defmodule Pipeline do
  @moduledoc """
  Post-export markdown transformation pipeline.

  Applies a list of transform modules sequentially over markdown files.
  Each transform module must implement the `Pipeline.Transform` behaviour.

  Transforms are applied in the order given. A file is rewritten only
  when at least one transform mutates its content (checked via equality).

  ## Usage

      opts = %{
        zotero_url: "http://localhost:23119",
        bibtex_file: System.get_env("BIBTEX_FILE"),
        citation_mode: :warn   # :silent | :warn | :strict
      }

      # Batch: all .md files in a directory
      Pipeline.run(content_dir, [Pipeline.Transforms.Citations], opts)

      # Targeted: specific files only
      Pipeline.run_on_files(["content/foo.md"], [Pipeline.Transforms.Citations], opts)

      # With pre-initialized transforms (for watch mode, avoids re-init)
      initialized = Pipeline.init_transforms([Pipeline.Transforms.Citations], opts)
      Pipeline.run_on_files_with(["content/foo.md"], initialized, opts)
  """

  require Logger

  @type transform :: module()
  @type initialized_transform :: {module(), term()}
  @type opts :: map()

  @doc """
  Initialize transform modules. Returns a list of `{module, state}` tuples.

  Call this once and reuse the result with `run_on_files_with/3` to avoid
  re-initializing transforms on every file change (e.g., in watch mode).
  """
  @spec init_transforms([transform()], opts()) :: [initialized_transform()]
  def init_transforms(transforms, opts) do
    Enum.map(transforms, fn mod ->
      state = mod.init(opts)
      {mod, state}
    end)
  end

  @doc """
  Tear down previously initialized transforms, releasing any resources.
  """
  @spec teardown_transforms([initialized_transform()]) :: :ok
  def teardown_transforms(initialized) do
    Enum.each(initialized, fn {mod, state} ->
      if function_exported?(mod, :teardown, 1) do
        mod.teardown(state)
      end
    end)

    :ok
  end

  @doc """
  Run all transforms over every `.md` file under `content_dir`.

  Initializes and tears down transforms automatically.
  Returns `{:ok, stats}` where stats maps each transform to a count of files it changed.
  """
  @spec run(String.t(), [transform()], opts()) :: {:ok, map()}
  def run(content_dir, transforms, opts \\ %{}) do
    md_files =
      content_dir
      |> Path.join("**/*.md")
      |> Path.wildcard()

    if md_files == [] do
      Logger.warning("Pipeline: no .md files found in #{content_dir}")
      {:ok, %{}}
    else
      Logger.info(
        "Pipeline: processing #{length(md_files)} markdown files " <>
          "with #{length(transforms)} transform(s)"
      )

      initialized = init_transforms(transforms, opts)
      stats = apply_transforms(md_files, initialized, opts)
      teardown_transforms(initialized)
      {:ok, stats}
    end
  end

  @doc """
  Run all transforms over specific `.md` files only.

  Initializes and tears down transforms automatically.
  Files that don't exist are silently skipped.
  """
  @spec run_on_files([String.t()], [transform()], opts()) :: {:ok, map()}
  def run_on_files(file_paths, transforms, opts \\ %{}) do
    existing = Enum.filter(file_paths, &File.exists?/1)

    if existing == [] do
      Logger.debug("Pipeline: no files to process")
      {:ok, %{}}
    else
      Logger.info("Pipeline: processing #{length(existing)} file(s)")
      initialized = init_transforms(transforms, opts)
      stats = apply_transforms(existing, initialized, opts)
      teardown_transforms(initialized)
      {:ok, stats}
    end
  end

  @doc """
  Run pre-initialized transforms over specific `.md` files.

  Does NOT call `init` or `teardown` — the caller manages the transform
  lifecycle. Use this in watch mode to avoid re-initializing on every change.
  """
  @spec run_on_files_with([String.t()], [initialized_transform()], opts()) :: {:ok, map()}
  def run_on_files_with(file_paths, initialized, opts) do
    existing = Enum.filter(file_paths, &File.exists?/1)

    if existing == [] do
      Logger.debug("Pipeline: no files to process")
      {:ok, %{}}
    else
      stats = apply_transforms(existing, initialized, opts)
      {:ok, stats}
    end
  end

  # -------------------------------------------------------------------
  # Private
  # -------------------------------------------------------------------

  defp apply_transforms(md_files, initialized, opts) do
    Enum.reduce(md_files, %{}, fn path, acc ->
      original = File.read!(path)

      {transformed, file_stats} =
        Enum.reduce(initialized, {original, %{}}, fn {mod, state}, {content, fstats} ->
          result = mod.apply(content, state, opts)
          changed = result != content

          {result,
           Map.update(
             fstats,
             mod,
             if(changed, do: 1, else: 0),
             &(&1 + if(changed, do: 1, else: 0))
           )}
        end)

      if transformed != original do
        File.write!(path, transformed)
        Logger.debug("Pipeline: updated #{Path.relative_to_cwd(path)}")
      end

      Map.merge(acc, file_stats, fn _k, a, b -> a + b end)
    end)
  end
end