defmodule OrgGarden.Resolvers.BibTeX do
  @moduledoc """
  Resolves citation keys from a local BibTeX (.bib) file.

  Configured via the `BIBTEX_FILE` environment variable, or passed directly
  as `opts.bibtex_file`. The file is parsed once at init time and the
  resulting entry map is reused for all lookups.

  Supports extracting: author last names, year, title, DOI, URL.

  BibTeX entry format parsed:

      @type{citationkey,
        author  = {Last, First and Last2, First2},
        year    = {2021},
        title   = {Some Title},
        doi     = {10.xxxx/yyyy},
        url     = {https://example.com},
      }

  Returns `{:ok, %{label: "Author, Year", url: "..."}}` or `:error`.
  """

  require Logger

  # ------------------------------------------------------------------
  # Public API
  # ------------------------------------------------------------------

  @doc """
  Parse a .bib file and return a map of `%{citation_key => entry_map}`.
  Returns `{:ok, entries}` or `{:error, reason}`.
  """
  @spec load(String.t()) :: {:ok, map()} | {:error, term()}
  def load(path) do
    case File.read(path) do
      {:ok, content} ->
        entries = parse_entries(content)
        Logger.info("BibTeX: loaded #{map_size(entries)} entries from #{path}")
        {:ok, entries}

      {:error, reason} ->
        {:error, reason}
    end
  end

  @doc """
  Resolve a citation key from pre-loaded BibTeX entries.
  """
  @spec resolve(String.t(), map()) :: {:ok, map()} | :error
  def resolve(key, entries) do
    case Map.fetch(entries, key) do
      {:ok, entry} ->
        label = build_label(entry)
        url = build_url(entry)
        {:ok, %{label: label, url: url}}

      :error ->
        :error
    end
  end

  # ------------------------------------------------------------------
  # Parsing
  # ------------------------------------------------------------------

  # Match @type{key, ...fields...}
  # We handle nested braces by scanning character by character after
  # finding the opening, rather than relying on a single regex.
  @entry_header ~r/@\w+\s*\{\s*([^,\s]+)\s*,/

  defp parse_entries(content) do
    # Split on "@" boundaries, then parse each chunk
    content
    |> String.split(~r/(?=@\w+\s*\{)/, trim: true)
    |> Enum.reduce(%{}, fn chunk, acc ->
      case Regex.run(@entry_header, chunk) do
        [_, key] ->
          fields = parse_fields(chunk)
          Map.put(acc, String.trim(key), fields)

        _ ->
          acc
      end
    end)
  end

  # Extract key = {value} or key = "value" pairs from an entry block.
  # Handles simple single-depth braces; good enough for common fields.
  @field_regex ~r/(\w+)\s*=\s*(?:\{([^{}]*(?:\{[^{}]*\}[^{}]*)*)\}|"([^"]*)")/

  defp parse_fields(chunk) do
    @field_regex
    |> Regex.scan(chunk)
    |> Enum.reduce(%{}, fn match, acc ->
      field_name = Enum.at(match, 1) |> String.downcase()
      # Value is in capture group 2 (braces) or 3 (quotes)
      value =
        case {Enum.at(match, 2, ""), Enum.at(match, 3, "")} do
          {"", q} -> q
          {b, _} -> b
        end

      Map.put(acc, field_name, String.trim(value))
    end)
  end

  # ------------------------------------------------------------------
  # Label & URL building
  # ------------------------------------------------------------------

  defp build_label(entry) do
    author_part =
      entry
      |> Map.get("author", "")
      |> parse_authors()
      |> format_authors()

    year = Map.get(entry, "year", Map.get(entry, "date", ""))
    year = extract_year(year)

    if year && author_part != "", do: "#{author_part}, #{year}", else: author_part
  end

  defp parse_authors(""), do: []

  defp parse_authors(author_str) do
    author_str
    |> String.split(" and ", trim: true)
    |> Enum.map(&extract_last_name/1)
    |> Enum.reject(&(&1 == ""))
  end

  # Handles "Last, First" and "First Last" formats
  defp extract_last_name(name) do
    name = String.trim(name)

    cond do
      String.contains?(name, ",") ->
        name |> String.split(",") |> List.first() |> String.trim()

      String.contains?(name, " ") ->
        name |> String.split(" ") |> List.last() |> String.trim()

      true ->
        name
    end
  end

  defp format_authors([]), do: "Unknown"
  defp format_authors([single]), do: single
  defp format_authors([first | rest]), do: "#{first} & #{List.last(rest)}"

  defp extract_year(""), do: nil

  defp extract_year(str) do
    case Regex.run(~r/\b(\d{4})\b/, str) do
      [_, year] -> year
      _ -> nil
    end
  end

  defp build_url(entry) do
    cond do
      doi = Map.get(entry, "doi", "") |> non_empty() ->
        "https://doi.org/#{doi}"

      url = Map.get(entry, "url", "") |> non_empty() ->
        url

      true ->
        nil
    end
  end

  defp non_empty(""), do: nil
  defp non_empty(v), do: v
end