defmodule Eyeon.TextParser.Number do
  @moduledoc false

  # Parses Ion numeric literals: integers (decimal, hex, binary), floats, and decimals.
  # Each function takes (binary, pos, opts) and returns {value, rest, pos} or {:error, reason}.

  alias Eyeon.TextDecoder.Timestamp
  alias Eyeon.TextDecoder.Validation

  @doc """
  Parse a number starting with a digit or `-`.
  The sign (if negative) has already been consumed; `negative` indicates sign.
  `first_digit` is the first digit byte already consumed.
  """
  @spec parse_number(byte(), binary(), non_neg_integer(), boolean()) ::
          {any(), binary(), non_neg_integer()} | {:error, String.t()}
  def parse_number(?0, rest, pos, negative) do
    case rest do
      <<?x, rest2::binary>> ->
        parse_hex_int(rest2, pos + 1, negative)

      <<?X, rest2::binary>> ->
        parse_hex_int(rest2, pos + 1, negative)

      <<?b, rest3::binary>> ->
        parse_bin_int(rest3, pos + 1, negative)

      <<?B, rest3::binary>> ->
        parse_bin_int(rest3, pos + 1, negative)

      _ ->
        # Could be 0, 0.xxx, 0eN, 0dN, or a timestamp starting with 0
        parse_decimal_digits(rest, pos, negative, [?0], 1)
    end
  end

  def parse_number(digit, rest, pos, negative) when digit in ?1..?9 do
    parse_decimal_digits(rest, pos, negative, [digit], 1)
  end

  # Parse decimal integer digits, then disambiguate
  defp parse_decimal_digits(<<digit, rest::binary>>, pos, negative, acc, count)
       when digit in ?0..?9 do
    parse_decimal_digits(rest, pos + 1, negative, [digit | acc], count + 1)
  end

  defp parse_decimal_digits(<<?_, digit, rest::binary>>, pos, negative, acc, count)
       when digit in ?0..?9 do
    # Underscore followed by digit
    parse_decimal_digits(rest, pos + 2, negative, [digit, ?_ | acc], count + 2)
  end

  # Underscore at end or followed by underscore - keep in acc for validation later
  defp parse_decimal_digits(<<?_, rest::binary>>, pos, negative, acc, count) do
    parse_decimal_digits(rest, pos + 1, negative, [?_ | acc], count + 1)
  end

  # After 4 digits, check for timestamp (hyphen after YYYY)
  defp parse_decimal_digits(<<?-, rest::binary>>, pos, negative, acc, _count) do
    digits_str = acc |> Enum.reverse() |> IO.iodata_to_binary()
    clean = String.replace(digits_str, "_", "")

    if String.length(clean) == 4 and not negative do
      # This is a timestamp: YYYY-...
      parse_timestamp_after_year(clean, rest, pos + 1)
    else
      # This is just an integer followed by a minus (separate tokens)
      finalize_integer(digits_str, negative, <<?-, rest::binary>>, pos)
    end
  end

  # Dot -> could be decimal or float
  defp parse_decimal_digits(<<?., rest::binary>>, pos, negative, acc, _count) do
    int_str = acc |> Enum.reverse() |> IO.iodata_to_binary()
    parse_fractional(rest, pos + 1, negative, int_str)
  end

  # e/E -> float
  defp parse_decimal_digits(<<e, rest::binary>>, pos, negative, acc, _count)
       when e in [?e, ?E] do
    int_str = acc |> Enum.reverse() |> IO.iodata_to_binary()
    parse_float_exponent(rest, pos + 1, negative, int_str, "")
  end

  # d/D -> decimal with exponent
  defp parse_decimal_digits(<<d, rest::binary>>, pos, negative, acc, _count)
       when d in [?d, ?D] do
    int_str = acc |> Enum.reverse() |> IO.iodata_to_binary()
    parse_decimal_exponent(rest, pos + 1, negative, int_str, "")
  end

  # T after digits -> could be year-only timestamp (YYYYT)
  defp parse_decimal_digits(<<?T, rest::binary>>, pos, negative, acc, _count) do
    digits_str = acc |> Enum.reverse() |> IO.iodata_to_binary()
    clean = String.replace(digits_str, "_", "")

    if String.length(clean) == 4 and not negative do
      parse_timestamp_year_only(clean, rest, pos + 1)
    else
      finalize_integer(digits_str, negative, <<?T, rest::binary>>, pos)
    end
  end

