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// MIT License

// Copyright (c) 2023 Jakub Wojnowski

// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:

// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.

// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.
package tasks.util

import fs2.Chunk
import fs2.Pipe
import fs2.Pull
import fs2.RaiseThrowable
import fs2.Stream

import cats.effect.Sync
import cats.effect.std.SecureRandom
import cats.syntax.all._

import scala.util.Try
import scala.util.control.NonFatal

import java.nio.ByteBuffer

import javax.crypto.Cipher
import javax.crypto.SecretKey
import javax.crypto.spec.GCMParameterSpec
import javax.crypto.spec.SecretKeySpec

object Aes {

  private val IntSizeInBytes = 4
  private val IvLengthBytes = 12
  private val AuthTagLengthBytes = 16
  private val DefaultChunkSize = 4 * 1024 * 1024

  private val transformation = "AES/GCM/NoPadding"
  private val keyAlgorithm = "AES"

  def generateKeyHexString(implicit
      ev: SecureRandom[cats.effect.IO]
  ): cats.effect.IO[String] = {
    SecureRandom[cats.effect.IO]
      .nextBytes(32) // 256 bits
      .map(_.map("%02x".format(_)).mkString)
  }

  def decrypt[F[_]: Sync](key: SecretKey): Pipe[F, Byte, Byte] =
    (stream: Stream[F, Byte]) =>
      readFirstN(IntSizeInBytes, stream) { (chunkSizeBytes, remainingStream) =>
        val chunkSize = bytesToChunkSize(chunkSizeBytes)

        remainingStream
          .chunkN(IvLengthBytes + chunkSize + AuthTagLengthBytes)
          .flatMap { chunk =>
            readFirstN(IvLengthBytes, Stream.chunk(chunk).covary[F]) {
              (ivChunk, stream) =>
                Stream
                  .eval(createCipher(Cipher.DECRYPT_MODE, key, ivChunk.toArray))
                  .flatMap { cipher =>
                    stream.mapChunks(cipherChunk(cipher, _))
                  }
            }
          }
          .adaptError { case NonFatal(throwable) =>
            Error.DecryptionError(throwable)
          }

      }

  def encrypt[F[_]: Sync: SecureRandom](
      key: SecretKey,
      chunkSize: Int = DefaultChunkSize
  ): Pipe[F, Byte, Byte] =
    (stream: Stream[F, Byte]) =>
      Stream.chunk(chunkSizeToBytes(chunkSize)) ++
        stream
          .chunkN(chunkSize)
          .flatMap { chunk =>
            Stream.eval(SecureRandom[F].nextBytes(IvLengthBytes)).flatMap {
              ivBytes =>
                Stream
                  .eval(createCipher[F](Cipher.ENCRYPT_MODE, key, ivBytes))
                  .flatMap { cipher =>
                    Stream.chunk(Chunk.array(ivBytes)) ++ Stream
                      .chunk(cipherChunk(cipher, chunk))
                  }
            }
          }
          .adaptError { case NonFatal(throwable) =>
            Error.EncryptionError(throwable)
          }

  def keyFromHex(hexString: String): Option[SecretKey] =
    Try {
      new SecretKeySpec(
        hexString.sliding(2, 2).toArray.map(Integer.parseInt(_, 16).toByte),
        keyAlgorithm
      )
    }.toOption

  private type Mode = Int

  private def createCipher[F[_]: Sync](
      mode: Mode,
      key: SecretKey,
      ivBytes: Array[Byte]
  ): F[Cipher] =
    Sync[F].delay {
      val cipher = Cipher.getInstance(transformation)
      cipher.init(
        mode,
        key,
        new GCMParameterSpec(AuthTagLengthBytes * 8, ivBytes)
      )
      cipher
    }

  private def readFirstN[F[_]: RaiseThrowable, A, B](
      n: Int,
      stream: Stream[F, A]
  )(
      f: (Chunk[A], Stream[F, A]) => Stream[F, B]
  ): Stream[F, B] =
    stream.pull
      .unconsN(n)
      .flatMap {
        case Some((chunk, stream)) =>
          f(chunk, stream).pull.echo
        case None =>
          Pull.raiseError(Error.DataTooShort)
      }
      .stream

  private def cipherChunk(cipher: Cipher, chunk: Chunk[Byte]): Chunk[Byte] = {
    val inputBuffer = chunk.toByteBuffer
    val outputBuffer =
      ByteBuffer.allocate(cipher.getOutputSize(inputBuffer.remaining()))
    cipher.doFinal(inputBuffer, outputBuffer)
    Chunk.byteBuffer(outputBuffer.rewind())
  }

  private def chunkSizeToBytes(chunkSize: Int): Chunk[Byte] = {
    val buffer = ByteBuffer.allocate(IntSizeInBytes)
    buffer.putInt(chunkSize)
    Chunk.byteBuffer(buffer.rewind())
  }

  private def bytesToChunkSize(bytes: Chunk[Byte]): Int =
    bytes.toByteBuffer.getInt

  sealed abstract class Error(message: String, cause: Option[Throwable] = None)
      extends Exception(message, cause.orNull)
      with Product
      with Serializable

  object Error {
    case object DataTooShort extends Error("Data too short")

    case class EncryptionError(cause: Throwable)
        extends Error(s"Error during encryption: $cause", cause.some)

    case class DecryptionError(cause: Throwable)
        extends Error(s"Error during decryption: $cause", cause.some)
  }

}