defmodule ToweropsWeb.ChangelogParser do
  @moduledoc """
  Reads and parses priv/static/changelog.txt into structured entries.

  Parses changelog entries with format:
  - "YYYY-MM-DD — Title" or "YYYY-MM-DD – Title" (with em/en dash)
  - "YYYY-MM-DD" (date only, no title)
  - "* Item text" (bullet items belonging to the most recent date)
  """

  @doc "Returns a list of %{date: Date.t(), title: String.t() | nil, items: [String.t()]}"
  def parse do
    path = Application.app_dir(:towerops, "priv/static/changelog.txt")

    case File.read(path) do
      {:ok, content} ->
        content
        |> parse_content()
        |> Enum.map(&convert_entry/1)

      {:error, _} ->
        []
    end
  end

  @doc """
  Parse changelog content string into a list of entries.
  Returns internal tuples: {:changelog_entry, date, title_opt, items}
  """
  def parse_content(content) when is_binary(content) do
    content
    |> String.split("\n")
    |> Enum.reduce([], fn line, acc -> parse_line(line, acc) end)
    |> Enum.reverse()
    |> Enum.map(fn entry ->
      {:changelog_entry, entry.date, entry.title, Enum.reverse(entry.items)}
    end)
  end

  # Parse a single line and update the accumulator
  defp parse_line(line, acc) do
    # Check for date with title: "YYYY-MM-DD [—–] Title"
    case Regex.run(~r/^(\d{4}-\d{2}-\d{2})\s+(—|–)\s+(.+)/u, line) do
      [_, date_str, _dash, title] ->
        add_date_entry(acc, date_str, {:some, String.trim(title)})

      nil ->
        parse_line_no_title(line, acc)
    end
  end

  defp parse_line_no_title(line, acc) do
    cond do
      # Check for date only: "YYYY-MM-DD"
      Regex.match?(~r/^\d{4}-\d{2}-\d{2}\s*$/, line) ->
        add_date_entry(acc, line, :none)

      # Check for bullet item: "* Item text"
      String.starts_with?(String.trim(line), "* ") ->
        add_item_to_current_entry(acc, line)

      # Non-matching line - ignore
      true ->
        acc
    end
  end

  defp add_date_entry(acc, date_str, title) do
    [
      %{
        date: String.trim(date_str),
        title: title,
        items: []
      }
      | acc
    ]
  end

  defp add_item_to_current_entry(acc, line) do
    trimmed = String.trim(line)
    item = String.slice(trimmed, 2..-1//1)

    case acc do
      [current | rest] ->
        [%{current | items: [item | current.items]} | rest]

      [] ->
        # Orphan item before any date header - ignore
        []
    end
  end

  defp convert_entry({:changelog_entry, date_str, title_opt, items}) do
    %{
      date: Date.from_iso8601!(date_str),
      title: unwrap_option(title_opt),
      items: items
    }
  end

  defp unwrap_option({:some, value}), do: value
  defp unwrap_option(:none), do: nil
end