defmodule Sashite.Pmn.Parser do
  @moduledoc false

  # Secure parser for PMN (Portable Move Notation) strings.
  #
  # Designed for untrusted input: validates bounds first, identifies the
  # form by scanning for operator characters, then validates sub-tokens
  # by delegating to CELL and EPIN libraries.
  #
  # Parsing strategy — operator-first dispatch:
  #
  #   1. Check for "..." (pass) — exact match
  #   2. Scan for * or . — unambiguous drop operators (never in CELL/EPIN)
  #   3. Scan for -, +, ~, = — movement/mutation operators
  #   4. Split on operator, extract and validate sub-tokens
  #
  # The two-pass scan (drops first, then movements) resolves the ambiguity
  # where - and + can appear as EPIN state modifiers before * or . operators.
  #
  # Time complexity: O(n) where n ≤ 25 (bounded by max_string_length).
  # Space complexity: O(1) — binary slicing via binary_part (no copies).

  @max_string_length 25

  # ── Public API ────────────────────────────────────────────────────────

  @doc false
  @spec parse(String.t()) :: {:ok, map()} | {:error, atom()}
  def parse(<<>>), do: {:error, :empty_input}

  def parse(input) when is_binary(input) do
    if byte_size(input) > @max_string_length do
      {:error, :input_too_long}
    else
      identify_and_parse(input)
    end
  end

  def parse(_), do: {:error, :not_a_string}

  @doc false
  @spec parse!(String.t()) :: map()
  def parse!(input) do
    case parse(input) do
      {:ok, move} -> move
      {:error, reason} -> raise ArgumentError, Atom.to_string(reason)
    end
  end

  # ── Form identification ───────────────────────────────────────────────
  #
  # Pass is matched exactly. Then drop operators (* and .) are scanned
  # first because - and + can appear as EPIN state modifiers before them.
  # Once drops are ruled out, -, +, ~, = are unambiguous.

  defp identify_and_parse("..."), do: {:ok, %{form: :pass}}

  defp identify_and_parse(input) do
    case scan_drop_operator(input, 0) do
      {?*, pos} -> parse_drop_quiet(input, pos)
      {?., pos} -> parse_drop_capture(input, pos)
      :none -> identify_non_drop(input)
    end
  end

  defp identify_non_drop(input) do
    case scan_movement_operator(input, 0) do
      {?-, pos} -> parse_move_quiet(input, pos)
      {?+, 0} -> parse_static_capture(input)
      {?+, pos} -> parse_move_capture(input, pos)
      {?~, pos} -> parse_move_special(input, pos)
      {?=, pos} -> parse_in_place_mutation(input, pos)
      :none -> {:error, :unrecognized_form}
    end
  end

  # ── Operator scanning ─────────────────────────────────────────────────
  #
  # Two separate scanners: drops first (*, .), then movements (-, +, ~, =).
  # Guards in clause heads for native BEAM dispatch.

  defp scan_drop_operator(<<>>, _pos), do: :none
  defp scan_drop_operator(<<?*, _::binary>>, pos), do: {?*, pos}
  defp scan_drop_operator(<<?., _::binary>>, pos), do: {?., pos}
  defp scan_drop_operator(<<_, rest::binary>>, pos), do: scan_drop_operator(rest, pos + 1)

  defp scan_movement_operator(<<>>, _pos), do: :none
  defp scan_movement_operator(<<?-, _::binary>>, pos), do: {?-, pos}
  defp scan_movement_operator(<<?+, _::binary>>, pos), do: {?+, pos}
  defp scan_movement_operator(<<?~, _::binary>>, pos), do: {?~, pos}
  defp scan_movement_operator(<<?=, _::binary>>, pos), do: {?=, pos}
  defp scan_movement_operator(<<_, rest::binary>>, pos), do: scan_movement_operator(rest, pos + 1)

  # ── Form parsers: movements ──────────────────────────────────────────

  # src-dst[=actor]
  defp parse_move_quiet(input, op_pos) do
    source = binary_part(input, 0, op_pos)
    rest = binary_slice_after(input, op_pos)

    with {:ok, dst, actor} <- extract_dst_actor(rest),
         :ok <- validate_cell(source),
         :ok <- validate_cell(dst),
         :ok <- validate_optional_epin(actor) do
      {:ok, %{form: :move_quiet, source: source, destination: dst, actor: actor}}
    end
  end

  # src+dst[=actor][/captured]
  defp parse_move_capture(input, op_pos) do
    source = binary_part(input, 0, op_pos)
    rest = binary_slice_after(input, op_pos)

    with {:ok, dst, actor, captured} <- extract_dst_actor_captured(rest),
         :ok <- validate_cell(source),
         :ok <- validate_cell(dst),
         :ok <- validate_optional_epin(actor),
         :ok <- validate_optional_epin(captured) do
      {:ok,
       %{form: :move_capture, source: source, destination: dst, actor: actor, captured: captured}}
    end
  end