  # End of digits -> integer
  defp parse_decimal_digits(rest, pos, negative, acc, _count) do
    digits_str = acc |> Enum.reverse() |> IO.iodata_to_binary()
    finalize_integer(digits_str, negative, rest, pos)
  end

  defp finalize_integer(digits_str, negative, rest, pos) do
    full_str = if negative, do: "-" <> digits_str, else: digits_str

    with :ok <- validate_underscores(full_str, :dec_int),
         :ok <- validate_no_leading_zeros(full_str),
         :ok <- expect_separator(rest, pos) do
      clean = String.replace(full_str, "_", "")
      {String.to_integer(clean), rest, pos}
    end
  end

  # Parse fractional part after "."
  defp parse_fractional(data, pos, negative, int_str) do
    {frac_str, rest, pos2} = scan_frac_digits(data, pos, [])

    case rest do
      <<e, rest2::binary>> when e in [?e, ?E] ->
        parse_float_exponent(rest2, pos2 + 1, negative, int_str, frac_str)

      <<d, rest2::binary>> when d in [?d, ?D] ->
        parse_decimal_exponent(rest2, pos2 + 1, negative, int_str, frac_str)

      _ ->
        # Decimal (no exponent)
        finalize_decimal(int_str, frac_str, "", negative, rest, pos2)
    end
  end

  defp scan_frac_digits(<<digit, rest::binary>>, pos, acc) when digit in ?0..?9 do
    scan_frac_digits(rest, pos + 1, [digit | acc])
  end

  defp scan_frac_digits(<<?_, digit, rest::binary>>, pos, acc) when digit in ?0..?9 do
    scan_frac_digits(rest, pos + 2, [digit, ?_ | acc])
  end

  defp scan_frac_digits(<<?_, rest::binary>>, pos, acc) do
    scan_frac_digits(rest, pos + 1, [?_ | acc])
  end

  defp scan_frac_digits(rest, pos, acc) do
    str = acc |> Enum.reverse() |> IO.iodata_to_binary()
    {str, rest, pos}
  end

  # Parse float exponent: [+-]?[0-9]+
  defp parse_float_exponent(data, pos, negative, int_str, frac_str) do
    {sign, data2, pos2} =
      case data do
        <<?+, rest::binary>> -> {"+", rest, pos + 1}
        <<?-, rest::binary>> -> {"-", rest, pos + 1}
        _ -> {"", data, pos}
      end

    {exp_digits, rest, pos3} = scan_exp_digits(data2, pos2, [])

    if exp_digits == "" do
      {:error, "expected digits in float exponent"}
    else
      finalize_float(int_str, frac_str, sign <> exp_digits, negative, rest, pos3)
    end
  end

  # Parse decimal exponent: [+-]?[0-9]+
  defp parse_decimal_exponent(data, pos, negative, int_str, frac_str) do
    {sign, data2, pos2} =
      case data do
        <<?+, rest::binary>> -> {"+", rest, pos + 1}
        <<?-, rest::binary>> -> {"-", rest, pos + 1}
        _ -> {"", data, pos}
      end

    {exp_digits, rest, pos3} = scan_exp_digits(data2, pos2, [])

    if exp_digits == "" do
      {:error, "expected digits in decimal exponent"}
    else
      finalize_decimal(int_str, frac_str, sign <> exp_digits, negative, rest, pos3)
    end
  end

  defp scan_exp_digits(<<digit, rest::binary>>, pos, acc) when digit in ?0..?9 do
    scan_exp_digits(rest, pos + 1, [digit | acc])
  end

  defp scan_exp_digits(<<?_, digit, rest::binary>>, pos, acc) when digit in ?0..?9 do
    scan_exp_digits(rest, pos + 2, [digit, ?_ | acc])
  end

  defp scan_exp_digits(rest, pos, acc) do
    str = acc |> Enum.reverse() |> IO.iodata_to_binary()
    {str, rest, pos}
  end

  defp finalize_float(int_str, frac_str, exp_str, negative, rest, pos) do
    mantissa =
      if frac_str == "" do
        int_str
      else
        int_str <> "." <> frac_str
      end

    full = if negative, do: "-" <> mantissa <> "e" <> exp_str, else: mantissa <> "e" <> exp_str

    with :ok <- validate_underscores(full, :float),
         :ok <- validate_no_leading_zeros(full),
         :ok <- expect_separator(rest, pos) do
      clean = String.replace(full, "_", "")

      # Normalize for Elixir float parsing
      normalized =
        cond do
          String.match?(clean, ~r/\.[eE]/) ->
            String.replace(clean, ~r/\.([eE])/, ".0\\1")

