// Copyright 2018-2024 the Deno authors. All rights reserved. MIT license.

use std::collections::BTreeMap;
use std::env::current_exe;
use std::fs;
use std::future::Future;
use std::io::Read;
use std::io::Seek;
use std::io::SeekFrom;
use std::io::Write;
use std::path::Path;
use std::path::PathBuf;
use std::process::Command;

use deno_ast::ModuleSpecifier;
use deno_core::anyhow::bail;
use deno_core::anyhow::Context;
use deno_core::error::AnyError;
use deno_core::futures::io::AllowStdIo;
use deno_core::futures::AsyncReadExt;
use deno_core::futures::AsyncSeekExt;
use deno_core::serde_json;
use deno_core::url::Url;
use deno_npm::NpmSystemInfo;
use deno_runtime::permissions::PermissionsOptions;
use deno_semver::package::PackageReq;
use deno_semver::VersionReqSpecifierParseError;
use log::Level;
use serde::Deserialize;
use serde::Serialize;

use crate::args::package_json::PackageJsonDepValueParseError;
use crate::args::package_json::PackageJsonDeps;
use crate::args::CaData;
use crate::args::CliOptions;
use crate::args::CompileFlags;
use crate::args::PackageJsonDepsProvider;
use crate::args::UnstableConfig;
use crate::cache::DenoDir;
use crate::file_fetcher::FileFetcher;
use crate::http_util::HttpClient;
use crate::npm::CliNpmResolver;
use crate::npm::InnerCliNpmResolverRef;
use crate::util::progress_bar::ProgressBar;
use crate::util::progress_bar::ProgressBarStyle;

use super::virtual_fs::FileBackedVfs;
use super::virtual_fs::VfsBuilder;
use super::virtual_fs::VfsRoot;
use super::virtual_fs::VirtualDirectory;

const MAGIC_TRAILER: &[u8; 8] = b"d3n0l4nd";

#[derive(Serialize, Deserialize)]
enum SerializablePackageJsonDepValueParseError {
  Specifier(String),
  Unsupported { scheme: String },
}

impl SerializablePackageJsonDepValueParseError {
  pub fn from_err(err: PackageJsonDepValueParseError) -> Self {
    match err {
      PackageJsonDepValueParseError::Specifier(err) => {
        Self::Specifier(err.source.to_string())
      }
      PackageJsonDepValueParseError::Unsupported { scheme } => {
        Self::Unsupported { scheme }
      }
    }
  }

  pub fn into_err(self) -> PackageJsonDepValueParseError {
    match self {
      SerializablePackageJsonDepValueParseError::Specifier(source) => {
        PackageJsonDepValueParseError::Specifier(
          VersionReqSpecifierParseError {
            source: monch::ParseErrorFailureError::new(source),
          },
        )
      }
      SerializablePackageJsonDepValueParseError::Unsupported { scheme } => {
        PackageJsonDepValueParseError::Unsupported { scheme }
      }
    }
  }
}

#[derive(Serialize, Deserialize)]
pub struct SerializablePackageJsonDeps(
  BTreeMap<
    String,
    Result<PackageReq, SerializablePackageJsonDepValueParseError>,
  >,
);

impl SerializablePackageJsonDeps {
  pub fn from_deps(deps: PackageJsonDeps) -> Self {
    Self(
      deps
        .into_iter()
        .map(|(name, req)| {
          let res =
            req.map_err(SerializablePackageJsonDepValueParseError::from_err);
          (name, res)
        })
        .collect(),
    )
  }

  pub fn into_deps(self) -> PackageJsonDeps {
    self
      .0
      .into_iter()
      .map(|(name, res)| (name, res.map_err(|err| err.into_err())))
      .collect()
  }
}

#[derive(Deserialize, Serialize)]
pub enum NodeModules {
  Managed {
    /// Whether this uses a node_modules directory (true) or the global cache (false).
    node_modules_dir: bool,
    package_json_deps: Option<SerializablePackageJsonDeps>,
  },
  Byonm {
    package_json_deps: Option<SerializablePackageJsonDeps>,
  },
}