  # src~dst[=actor][/captured]
  defp parse_move_special(input, op_pos) do
    source = binary_part(input, 0, op_pos)
    rest = binary_slice_after(input, op_pos)

    with {:ok, dst, actor, captured} <- extract_dst_actor_captured(rest),
         :ok <- validate_cell(source),
         :ok <- validate_cell(dst),
         :ok <- validate_optional_epin(actor),
         :ok <- validate_optional_epin(captured) do
      {:ok,
       %{form: :move_special, source: source, destination: dst, actor: actor, captured: captured}}
    end
  end

  # ── Form parsers: static capture ─────────────────────────────────────

  # +square[/captured]
  defp parse_static_capture(input) do
    rest = binary_slice_after(input, 0)

    with {:ok, square, captured} <- extract_square_captured(rest),
         :ok <- validate_cell(square),
         :ok <- validate_optional_epin(captured) do
      {:ok, %{form: :static_capture, square: square, captured: captured}}
    end
  end

  # ── Form parsers: drops ──────────────────────────────────────────────

  # [piece]*dst[=actor]
  defp parse_drop_quiet(input, op_pos) do
    piece = if op_pos > 0, do: binary_part(input, 0, op_pos), else: nil
    rest = binary_slice_after(input, op_pos)

    with :ok <- validate_optional_epin(piece),
         {:ok, dst, actor} <- extract_dst_actor(rest),
         :ok <- validate_cell(dst),
         :ok <- validate_optional_epin(actor) do
      {:ok, %{form: :drop_quiet, piece: piece, destination: dst, actor: actor}}
    end
  end

  # [piece].dst[=actor][/captured]
  defp parse_drop_capture(input, op_pos) do
    piece = if op_pos > 0, do: binary_part(input, 0, op_pos), else: nil
    rest = binary_slice_after(input, op_pos)

    with :ok <- validate_optional_epin(piece),
         {:ok, dst, actor, captured} <- extract_dst_actor_captured(rest),
         :ok <- validate_cell(dst),
         :ok <- validate_optional_epin(actor),
         :ok <- validate_optional_epin(captured) do
      {:ok,
       %{form: :drop_capture, piece: piece, destination: dst, actor: actor, captured: captured}}
    end
  end

  # ── Form parsers: in-place mutation ──────────────────────────────────

  # square=piece
  defp parse_in_place_mutation(input, op_pos) do
    square = binary_part(input, 0, op_pos)
    piece = binary_slice_after(input, op_pos)

    with :ok <- validate_cell(square),
         :ok <- validate_epin(piece) do
      {:ok, %{form: :in_place_mutation, square: square, piece: piece}}
    end
  end

  # ── Suffix extraction ────────────────────────────────────────────────
  #
  # Since = and / never appear in CELL tokens, they unambiguously
  # delimit suffixes. Split on / first (rightmost boundary), then
  # on = within the remaining prefix.

  # Extract dst[=actor] — for move_quiet and drop_quiet
  defp extract_dst_actor(rest) do
    case split_on(rest, <<?=>>) do
      {dst, actor} -> {:ok, dst, actor}
      :nomatch -> {:ok, rest, nil}
    end
  end

  # Extract dst[=actor][/captured] — for move_capture, move_special, drop_capture
  defp extract_dst_actor_captured(rest) do
    {before_slash, captured} =
      case split_on(rest, <<?/>>) do
        {before, after_slash} -> {before, after_slash}
        :nomatch -> {rest, nil}
      end

    case split_on(before_slash, <<?=>>) do
      {dst, actor} -> {:ok, dst, actor, captured}
      :nomatch -> {:ok, before_slash, nil, captured}
    end
  end

  # Extract square[/captured] — for static_capture
  defp extract_square_captured(rest) do
    case split_on(rest, <<?/>>) do
      {square, captured} -> {:ok, square, captured}
      :nomatch -> {:ok, rest, nil}
    end
  end

  # ── Validation helpers ───────────────────────────────────────────────

  defp validate_cell(token) do
    if Sashite.Cell.valid?(token), do: :ok, else: {:error, :invalid_cell_token}
  end

  defp validate_epin(token) do
    if Sashite.Epin.valid?(token), do: :ok, else: {:error, :invalid_epin_token}
  end

  defp validate_optional_epin(nil), do: :ok
  defp validate_optional_epin(token), do: validate_epin(token)

  # ── Binary helpers ───────────────────────────────────────────────────

  # Split binary on the first occurrence of a single-byte pattern.
  # Returns {before, after} or :nomatch.
  defp split_on(binary, pattern) do
    case :binary.match(binary, pattern) do
      {pos, 1} ->
        {binary_part(binary, 0, pos), binary_part(binary, pos + 1, byte_size(binary) - pos - 1)}

      :nomatch ->
        :nomatch
    end
  end

  # Slice binary after the byte at the given position (skipping it).
  defp binary_slice_after(binary, pos) do
    binary_part(binary, pos + 1, byte_size(binary) - pos - 1)
  end
end