          String.contains?(clean, ".") ->
            clean

          true ->
            String.replace(clean, ~r/([eE])/, ".0\\1")
        end

      case Float.parse(normalized) do
        {val, ""} -> {val, rest, pos}
        _ -> parse_float_overflow(normalized, rest, pos)
      end
    end
  end

  defp parse_float_overflow(str, rest, pos) do
    if String.starts_with?(str, "-") do
      {:neg_infinity, rest, pos}
    else
      {:infinity, rest, pos}
    end
  end

  defp finalize_decimal(int_str, frac_str, exp_str, negative, rest, pos) do
    mantissa =
      if frac_str == "" do
        int_str <> "."
      else
        int_str <> "." <> frac_str
      end

    full_for_validation =
      if exp_str == "" do
        if negative, do: "-" <> mantissa, else: mantissa
      else
        if negative, do: "-" <> mantissa <> "d" <> exp_str, else: mantissa <> "d" <> exp_str
      end

    with :ok <- validate_underscores(full_for_validation, :decimal),
         :ok <- validate_no_leading_zeros(full_for_validation),
         :ok <- expect_separator(rest, pos) do
      clean_mantissa = String.replace(if(negative, do: "-" <> mantissa, else: mantissa), "_", "")

      if exp_str == "" do
        dec = Decimal.new(clean_mantissa)
        {dec, rest, pos}
      else
        clean_exp = String.replace(exp_str, "_", "")
        explicit_exp = String.to_integer(clean_exp)

        mantissa_dec =
          Decimal.new(
            if(String.contains?(clean_mantissa, "."),
              do: clean_mantissa,
              else: clean_mantissa <> "."
            )
          )

        dec = %Decimal{mantissa_dec | exp: mantissa_dec.exp + explicit_exp}
        {dec, rest, pos}
      end
    end
  end

  # ---- Hex integer ----

  defp parse_hex_int(data, pos, negative) do
    {hex_str, rest, pos2} = scan_hex_digits(data, pos, [])

    if hex_str == "" do
      {:error, "expected hex digits after 0x"}
    else
      full = if negative, do: "-0x" <> hex_str, else: "0x" <> hex_str

      with :ok <- validate_underscores(full, :hex_int),
           :ok <- expect_separator(rest, pos2) do
        clean = String.replace(hex_str, "_", "")
        result = String.to_integer(clean, 16)
        val = if negative, do: -result, else: result
        {val, rest, pos2}
      end
    end
  end

  defp scan_hex_digits(<<digit, rest::binary>>, pos, acc)
       when digit in ?0..?9 or digit in ?a..?f or digit in ?A..?F do
    scan_hex_digits(rest, pos + 1, [digit | acc])
  end

  defp scan_hex_digits(<<?_, rest::binary>>, pos, acc) do
    scan_hex_digits(rest, pos + 1, [?_ | acc])
  end

  defp scan_hex_digits(rest, pos, acc) do
    str = acc |> Enum.reverse() |> IO.iodata_to_binary()
    {str, rest, pos}
  end

  # ---- Binary integer ----

  defp parse_bin_int(data, pos, negative) do
    {bin_str, rest, pos2} = scan_bin_digits(data, pos, [])

    if bin_str == "" do
      {:error, "expected binary digits after 0b"}
    else
      full = if negative, do: "-0b" <> bin_str, else: "0b" <> bin_str

      with :ok <- validate_underscores(full, :bin_int),
           :ok <- expect_separator(rest, pos2) do
        clean = String.replace(bin_str, "_", "")
        result = String.to_integer(clean, 2)
        val = if negative, do: -result, else: result
        {val, rest, pos2}
      end
    end
  end

  defp scan_bin_digits(<<digit, rest::binary>>, pos, acc) when digit in [?0, ?1] do
    scan_bin_digits(rest, pos + 1, [digit | acc])
  end

  defp scan_bin_digits(<<?_, rest::binary>>, pos, acc) do
    scan_bin_digits(rest, pos + 1, [?_ | acc])
  end

  defp scan_bin_digits(rest, pos, acc) do
    str = acc |> Enum.reverse() |> IO.iodata_to_binary()
    {str, rest, pos}
  end

  # ---- Timestamp parsing ----

  # After YYYY- has been consumed
  defp parse_timestamp_after_year(year_str, data, pos) do
    # Expect MM
    case data do
      <<m1, m2, rest::binary>> when m1 in ?0..?9 and m2 in ?0..?9 ->
        month_str = <<m1, m2>>

        case rest do
          <<?T, rest2::binary>> ->
            # YYYY-MMT (year-month)
            ts_str = year_str <> "-" <> month_str <> "T"
            validate_and_return_timestamp(ts_str, rest2, pos + 3)

          <<?-, rest2::binary>> ->
            # YYYY-MM-DD...
            parse_timestamp_after_month(year_str, month_str, rest2, pos + 3)

          _ ->
            # Not a valid timestamp, treat YYYY as integer and - as separate
            # Actually this shouldn't happen since we only get here if we had YYYY-
            {:error, "invalid timestamp: expected day or 'T' after month"}
        end

