol = strings.ToLower(strings.TrimSpace(cfg.Protocol))
	if cfg.Protocol == "" {
		cfg.Protocol = domain.FTPProtocolFTP
	}
	if cfg.Protocol != domain.FTPProtocolFTP && cfg.Protocol != domain.FTPProtocolSFTP {
		return domain.FTPConfig{}, fmt.Errorf("file transfer: protocol must be ftp or sftp")
	}
	cfg.Host = strings.TrimSpace(cfg.Host)
	cfg.User = strings.TrimSpace(cfg.User)
	if cfg.Host == "" || cfg.User == "" {
		return domain.FTPConfig{}, fmt.Errorf("file transfer: host and user are required")
	}
	if cfg.Port <= 0 {
		if cfg.Protocol == domain.FTPProtocolSFTP {
			cfg.Port = 22
		} else {
			cfg.Port = 21
		}
	}
	if cfg.Port > 65535 {
		return domain.FTPConfig{}, fmt.Errorf("file transfer: port must be between 1 and 65535")
	}
	if cfg.ConnectTimeoutSecs <= 0 {
		cfg.ConnectTimeoutSecs = 10
	}
	if cfg.AuthMethod == "" {
		cfg.AuthMethod = domain.SSHAuthPassword
	}
	if cfg.Protocol == domain.FTPProtocolSFTP &&
		cfg.AuthMethod != domain.SSHAuthPassword && cfg.AuthMethod != domain.SSHAuthKey {
		return domain.FTPConfig{}, fmt.Errorf("sftp: auth method must be password or key")
	}
	return cfg, nil
}

func (u *plainUploader) dial(ctx context.Context) (*ftpclient.ServerConn, error) {
	timeout := time.Duration(u.cfg.ConnectTimeoutSecs) * time.Second
	addr := net.JoinHostPort(u.cfg.Host, fmt.Sprintf("%d", u.cfg.Port))
	ftpLog().Info("FTP dial start",
		"addr", addr,
		"user", u.cfg.User,
		"timeout", timeout,
		"has_password", u.cfg.Password != "")
	conn, err := ftpclient.Dial(
		addr,
		ftpclient.DialWithDialFunc(contextDialFunc(ctx, timeout)),
		ftpclient.DialWithShutTimeout(timeout),
	)
	if err != nil {
		return nil, fmt.Errorf("ftp: dial %s: %w", addr, err)
	}
	if err := conn.Login(u.cfg.User, u.cfg.Password); err != nil {
		_ = conn.Quit()
		return nil, fmt.Errorf("ftp: login: %w", err)
	}
	return conn, nil
}

// Upload stores one screenshot through plain FTP.
func (u *plainUploader) Upload(ctx context.Context, remoteRelPath string, data []byte) error {
	cleaned, err := validateRemoteRelativePath(remoteRelPath)
	if err != nil {
		return err
	}
	conn, err := u.dial(ctx)
	if err != nil {
		return err
	}
	defer func() { _ = conn.Quit() }()

	if err := enterFTPDirectory(conn, path.Dir(cleaned)); err != nil {
		return err
	}
	base := path.Base(cleaned)

	tempName := temporaryRemoteName(base)
	removeTemp := true
	defer func() {
		if removeTemp {
			_ = conn.Delete(tempName)
		}
	}()

	if err := conn.Stor(tempName, &contextReader{ctx: ctx, reader: bytes.NewReader(data)}); err != nil {
		if ctx.Err() != nil {
			return ctx.Err()
		}
		return fmt.Errorf("ftp: store %s: %w", cleaned, err)
	}
	if err := conn.Rename(tempName, base); err != nil {
		return fmt.Errorf("ftp: commit %s: %w", cleaned, err)
	}
	removeTemp = false
	ftpLog().Info("FTP upload ok", "remote_path", cleaned, "size", len(data))
	return nil
}