#[derive(Deserialize, Serialize)]
pub struct Metadata {
  pub argv: Vec<String>,
  pub seed: Option<u64>,
  pub permissions: PermissionsOptions,
  pub location: Option<Url>,
  pub v8_flags: Vec<String>,
  pub log_level: Option<Level>,
  pub ca_stores: Option<Vec<String>>,
  pub ca_data: Option<Vec<u8>>,
  pub unsafely_ignore_certificate_errors: Option<Vec<String>>,
  pub maybe_import_map: Option<(Url, String)>,
  pub entrypoint: ModuleSpecifier,
  pub node_modules: Option<NodeModules>,
  pub disable_deprecated_api_warning: bool,
  pub unstable_config: UnstableConfig,
}

pub fn load_npm_vfs(root_dir_path: PathBuf) -> Result<FileBackedVfs, AnyError> {
  let file_path = current_exe().unwrap();
  let mut file = std::fs::File::open(file_path)?;
  file.seek(SeekFrom::End(-(TRAILER_SIZE as i64)))?;
  let mut trailer = [0; TRAILER_SIZE];
  file.read_exact(&mut trailer)?;
  let trailer = Trailer::parse(&trailer)?.unwrap();
  file.seek(SeekFrom::Start(trailer.npm_vfs_pos))?;
  let mut vfs_data = vec![0; trailer.npm_vfs_len() as usize];
  file.read_exact(&mut vfs_data)?;
  let mut dir: VirtualDirectory = serde_json::from_slice(&vfs_data)?;

  // align the name of the directory with the root dir
  dir.name = root_dir_path
    .file_name()
    .unwrap()
    .to_string_lossy()
    .to_string();

  let fs_root = VfsRoot {
    dir,
    root_path: root_dir_path,
    start_file_offset: trailer.npm_files_pos,
  };
  Ok(FileBackedVfs::new(file, fs_root))
}

fn write_binary_bytes(
  writer: &mut impl Write,
  original_bin: Vec<u8>,
  metadata: &Metadata,
  eszip: eszip::EszipV2,
  npm_vfs: Option<&VirtualDirectory>,
  npm_files: &Vec<Vec<u8>>,
) -> Result<(), AnyError> {
  let metadata = serde_json::to_string(metadata)?.as_bytes().to_vec();
  let npm_vfs = serde_json::to_string(&npm_vfs)?.as_bytes().to_vec();
  let eszip_archive = eszip.into_bytes();

  writer.write_all(&original_bin)?;
  writer.write_all(&eszip_archive)?;
  writer.write_all(&metadata)?;
  writer.write_all(&npm_vfs)?;
  for file in npm_files {
    writer.write_all(file)?;
  }

  // write the trailer, which includes the positions
  // of the data blocks in the file
  writer.write_all(&{
    let eszip_pos = original_bin.len() as u64;
    let metadata_pos = eszip_pos + (eszip_archive.len() as u64);
    let npm_vfs_pos = metadata_pos + (metadata.len() as u64);
    let npm_files_pos = npm_vfs_pos + (npm_vfs.len() as u64);
    Trailer {
      eszip_pos,
      metadata_pos,
      npm_vfs_pos,
      npm_files_pos,
    }
    .as_bytes()
  })?;

  Ok(())
}

pub fn is_standalone_binary(exe_path: &Path) -> bool {
  let Ok(mut output_file) = std::fs::File::open(exe_path) else {
    return false;
  };
  if output_file
    .seek(SeekFrom::End(-(TRAILER_SIZE as i64)))
    .is_err()
  {
    // This seek may fail because the file is too small to possibly be
    // `deno compile` output.
    return false;
  }
  let mut trailer = [0; TRAILER_SIZE];
  if output_file.read_exact(&mut trailer).is_err() {
    return false;
  };
  let (magic_trailer, _) = trailer.split_at(8);
  magic_trailer == MAGIC_TRAILER
}

/// This function will try to run this binary as a standalone binary
/// produced by `deno compile`. It determines if this is a standalone
/// binary by skipping over the trailer width at the end of the file,
/// then checking for the magic trailer string `d3n0l4nd`. If found,
/// the bundle is executed. If not, this function exits with `Ok(None)`.
pub fn extract_standalone(
  exe_path: &Path,
  cli_args: Vec<String>,
) -> Result<
  Option<impl Future<Output = Result<(Metadata, eszip::EszipV2), AnyError>>>,
  AnyError,
> {
  // We do the first part sync so it can complete quickly
  let mut file = std::fs::File::open(exe_path)?;
  file.seek(SeekFrom::End(-(TRAILER_SIZE as i64)))?;
  let mut trailer = [0; TRAILER_SIZE];
  file.read_exact(&mut trailer)?;
  let trailer = match Trailer::parse(&trailer)? {
    None => return Ok(None),
    Some(trailer) => trailer,
  };

  file.seek(SeekFrom::Start(trailer.eszip_pos))?;