      _ ->
        {:error, "invalid timestamp: expected month digits"}
    end
  end

  defp parse_timestamp_after_month(year_str, month_str, data, pos) do
    case data do
      <<d1, d2, rest::binary>> when d1 in ?0..?9 and d2 in ?0..?9 ->
        day_str = <<d1, d2>>

        case rest do
          <<?T, rest2::binary>> ->
            # Could be YYYY-MM-DDT (date with T) or YYYY-MM-DDThh:mm...
            case rest2 do
              <<h1, h2, ?:, _::binary>> when h1 in ?0..?9 and h2 in ?0..?9 ->
                # Has time component
                parse_timestamp_time(
                  year_str <> "-" <> month_str <> "-" <> day_str <> "T",
                  rest2,
                  pos + 3
                )

              _ ->
                # YYYY-MM-DDT (date only with trailing T)
                ts_str = year_str <> "-" <> month_str <> "-" <> day_str <> "T"
                validate_and_return_timestamp(ts_str, rest2, pos + 3)
            end

          _ ->
            # YYYY-MM-DD (date only, no T)
            ts_str = year_str <> "-" <> month_str <> "-" <> day_str
            validate_and_return_timestamp(ts_str, rest, pos + 2)
        end

      _ ->
        {:error, "invalid timestamp: expected day digits"}
    end
  end

  defp parse_timestamp_time(prefix, data, pos) do
    # Parse hh:mm
    case data do
      <<h1, h2, ?:, m1, m2, rest::binary>>
      when h1 in ?0..?9 and h2 in ?0..?9 and m1 in ?0..?9 and m2 in ?0..?9 ->
        time_prefix = prefix <> <<h1, h2, ?:, m1, m2>>

        case rest do
          <<?:, s1, s2, rest2::binary>> when s1 in ?0..?9 and s2 in ?0..?9 ->
            # Has seconds
            seconds_str = <<s1, s2>>

            case rest2 do
              <<?., rest3::binary>> ->
                # Fractional seconds
                {frac, rest4, pos4} = scan_frac_digits(rest3, pos + 9, [])
                ts_str = time_prefix <> ":" <> seconds_str <> "." <> frac
                parse_timestamp_offset(ts_str, rest4, pos4)

              _ ->
                ts_str = time_prefix <> ":" <> seconds_str
                parse_timestamp_offset(ts_str, rest2, pos + 8)
            end

          _ ->
            # hh:mm with no seconds
            parse_timestamp_offset(time_prefix, rest, pos + 5)
        end

      _ ->
        {:error, "invalid timestamp: expected time"}
    end
  end

  defp parse_timestamp_offset(prefix, data, pos) do
    case data do
      <<?Z, rest::binary>> ->
        validate_and_return_timestamp(prefix <> "Z", rest, pos + 1)

      <<sign, oh1, oh2, ?:, om1, om2, rest::binary>>
      when sign in [?+, ?-] and oh1 in ?0..?9 and oh2 in ?0..?9 and om1 in ?0..?9 and
             om2 in ?0..?9 ->
        offset_str = <<sign, oh1, oh2, ?:, om1, om2>>
        validate_and_return_timestamp(prefix <> offset_str, rest, pos + 6)

      _ ->
        {:error, "invalid timestamp: expected timezone offset"}
    end
  end

  defp parse_timestamp_year_only(year_str, rest, pos) do
    ts_str = year_str <> "T"
    validate_and_return_timestamp(ts_str, rest, pos)
  end

  defp validate_and_return_timestamp(ts_str, rest, pos) do
    with :ok <- Timestamp.validate_timestamp(ts_str),
         :ok <- expect_separator(rest, pos) do
      {{:timestamp, ts_str}, rest, pos}
    end
  end

  # ---- Validation helpers (inline versions) ----

  defp validate_underscores(str, type) do
    Validation.validate_underscores(str, type)
  end

  defp validate_no_leading_zeros(str) do
    Validation.validate_no_leading_zeros(str)
  end

  defp expect_separator(<<>>, _pos), do: :ok

  defp expect_separator(<<byte, _::binary>>, _pos)
       when byte in [?\s, ?\t, ?\n, ?\r, ?\v, ?\f, ?,, ?:, ?), ?], ?}, ?(, ?[, ?{, ?", ?'] do
    :ok
  end

  defp expect_separator(<<?/, next, _::binary>>, _pos) when next in [?/, ?*], do: :ok

  defp expect_separator(<<byte, _::binary>>, pos) do
    {:error, "unexpected character '#{<<byte>>}' at position #{pos} after number"}
  end
end