defmodule Mix.Tasks.LocalizeMf2Treesitter.UpdateRuntime do
  @moduledoc """
  Refresh the vendored tree-sitter C runtime from the canonical
  upstream at [`tree-sitter/tree-sitter`](https://github.com/tree-sitter/tree-sitter).

  This package embeds the tree-sitter runtime as a single-TU
  amalgamation under `c_src/runtime/`. That runtime is compiled
  alongside the generated `c_src/grammar/parser.c` into the NIF
  `.so`. The runtime's supported ABI version must be ≥ the version
  used by `parser.c`, or every parse fails at `ts_parser_set_language`.

  When the grammar sync (`mix localize_mf2_treesitter.sync`) pulls a
  `parser.c` generated by a newer tree-sitter CLI than the vendored
  runtime supports, the runtime needs to catch up. This task handles
  that: it downloads a specific tree-sitter release tarball, extracts
  the `lib/` tree, and overlays it onto `c_src/runtime/`.

  The pinned version lives at the top of this module as
  `@runtime_version`. Bump it and re-run the task to move to a
  newer runtime release. A good default is to match whatever
  tree-sitter CLI version was used to regenerate the grammar —
  check `src/parser.c`'s first-line comment.

  ## Usage

      # Download and overlay the pinned runtime version.
      mix localize_mf2_treesitter.update_runtime

      # Fail (exit 1) if anything has drifted from the pinned
      # version; do not modify files. Intended for CI.
      mix localize_mf2_treesitter.update_runtime --check

  ## What gets replaced (and what doesn't)

  Replaced:

    * `c_src/runtime/include/tree_sitter/*.h`
    * `c_src/runtime/src/*.c`, `c_src/runtime/src/*.h`
    * `c_src/runtime/src/portable/`, `unicode/`, `wasm/` subtrees
    * `c_src/runtime/LICENSE` (tree-sitter's MIT licence)

  Preserved (hand-maintained):

    * `c_src/runtime/src/lib.c` — our amalgamation wrapper. Adds
      `#define _POSIX_C_SOURCE 200112L` and `#include`s each of the
      runtime's `.c` files. If the upstream adds a new `.c` file,
      this task warns and prompts the maintainer to extend `lib.c`.

  ## Network dependency

  Downloads the release tarball from
  `https://github.com/tree-sitter/tree-sitter/archive/refs/tags/v<VERSION>.tar.gz`
  over verified HTTPS. No API keys, no authentication required.
  """
  use Mix.Task

  @shortdoc "Refresh the vendored tree-sitter runtime from upstream."

  # Pinned tree-sitter runtime version. Bump and re-run the task to
  # pull a newer runtime. Match this to the tree-sitter CLI version
  # used to regenerate `c_src/grammar/parser.c` (check its first line).
  @runtime_version "0.26.8"

  @upstream_repo "tree-sitter/tree-sitter"

  @tarball_url "https://github.com/#{@upstream_repo}/archive/refs/tags/v#{@runtime_version}.tar.gz"

  # Files from the extracted tarball that we overlay onto c_src/runtime/.
  # Each entry is `{source_rel, dest_rel}` where source_rel is inside the
  # extracted `tree-sitter-<VERSION>/` directory and dest_rel is relative
  # to `c_src/runtime/`. Subtrees are walked recursively by the overlay
  # function, so we just list the top-level roots here.
  @subtrees [
    {"lib/include/tree_sitter", "include/tree_sitter"},
    {"lib/src", "src"}
  ]

  # Files we do NOT overwrite during the overlay. `lib.c` is our
  # hand-maintained amalgamation wrapper.
  @preserved_files MapSet.new([
                     "src/lib.c"
                   ])

  @top_level_files [
    {"LICENSE", "LICENSE"}
  ]

  @switches [check: :boolean]

  @impl Mix.Task
  def run(argv) do
    {opts, _rest} = OptionParser.parse!(argv, switches: @switches)
    check? = opts[:check] == true

    runtime_dir = runtime_dir()

    Mix.shell().info("[runtime] source:    tree-sitter v#{@runtime_version}")
    Mix.shell().info("[runtime] destination: #{runtime_dir}")

    {:ok, work_dir} = fetch_and_extract()

    try do
      drift = overlay(work_dir, runtime_dir, check?)
      missing = check_lib_c(work_dir, runtime_dir)

      cond do
        check? and drift != [] ->
          Mix.shell().error("[runtime] drift detected in #{length(drift)} file(s):")

          Enum.each(drift, fn path ->
            Mix.shell().error("  - #{path}")
          end)

          Mix.shell().error(
            "Run `mix localize_mf2_treesitter.update_runtime` to update."
          )

          exit({:shutdown, 1})

        check? ->
          Mix.shell().info(
            "[runtime] check: all vendored files match tree-sitter v#{@runtime_version}."
          )

        drift == [] ->
          Mix.shell().info("[runtime] already in sync; no files changed.")

        true ->
          Mix.shell().info("[runtime] updated #{length(drift)} file(s).")