  // If we have an eszip, read it out
  Ok(Some(async move {
    let bufreader =
      deno_core::futures::io::BufReader::new(AllowStdIo::new(file));

    let (eszip, loader) = eszip::EszipV2::parse(bufreader)
      .await
      .context("Failed to parse eszip header")?;

    let mut bufreader =
      loader.await.context("Failed to parse eszip archive")?;

    bufreader
      .seek(SeekFrom::Start(trailer.metadata_pos))
      .await?;

    let mut metadata = String::new();

    bufreader
      .take(trailer.metadata_len())
      .read_to_string(&mut metadata)
      .await
      .context("Failed to read metadata from the current executable")?;

    let mut metadata: Metadata = serde_json::from_str(&metadata).unwrap();
    metadata.argv.append(&mut cli_args[1..].to_vec());

    Ok((metadata, eszip))
  }))
}

const TRAILER_SIZE: usize = std::mem::size_of::<Trailer>() + 8; // 8 bytes for the magic trailer string

struct Trailer {
  eszip_pos: u64,
  metadata_pos: u64,
  npm_vfs_pos: u64,
  npm_files_pos: u64,
}

impl Trailer {
  pub fn parse(trailer: &[u8]) -> Result<Option<Trailer>, AnyError> {
    let (magic_trailer, rest) = trailer.split_at(8);
    if magic_trailer != MAGIC_TRAILER {
      return Ok(None);
    }

    let (eszip_archive_pos, rest) = rest.split_at(8);
    let (metadata_pos, rest) = rest.split_at(8);
    let (npm_vfs_pos, npm_files_pos) = rest.split_at(8);
    let eszip_archive_pos = u64_from_bytes(eszip_archive_pos)?;
    let metadata_pos = u64_from_bytes(metadata_pos)?;
    let npm_vfs_pos = u64_from_bytes(npm_vfs_pos)?;
    let npm_files_pos = u64_from_bytes(npm_files_pos)?;
    Ok(Some(Trailer {
      eszip_pos: eszip_archive_pos,
      metadata_pos,
      npm_vfs_pos,
      npm_files_pos,
    }))
  }

  pub fn metadata_len(&self) -> u64 {
    self.npm_vfs_pos - self.metadata_pos
  }

  pub fn npm_vfs_len(&self) -> u64 {
    self.npm_files_pos - self.npm_vfs_pos
  }

  pub fn as_bytes(&self) -> Vec<u8> {
    let mut trailer = MAGIC_TRAILER.to_vec();
    trailer.write_all(&self.eszip_pos.to_be_bytes()).unwrap();
    trailer.write_all(&self.metadata_pos.to_be_bytes()).unwrap();
    trailer.write_all(&self.npm_vfs_pos.to_be_bytes()).unwrap();
    trailer
      .write_all(&self.npm_files_pos.to_be_bytes())
      .unwrap();
    trailer
  }
}

fn u64_from_bytes(arr: &[u8]) -> Result<u64, AnyError> {
  let fixed_arr: &[u8; 8] = arr
    .try_into()
    .context("Failed to convert the buffer into a fixed-size array")?;
  Ok(u64::from_be_bytes(*fixed_arr))
}

pub fn unpack_into_dir(
  exe_name: &str,
  archive_name: &str,
  archive_data: Vec<u8>,
  is_windows: bool,
  temp_dir: &tempfile::TempDir,
) -> Result<PathBuf, AnyError> {
  let temp_dir_path = temp_dir.path();
  let exe_ext = if is_windows { "exe" } else { "" };
  let archive_path = temp_dir_path.join(exe_name).with_extension("zip");
  let exe_path = temp_dir_path.join(exe_name).with_extension(exe_ext);
  assert!(!exe_path.exists());

