package gs.std.v0

import java.security.MessageDigest
import java.nio.charset.Charset
import java.nio.charset.StandardCharsets

/**
  * Opaque type representing a MD5 hash.
  */
opaque type MD5 = Array[Byte]

/**
  * Opaque type representing a MD5 hash.
  */
object MD5:

  /**
   * MD5 hashes are exactly 16 bytes.
   */
  final val NumberOfBytes: ByteCount = ByteCount(16)

  /**
   * The algorithm name is "MD5".
   */
  final val Algorithm: String = "MD5"

  /**
   * Instantiate a [[MD5]] from the given byte array. This function does not
   * know whether the array is actually a calculated hash.
   *
   * Typically used for loading pre-validated hashes (e.g. from a database).
   *
   * @param bytes The bytes - must contain exactly 16 bytes.
   * @return The new [[MD5]] instance.
   */
  def fromBytes(bytes: Array[Byte]): MD5 = 
    if NumberOfBytes.equal(bytes.length) then
      bytes
    else
      throw IllegalArgumentException(s"MD5 values must be exactly $NumberOfBytes bytes.")

  /**
   * Calculate the MD5 hash for the given byte array.
   *
   * @param data The byte array.
   * @return The calculated [[MD5]].
   */
  def calculate(data: Array[Byte]): MD5 = 
    MessageDigest.getInstance(Algorithm).digest(data)

  /*
   * Calculate the MD5 hash for the given string.
   *
   * @param data The string data.
   * @param charset The character set of the string. Defaults to UTF-8.
   * @return The calculated [[MD5]].
   */
  def calculate(data: String, charset: Charset = StandardCharsets.UTF_8): MD5 =
    calculate(data.getBytes(charset))

  extension (md5: MD5)
    /**
     * @return The underlying byte array.
     */
    def unwrap(): Array[Byte] = md5

    /**
     * @return The underlying byte array.
     */
    def toBytes(): Array[Byte] = md5

    /**
     * Get the byte at the given index (0 to 15).
     *
     * Throws an exception if an out-of-bound index is given.
     *
     * @param index The byte index (0 to 15).
     * @return The byte at the specified index.
     */
    def getByte(index: Int): Byte =
      if index < 0 || index >= NumberOfBytes.unwrap() then
        throw IndexOutOfBoundsException(s"Index $index out of MD5 bound of $NumberOfBytes bytes.")
      else
        md5.apply(index)

    /**
     * Determine if this hash is the same as some other hash. Compares each byte
     * in order.
     *
     * @param other The [[MD5]] to compare against.
     * @return True if the hashes are identical, false otherwise.
     */
    def isSame(other: MD5): Boolean = md5.sameElements(other)

    /**
     * @return This hash encoded to a Base64 string.
     */
    def base64(): EncodedString = Base64Encoder.encode(md5)

    /**
     * @return This hash encoded to a Hexadecimal string.
     */
    def hex(): EncodedString = HexEncoder.encode(md5)

end MD5