space_dust

defmodule SpaceDust.Utils.Tle do
  @moduledoc """
  Utility functions for parsing two-line element sets (TLEs)
  """

  require Logger
  alias SpaceDust.Utils.TwoLineElementSet, as: TwoLineElementSet
  alias SpaceDust.Utils.Constants, as: Constants

  @microsecondsPerDay Constants.secondsPerDay() * 1_000_000

  @doc "parse a two line element set (TLE) from a string"
  def parseTLE(line1, line2) do
    # check to ensure each line is 69 characters
    case {String.length(line1), String.length(line2)} do
      # TLE has correct number of characters, proceed with parsing
      {69, 69} ->
        try do
          lastDigitsOfYear = String.to_integer(String.slice(line1, 18..19))
          currentYear = Date.utc_today().year

          epochYear =
            if lastDigitsOfYear > currentYear - 2000 do
              1900 + lastDigitsOfYear
            else
              2000 + lastDigitsOfYear
            end

          # day 1 is 0 days from the start of the year
          dayOfYear = String.to_float(String.slice(line1, 20..31)) - 1
          daysToAdd = trunc(dayOfYear)
          microsecondsToAdd = trunc((dayOfYear - daysToAdd) * @microsecondsPerDay)
          startOfYear = DateTime.new!(Date.new!(epochYear, 1, 1), Time.new!(0, 0, 0, 0))

          epoch =
            DateTime.add(startOfYear, daysToAdd, :day)
            |> DateTime.add(microsecondsToAdd, :microsecond)

          meanMotionDoubleDot =
            if String.at(line1, 44) == "-" do
              -1 * String.to_float(String.replace("0." <> String.slice(line1, 45..49), " ", "")) *
                Float.pow(10.0, String.to_integer(String.slice(line1, 50..51)))
            else
              String.to_float(String.replace("0." <> String.slice(line1, 45..49), " ", "")) *
                Float.pow(10.0, String.to_integer(String.slice(line1, 50..51)))
            end

          meanMotionDot =
            if String.at(line1, 33) == "-" do
              -1 * String.to_float(String.replace("0" <> String.slice(line1, 34..42), " ", ""))
            else
              String.to_float(String.replace("0" <> String.slice(line1, 34..42), " ", ""))
            end

          bStar =
            if String.at(line1, 53) == "-" do
              -1 * String.to_float(String.replace("0." <> String.slice(line1, 54..58), " ", "")) *
                Float.pow(10.0, String.to_integer(String.slice(line1, 59..60)))
            else
              String.to_float(String.replace("0." <> String.slice(line1, 53..58), " ", "")) *
                Float.pow(10.0, String.to_integer(String.slice(line1, 59..60)))
            end

          tle = %TwoLineElementSet{
            # line 1 parameters
            line1: line1,
            catalogNumber: String.replace(String.slice(line1, 2..6), " ", ""),
            classification: String.at(line1, 7),
            internationalDesignator: String.replace(String.slice(line1, 9..16), " ", ""),
            epoch: epoch,
            meanMotionDot: meanMotionDot,
            meanMotionDoubleDot: meanMotionDoubleDot,
            bStar: bStar,
            ephemerisType: String.to_integer(String.at(line1, 62)),
            elementSetNumber:
              String.to_integer(String.replace(String.slice(line1, 64..67), " ", "")),

            # line 2 parameters
            line2: line2,
            inclinationDeg: String.to_float(String.replace(String.slice(line2, 8..15), " ", "")),
            raanDeg: String.to_float(String.replace(String.slice(line2, 17..24), " ", "")),
            eccentricity:
              String.to_float(String.replace("0." <> String.slice(line2, 26..32), " ", "")),
            argPerigeeDeg: String.to_float(String.replace(String.slice(line2, 34..41), " ", "")),
            meanAnomalyDeg: String.to_float(String.replace(String.slice(line2, 43..50), " ", "")),
            meanMotion: String.to_float(String.replace(String.slice(line2, 52..62), " ", "")),
            revNumber: String.to_integer(String.replace(String.slice(line2, 63..67), " ", ""))
          }

          {:ok, tle}
        rescue
          ArgumentError -> {:error, "Unable to parse TLE- check all fields are correctly spaced"}
        end

      _ ->
        {:error, "Unable to parse TLE- line length is incorrect"}
    end
  end

  @doc """
  Get the position and velocity (in the TEME frame) of a satellite at a given time.
  Uses the sgp4_ex library for fast NIF-based propagation.
  """
  def getRVatTime(twoLineElementSet, utcEpoch) do
    case Sgp4Ex.parse_tle(twoLineElementSet.line1, twoLineElementSet.line2) do
      {:ok, sgp4_tle} ->
        case Sgp4Ex.propagate_tle_to_epoch(sgp4_tle, utcEpoch) do
          {:ok, %Sgp4Ex.TemeState{position: {rx, ry, rz}, velocity: {vx, vy, vz}}} ->
            {{rx, ry, rz}, {vx, vy, vz}}

          {:error, reason} ->
            Logger.error("SGP4 propagation failed: #{reason}")
            {:error, reason}
        end

      {:error, reason} ->
        Logger.error("Failed to parse TLE: #{reason}")
        {:error, reason}
    end
  end
end