  let archive_ext = Path::new(archive_name)
    .extension()
    .and_then(|ext| ext.to_str())
    .unwrap();
  let unpack_status = match archive_ext {
    "zip" if cfg!(windows) => {
      fs::write(&archive_path, &archive_data)?;
      Command::new("tar.exe")
        .arg("xf")
        .arg(&archive_path)
        .arg("-C")
        .arg(temp_dir_path)
        .spawn()
        .map_err(|err| {
          if err.kind() == std::io::ErrorKind::NotFound {
            std::io::Error::new(
              std::io::ErrorKind::NotFound,
              "`tar.exe` was not found in your PATH",
            )
          } else {
            err
          }
        })?
        .wait()?
    }
    "zip" => {
      fs::write(&archive_path, &archive_data)?;
      Command::new("unzip")
        .current_dir(temp_dir_path)
        .arg(&archive_path)
        .spawn()
        .map_err(|err| {
          if err.kind() == std::io::ErrorKind::NotFound {
            std::io::Error::new(
              std::io::ErrorKind::NotFound,
              "`unzip` was not found in your PATH, please install `unzip`",
            )
          } else {
            err
          }
        })?
        .wait()?
    }
    ext => bail!("Unsupported archive type: '{ext}'"),
  };
  if !unpack_status.success() {
    bail!("Failed to unpack archive.");
  }
  assert!(exe_path.exists());
  fs::remove_file(&archive_path)?;
  Ok(exe_path)
}
pub struct DenoCompileBinaryWriter<'a> {
  file_fetcher: &'a FileFetcher,
  client: &'a HttpClient,
  deno_dir: &'a DenoDir,
  npm_resolver: &'a dyn CliNpmResolver,
  npm_system_info: NpmSystemInfo,
  package_json_deps_provider: &'a PackageJsonDepsProvider,
}

impl<'a> DenoCompileBinaryWriter<'a> {
  #[allow(clippy::too_many_arguments)]
  pub fn new(
    file_fetcher: &'a FileFetcher,
    client: &'a HttpClient,
    deno_dir: &'a DenoDir,
    npm_resolver: &'a dyn CliNpmResolver,
    npm_system_info: NpmSystemInfo,
    package_json_deps_provider: &'a PackageJsonDepsProvider,
  ) -> Self {
    Self {
      file_fetcher,
      client,
      deno_dir,
      npm_resolver,
      npm_system_info,
      package_json_deps_provider,
    }
  }

  pub async fn write_bin(
    &self,
    writer: &mut impl Write,
    eszip: eszip::EszipV2,
    module_specifier: &ModuleSpecifier,
    compile_flags: &CompileFlags,
    cli_options: &CliOptions,
  ) -> Result<(), AnyError> {
    // Select base binary based on target
    let mut original_binary = self.get_base_binary(compile_flags).await?;

    if compile_flags.no_terminal {
      let target = compile_flags.resolve_target();
      if !target.contains("windows") {
        bail!(
          "The `--no-terminal` flag is only available when targeting Windows (current: {})",
          target,
        )
      }
      set_windows_binary_to_gui(&mut original_binary)?;
    }

    self
      .write_standalone_binary(
        writer,
        original_binary,
        eszip,
        module_specifier,
        cli_options,
        compile_flags,
      )
      .await
  }

  async fn get_base_binary(
    &self,
    compile_flags: &CompileFlags,
  ) -> Result<Vec<u8>, AnyError> {
    // Used for testing.
    //
    // Phase 2 of the 'min sized' deno compile RFC talks
    // about adding this as a flag.
    if let Some(path) = std::env::var_os("DENORT_BIN") {
      return Ok(std::fs::read(path)?);
    }

    let target = compile_flags.resolve_target();
    let binary_name = format!("denort-{target}.zip");

    let binary_path_suffix = if crate::version::is_canary() {
      format!("canary/{}/{}", crate::version::GIT_COMMIT_HASH, binary_name)
    } else {
      format!("release/v{}/{}", env!("CARGO_PKG_VERSION"), binary_name)
    };

    let download_directory = self.deno_dir.dl_folder_path();
    let binary_path = download_directory.join(&binary_path_suffix);

    if !binary_path.exists() {
      self
        .download_base_binary(&download_directory, &binary_path_suffix)
        .await?;
    }

    let archive_data = std::fs::read(binary_path)?;
    let temp_dir = tempfile::TempDir::new()?;
    let base_binary_path = unpack_into_dir(
      "denort",
      &binary_name,
      archive_data,
      target.contains("windows"),
      &temp_dir,
    )?;
    let base_binary = std::fs::read(base_binary_path)?;
    drop(temp_dir); // delete the temp dir
    Ok(base_binary)
  }