// TestConnection writes and removes a small probe under the configured path,
// proving that credentials and directory permissions are both usable.
func (u *plainUploader) TestConnection(ctx context.Context) error {
	probe, err := probeRemotePath(u.cfg.PathPrefix)
	if err != nil {
		return err
	}
	conn, err := u.dial(ctx)
	if err != nil {
		return err
	}
	defer func() { _ = conn.Quit() }()

	if err := enterFTPDirectory(conn, path.Dir(probe)); err != nil {
		return err
	}
	base := path.Base(probe)
	if err := conn.Stor(base, strings.NewReader("snapgo")); err != nil {
		return fmt.Errorf("ftp probe: write: %w", err)
	}
	if err := conn.Delete(base); err != nil {
		return fmt.Errorf("ftp probe: delete: %w", err)
	}
	return nil
}

// enterFTPDirectory walks to dir relative to the login root, creating missing
// segments.
func enterFTPDirectory(conn *ftpclient.ServerConn, dir string) error {
	dir = strings.Trim(dir, "/")
	if dir == "" || dir == "." {
		return nil
	}
	for _, segment := range strings.Split(dir, "/") {
		if err := conn.ChangeDir(segment); err == nil {
			continue
		}
		if err := conn.MakeDir(segment); err != nil {
			// A concurrent uploader may have created the directory after our
			// failed CWD. Retrying CWD distinguishes that race from a real error.
			if retryErr := conn.ChangeDir(segment); retryErr == nil {
				continue
			}
			return fmt.Errorf("ftp: create directory %q: %w", segment, err)
		}
		if err := conn.ChangeDir(segment); err != nil {
			return fmt.Errorf("ftp: enter directory %q: %w", segment, err)
		}
	}
	return nil
}

func validateRemoteRelativePath(value string) (string, error) {
	value = strings.TrimSpace(value)
	if value == "" || strings.HasPrefix(value, "/") || strings.HasPrefix(value, "~") {
		return "", fmt.Errorf("file transfer: remote path must be relative to the login directory")
	}
	if strings.ContainsAny(value, "\x00\r\n\\") {
		return "", fmt.Errorf("file transfer: remote path contains an invalid character")
	}
	for _, segment := range strings.Split(value, "/") {
		if segment == ".." {
			return "", fmt.Errorf("file transfer: remote path must not contain '..'")
		}
	}
	cleaned := path.Clean(value)
	if cleaned == "." || path.Base(cleaned) == "." || path.Base(cleaned) == ".." {
		return "", fmt.Errorf("file transfer: remote filename is empty")
	}
	return cleaned, nil
}

func probeRemotePath(prefix string) (string, error) {
	prefix = strings.TrimSpace(prefix)
	prefix = strings.TrimPrefix(prefix, "~")
	prefix = strings.TrimLeft(prefix, "/")
	probe := fmt.Sprintf(".snapgo-probe-%d", time.Now().UnixNano())
	return validateRemoteRelativePath(path.Join(prefix, probe))
}

func temporaryRemoteName(base string) string {
	return fmt.Sprintf(".%s.snapgo-upload-%d", base, time.Now().UnixNano())
}

type contextReader struct {
	ctx    context.Context
	reader io.Reader
}

func contextDialFunc(ctx context.Context, timeout time.Duration) func(string, string) (net.Conn, error) {
	dialer := &net.Dialer{Timeout: timeout}
	return func(network, address string) (net.Conn, error) {
		conn, err := dialer.DialContext(ctx, network, address)
		if err != nil {
			return nil, err
		}
		wrapped := &cancelableConn{
			Conn: conn,
			done: make(chan struct{}),
		}
		go func() {
			select {
			case <-ctx.Done():
				_ = wrapped.Close()
			case <-wrapped.done:
			}
		}()
		return wrapped, nil
	}
}

type cancelableConn struct {
	net.Conn
	done chan struct{}
	once sync.Once
}

func (c *cancelableConn) Close() error {
	var err error
	c.once.Do(func() {
		close(c.done)
		err = c.Conn.Close()
	})
	return err
}

func (r *contextReader) Read(p []byte) (int, error) {
	select {
	case <-r.ctx.Done():
		return 0, r.ctx.Err()
	default:
		return r.reader.Read(p)
	}
}