defmodule Mix.Tasks.Deps.Patch do
  @shortdoc "Applies local patches to fetched hex dependencies"

  @moduledoc """
  Applies every patch file in `priv/dep_patches/*.patch` to its
  corresponding dependency under `deps/`. Each patch name follows the
  convention `<dep_name>-<version>.patch` (e.g. `live_table-0.4.1.patch`).

  Applied idempotently — if the patched text is already present we skip
  the dep. Run after `mix deps.get` / `mix deps.compile`. Safe to invoke
  repeatedly.

  Why this exists: `live_table` 0.4.1 has a call to
  `LiveTable.LiveSelectHelpers.restore_live_select_from_params/2` whose
  return value is discarded inside the macro-expanded `handle_params/3`,
  tripping the strict dialyzer `:unmatched_return` flag at every `use`
  site. The patch adds an `_ =` binding. Upstream PR TODO — once merged
  and released, drop both the patch and this task.
  """
  use Mix.Task

  @patches_dir "priv/dep_patches"

  @impl Mix.Task
  def run(_args) do
    case File.ls(@patches_dir) do
      {:ok, entries} ->
        patches = entries |> Enum.filter(&String.ends_with?(&1, ".patch")) |> Enum.sort()

        Enum.each(patches, &apply_patch/1)

      {:error, :enoent} ->
        Mix.shell().info("deps.patch: no #{@patches_dir} directory — nothing to do")
    end
  end

  defp apply_patch(filename) do
    dep_name =
      filename
      |> Path.basename(".patch")
      |> String.split("-")
      |> Enum.drop(-1)
      |> Enum.join("-")

    dep_dir = Path.join("deps", dep_name)

    cond do
      not File.dir?(dep_dir) ->
        Mix.shell().info("deps.patch: skipping #{filename} — deps/#{dep_name} not present")

      patch_already_applied?(filename, dep_dir) ->
        Mix.shell().info("deps.patch: #{filename} already applied")

      true ->
        patch_path = Path.join(@patches_dir, filename)
        {output, exit_code} = System.cmd("patch", ["-p1", "-i", Path.absname(patch_path)], cd: dep_dir)

        if exit_code == 0 do
          Mix.shell().info("deps.patch: applied #{filename}")
          touch_sentinel(filename, dep_dir)
        else
          Mix.raise("deps.patch: failed to apply #{filename}:\n#{output}")
        end
    end
  end

  # Record that the patch landed by writing a sentinel file inside the
  # dep dir. `File.touch!/1` avoids spurious re-applications when the
  # patch contains text the dep happens to contain already.
  defp touch_sentinel(filename, dep_dir) do
    File.touch!(Path.join(dep_dir, ".#{filename}.applied"))
  end

  defp patch_already_applied?(filename, dep_dir) do
    File.exists?(Path.join(dep_dir, ".#{filename}.applied"))
  end
end