  async fn download_base_binary(
    &self,
    output_directory: &Path,
    binary_path_suffix: &str,
  ) -> Result<(), AnyError> {
    let download_url = format!("https://dl.deno.land/{binary_path_suffix}");
    let maybe_bytes = {
      let progress_bars = ProgressBar::new(ProgressBarStyle::DownloadBars);
      let progress = progress_bars.update(&download_url);

      self
        .client
        .download_with_progress(download_url, &progress)
        .await?
    };
    let bytes = match maybe_bytes {
      Some(bytes) => bytes,
      None => {
        log::info!("Download could not be found, aborting");
        std::process::exit(1)
      }
    };

    std::fs::create_dir_all(output_directory)?;
    let output_path = output_directory.join(binary_path_suffix);
    std::fs::create_dir_all(output_path.parent().unwrap())?;
    tokio::fs::write(output_path, bytes).await?;
    Ok(())
  }

  /// This functions creates a standalone deno binary by appending a bundle
  /// and magic trailer to the currently executing binary.
  async fn write_standalone_binary(
    &self,
    writer: &mut impl Write,
    original_bin: Vec<u8>,
    mut eszip: eszip::EszipV2,
    entrypoint: &ModuleSpecifier,
    cli_options: &CliOptions,
    compile_flags: &CompileFlags,
  ) -> Result<(), AnyError> {
    let ca_data = match cli_options.ca_data() {
      Some(CaData::File(ca_file)) => Some(
        std::fs::read(ca_file)
          .with_context(|| format!("Reading: {ca_file}"))?,
      ),
      Some(CaData::Bytes(bytes)) => Some(bytes.clone()),
      None => None,
    };
    let maybe_import_map = cli_options
      .resolve_import_map(self.file_fetcher)
      .await?
      .map(|import_map| (import_map.base_url().clone(), import_map.to_json()));
    let (npm_vfs, npm_files, node_modules) =
      match self.npm_resolver.as_inner() {
        InnerCliNpmResolverRef::Managed(managed) => {
          let snapshot =
            managed.serialized_valid_snapshot_for_system(&self.npm_system_info);
          if !snapshot.as_serialized().packages.is_empty() {
            let (root_dir, files) = self.build_vfs()?.into_dir_and_files();
            eszip.add_npm_snapshot(snapshot);
            (
              Some(root_dir),
              files,
              Some(NodeModules::Managed {
                node_modules_dir: self
                  .npm_resolver
                  .root_node_modules_path()
                  .is_some(),
                package_json_deps: self.package_json_deps_provider.deps().map(
                  |deps| SerializablePackageJsonDeps::from_deps(deps.clone()),
                ),
              }),
            )
          } else {
            (None, Vec::new(), None)
          }
        }
        InnerCliNpmResolverRef::Byonm(_) => {
          let (root_dir, files) = self.build_vfs()?.into_dir_and_files();
          (
            Some(root_dir),
            files,
            Some(NodeModules::Byonm {
              package_json_deps: self.package_json_deps_provider.deps().map(
                |deps| SerializablePackageJsonDeps::from_deps(deps.clone()),
              ),
            }),
          )
        }
      };

    let metadata = Metadata {
      argv: compile_flags.args.clone(),
      seed: cli_options.seed(),
      location: cli_options.location_flag().clone(),
      permissions: cli_options.permissions_options(),
      v8_flags: cli_options.v8_flags().clone(),
      unsafely_ignore_certificate_errors: cli_options
        .unsafely_ignore_certificate_errors()
        .clone(),
      log_level: cli_options.log_level(),
      ca_stores: cli_options.ca_stores().clone(),
      ca_data,
      entrypoint: entrypoint.clone(),
      maybe_import_map,
      node_modules,
      disable_deprecated_api_warning: cli_options
        .disable_deprecated_api_warning,
      unstable_config: UnstableConfig {
        legacy_flag_enabled: cli_options.legacy_unstable_flag(),
        bare_node_builtins: cli_options.unstable_bare_node_builtins(),
        byonm: cli_options.unstable_byonm(),
        sloppy_imports: cli_options.unstable_sloppy_imports(),
        features: cli_options.unstable_features(),
      },
    };