          Mix.shell().info(
            "[runtime] run `mix compile --force` to rebuild the NIF against the new runtime."
          )
      end

      warn_about_missing_includes(missing)
    after
      File.rm_rf!(work_dir)
    end

    :ok
  end

  # ── Download + extract ────────────────────────────────────────────

  defp fetch_and_extract do
    ensure_http_started()

    work_dir = Path.join(System.tmp_dir!(), "ts-runtime-#{System.unique_integer([:positive])}")
    File.mkdir_p!(work_dir)

    tarball_path = Path.join(work_dir, "archive.tar.gz")
    Mix.shell().info("[runtime] downloading #{@tarball_url}")
    download!(@tarball_url, tarball_path)

    # :erl_tar with :compressed handles gzip transparently.
    case :erl_tar.extract(String.to_charlist(tarball_path), [:compressed, {:cwd, String.to_charlist(work_dir)}]) do
      :ok ->
        extracted_root = Path.join(work_dir, "tree-sitter-#{@runtime_version}")

        unless File.dir?(extracted_root) do
          Mix.raise(
            "extracted tarball does not contain expected root dir: #{extracted_root}"
          )
        end

        {:ok, extracted_root}

      {:error, reason} ->
        Mix.raise("tarball extraction failed: #{inspect(reason)}")
    end
  end

  defp download!(url, target) do
    url_charlist = String.to_charlist(url)
    request = {url_charlist, [{~c"accept", ~c"application/octet-stream"}]}

    case :httpc.request(:get, request, [ssl: ssl_options(), autoredirect: true], body_format: :binary) do
      {:ok, {{_http, status, _reason}, _headers, body}} when status in 200..299 ->
        File.write!(target, body)

      {:ok, {{_http, status, reason}, _headers, _body}} ->
        Mix.raise("""
        HTTP #{status} #{reason} from #{url}

        Confirm that v#{@runtime_version} is a valid tree-sitter release tag
        (https://github.com/#{@upstream_repo}/releases).
        """)

      {:error, reason} ->
        Mix.raise("HTTP request to #{url} failed: #{inspect(reason)}")
    end
  end

  defp ensure_http_started do
    :ssl.start()
    :inets.start()
  end

  defp ssl_options do
    [
      verify: :verify_peer,
      cacerts: :public_key.cacerts_get(),
      customize_hostname_check: [
        match_fun: :public_key.pkix_verify_hostname_match_fun(:https)
      ]
    ]
  end

  # ── Overlay ───────────────────────────────────────────────────────

  # Walk the configured subtrees + top-level files and copy every
  # file that differs, skipping the preserved set. Returns the list of
  # dest paths that would change (or did change, when not --check).
  defp overlay(work_dir, runtime_dir, check?) do
    subtree_drift =
      for {src_rel, dst_rel} <- @subtrees,
          file <- list_files_recursive(Path.join(work_dir, src_rel)),
          do:
            overlay_file(
              src_root: Path.join(work_dir, src_rel),
              dst_root: Path.join(runtime_dir, dst_rel),
              src_absolute: file,
              dst_prefix: dst_rel,
              check?: check?
            )

    top_drift =
      for {src_rel, dst_rel} <- @top_level_files,
          do:
            overlay_single(
              src: Path.join(work_dir, src_rel),
              dst: Path.join(runtime_dir, dst_rel),
              dst_rel: dst_rel,
              check?: check?
            )

    (subtree_drift ++ top_drift) |> Enum.reject(&is_nil/1)
  end

  defp overlay_file(src_root: src_root, dst_root: dst_root, src_absolute: src_absolute, dst_prefix: dst_prefix, check?: check?) do
    rel = Path.relative_to(src_absolute, src_root)
    dst_rel_full = Path.join(dst_prefix, rel)

    if MapSet.member?(@preserved_files, dst_rel_full) do
      nil
    else
      dst = Path.join(dst_root, rel)
      write_if_changed(src_absolute, dst, dst_rel_full, check?)
    end
  end

  defp overlay_single(src: src, dst: dst, dst_rel: dst_rel, check?: check?) do
    write_if_changed(src, dst, dst_rel, check?)
  end

  defp write_if_changed(src, dst, dst_rel, check?) do
    new_bytes = File.read!(src)

    cond do
      not File.exists?(dst) ->
        write_or_record(dst, new_bytes, dst_rel, check?)

      File.read!(dst) != new_bytes ->
        write_or_record(dst, new_bytes, dst_rel, check?)

      true ->
        nil
    end
  end

  defp write_or_record(dst, bytes, dst_rel, check?) do
    unless check? do
      File.mkdir_p!(Path.dirname(dst))
      File.write!(dst, bytes)
      Mix.shell().info("[runtime] wrote #{dst_rel}")
    end

    dst_rel
  end

  defp list_files_recursive(path) do
    cond do
      File.regular?(path) -> [path]
      File.dir?(path) -> path |> File.ls!() |> Enum.flat_map(&list_files_recursive(Path.join(path, &1)))
      true -> []
    end
  end

  # ── lib.c amalgamation sanity check ───────────────────────────────

  # Verify our preserved lib.c `#include`s every .c file that upstream
  # ships under lib/src/. If upstream added a new .c, we need to extend
  # our lib.c or the runtime will link with symbols undefined. Returns
  # the list of missing includes; caller decides what to print.
  defp check_lib_c(work_dir, runtime_dir) do
    upstream_cs =
      Path.join(work_dir, "lib/src")
      |> File.ls!()
      |> Enum.filter(&String.ends_with?(&1, ".c"))
      |> Enum.reject(&(&1 == "lib.c"))

    our_lib_c = Path.join(runtime_dir, "src/lib.c") |> File.read!()

    Enum.reject(upstream_cs, fn c_file ->
      String.contains?(our_lib_c, "\"./#{c_file}\"")
    end)
  end

  defp warn_about_missing_includes([]), do: :ok

  defp warn_about_missing_includes(missing) do
    Mix.shell().info("")
    Mix.shell().error("[runtime] ⚠ upstream ships .c files not #include'd in src/lib.c:")

    Enum.each(missing, fn f -> Mix.shell().error("  - #{f}") end)

    Mix.shell().error("""

    Edit c_src/runtime/src/lib.c and add `#include "./<name>.c"` for
    each missing file, then rebuild. Without this, the amalgamation
    link step will produce undefined-symbol errors for whatever
    those new .c files define.
    """)
  end

  defp runtime_dir do
    Path.join([File.cwd!(), "c_src", "runtime"])
  end
end