package gs.hex.v0

object Hex:

  private val HexChars: Array[Char] = Array(
    '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e',
    'f'
  )

  // Leverage cache to avoid operations.
  private val Digits: Array[Int] = Array(
    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, -1,
    -1, -1, -1, -1, -1, -1, 10, 11, 12, 13, 14, 15, -1, -1, -1, -1, -1, -1, -1,
    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
    10, 11, 12, 13, 14, 15, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
    -1, -1, -1, -1, -1, -1, -1
  )

  /** Convert the given raw data to a hexadecimal string.
    *
    * @param data
    *   The data to convert.
    * @return
    *   The output hexadecimal string.
    */
  def toHexString(data: Array[Byte]): String =
    var index           = 0
    val ch: Array[Char] = new Array(data.length * 2)
    data.foreach { byte =>
      ch(index) = HexChars((byte & 0xf0) >> 4)
      ch(index + 1) = HexChars(byte & 0x0f)
      index = index + 2
    }
    new String(ch)

  /** Convert the given hexadecimal string to a byte array.
    *
    * @param encoded
    *   The hexadecimal string.
    * @return
    *   The encoded byte array, or nothing if the input is not a valid hex
    *   string.
    */
  def fromHexString(encoded: String): Option[Array[Byte]] =
    if encoded.isBlank() then Some(Array.empty)
    else if encoded.length() % 2 != 0 then None
    else
      val size              = encoded.length() / 2
      val data: Array[Byte] = new Array(size)

      // The "base" tracks the base index for character extraction.
      var base = 0

      // The "index" tracks the byte index being updated.
      var index = 0

      // Track if calculation failed for any reason.
      var failed = false

      // Populate each byte in the destination array.
      while !failed && index < size do
        // Extract the relevant characters.
        val digit1 = Digits(encoded.charAt(base).toInt)
        val digit2 = Digits(encoded.charAt(base + 1).toInt)

        if digit1 >= 0 && digit2 >= 0 then
          // Convert to byte and assign the result.
          data(index) = pairToByte(digit1, digit2)

          index = index + 1
          base = base + 2
        else
          index = size
          failed = true

      if !failed then Some(data) else None

  private inline def pairToByte(
    digit1: Int,
    digit2: Int
  ): Byte =
    ((digit1 << 4) + digit2).toByte

end Hex