    write_binary_bytes(
      writer,
      original_bin,
      &metadata,
      eszip,
      npm_vfs.as_ref(),
      &npm_files,
    )
  }

  fn build_vfs(&self) -> Result<VfsBuilder, AnyError> {
    fn maybe_warn_different_system(system_info: &NpmSystemInfo) {
      if system_info != &NpmSystemInfo::default() {
        log::warn!("{} The node_modules directory may be incompatible with the target system.", crate::colors::yellow("Warning"));
      }
    }

    match self.npm_resolver.as_inner() {
      InnerCliNpmResolverRef::Managed(npm_resolver) => {
        if let Some(node_modules_path) = npm_resolver.root_node_modules_path() {
          maybe_warn_different_system(&self.npm_system_info);
          let mut builder = VfsBuilder::new(node_modules_path.clone())?;
          builder.add_dir_recursive(node_modules_path)?;
          Ok(builder)
        } else {
          // DO NOT include the user's registry url as it may contain credentials,
          // but also don't make this dependent on the registry url
          let registry_url = npm_resolver.registry_base_url();
          let root_path =
            npm_resolver.registry_folder_in_global_cache(registry_url);
          let mut builder = VfsBuilder::new(root_path)?;
          for package in npm_resolver.all_system_packages(&self.npm_system_info)
          {
            let folder =
              npm_resolver.resolve_pkg_folder_from_pkg_id(&package.id)?;
            builder.add_dir_recursive(&folder)?;
          }
          // overwrite the root directory's name to obscure the user's registry url
          builder.set_root_dir_name("node_modules".to_string());
          Ok(builder)
        }
      }
      InnerCliNpmResolverRef::Byonm(npm_resolver) => {
        maybe_warn_different_system(&self.npm_system_info);
        // the root_node_modules directory will always exist for byonm
        let node_modules_path = npm_resolver.root_node_modules_path().unwrap();
        let parent_path = node_modules_path.parent().unwrap();
        let mut builder = VfsBuilder::new(parent_path.to_path_buf())?;
        let package_json_path = parent_path.join("package.json");
        if package_json_path.exists() {
          builder.add_file_at_path(&package_json_path)?;
        }
        if node_modules_path.exists() {
          builder.add_dir_recursive(node_modules_path)?;
        }
        Ok(builder)
      }
    }
  }
}

/// This function sets the subsystem field in the PE header to 2 (GUI subsystem)
/// For more information about the PE header: https://learn.microsoft.com/en-us/windows/win32/debug/pe-format
fn set_windows_binary_to_gui(bin: &mut [u8]) -> Result<(), AnyError> {
  // Get the PE header offset located in an i32 found at offset 60
  // See: https://learn.microsoft.com/en-us/windows/win32/debug/pe-format#ms-dos-stub-image-only
  let start_pe = u32::from_le_bytes((bin[60..64]).try_into()?);

  // Get image type (PE32 or PE32+) indicates whether the binary is 32 or 64 bit
  // The used offset and size values can be found here:
  // https://learn.microsoft.com/en-us/windows/win32/debug/pe-format#optional-header-image-only
  let start_32 = start_pe as usize + 28;
  let magic_32 =
    u16::from_le_bytes(bin[(start_32)..(start_32 + 2)].try_into()?);

  let start_64 = start_pe as usize + 24;
  let magic_64 =
    u16::from_le_bytes(bin[(start_64)..(start_64 + 2)].try_into()?);

  // Take the standard fields size for the current architecture (32 or 64 bit)
  // This is the ofset for the Windows-Specific fields
  let standard_fields_size = if magic_32 == 0x10b {
    28
  } else if magic_64 == 0x20b {
    24
  } else {
    bail!("Could not find a matching magic field in the PE header")
  };

  // Set the subsystem field (offset 68) to 2 (GUI subsystem)
  // For all possible options, see: https://learn.microsoft.com/en-us/windows/win32/debug/pe-format#optional-header-windows-specific-fields-image-only
  let subsystem_offset = 68;
  let subsystem_start =
    start_pe as usize + standard_fields_size + subsystem_offset;
  let subsystem: u16 = 2;
  bin[(subsystem_start)..(subsystem_start + 2)]
    .copy_from_slice(&subsystem.to_le_bytes());
  Ok(())
}