package explain

import (
	"crypto/tls"
	"crypto/x509"
	"errors"
	"fmt"
	"io"
	"io/fs"
	"net"
	"syscall"

	"ddc/internal/guard"
)

var keptAsIs = []error{
	guard.ErrBlockedAddress,
	guard.ErrBlockedPort,
	guard.ErrTooManyRedirects,
	guard.ErrUnsupportedScheme,
}

func Error(err error) string {
	if err == nil {
		return ""
	}
	for _, known := range keptAsIs {
		if errors.Is(err, known) {
			return capitalize(known.Error())
		}
	}
	if message := networkMessage(err); message != "" {
		return message
	}
	if message := fileSystemMessage(err); message != "" {
		return message
	}
	return capitalize(err.Error())
}

func networkMessage(err error) string {
	var dnsError *net.DNSError
	var certificateError *tls.CertificateVerificationError
	var unknownAuthority x509.UnknownAuthorityError
	var hostnameError x509.HostnameError
	var timeoutError net.Error

	switch {
	case errors.As(err, &dnsError):
		return fmt.Sprintf("Could not find the server %q. Check the link.", dnsError.Name)
	case errors.As(err, &certificateError), errors.As(err, &unknownAuthority), errors.As(err, &hostnameError):
		return "The server's security certificate is not valid."
	case errors.Is(err, syscall.ECONNREFUSED):
		return "The server refused the connection."
	case errors.Is(err, syscall.ECONNRESET), errors.Is(err, io.ErrUnexpectedEOF):
		return "The server closed the connection before the file was complete. Use Retry to continue."
	case errors.Is(err, syscall.ENETUNREACH), errors.Is(err, syscall.EHOSTUNREACH):
		return "The server could not be reached from this network."
	case errors.As(err, &timeoutError) && timeoutError.Timeout():
		return "The server took too long to respond."
	default:
		return ""
	}
}

func fileSystemMessage(err error) string {
	switch {
	case errors.Is(err, syscall.ENOSPC):
		return "The disk is full."
	case errors.Is(err, fs.ErrPermission):
		return "The server does not have permission to write there. Check the folder owner and permissions."
	case errors.Is(err, syscall.EROFS):
		return "The destination is on a read-only file system."
	default:
		return ""
	}
}

func capitalize(message string) string {
	if message == "" {
		return message
	}
	first := message[0]
	if first >= 'a' && first <= 'z' {
		return string(first-'a'+'A') + message[1:]
	}
	return message
}