package controllers.caching

import play.api.mvc.Results.NotModified
import play.api.mvc.{Request, Result}

import java.io.{ByteArrayOutputStream, DataOutputStream, OutputStream}
import java.security.{DigestOutputStream, MessageDigest}
import java.util.Base64
import scala.concurrent.{ExecutionContext, Future}
import scala.util.Using

trait Etagable[A]:
  extension (a: A) def etag: String


trait DigestEtagable[A] extends Etagable[A]:
  private val base64 = Base64.getUrlEncoder.withoutPadding

  protected def writeDigestData(a: A, s: DataOutputStream): Unit

  extension (a: A) {
    override def etag: String = {
      val md = MessageDigest.getInstance("SHA-1")
      Using(DataOutputStream(DigestOutputStream(OutputStream.nullOutputStream(), md))) { dos =>
        writeDigestData(a, dos)
        dos.flush()
      }

      base64.encodeToString(md.digest())
    }
  }


trait CheckFresh {
  protected def checkFreshAsync[A, E: Etagable](entity: E)(onStale: => Future[Result])(using request: Request[A], ec: ExecutionContext): Future[Result] = {
    compareEtag(request, entity).fold(
      entityEtag => onStale.map(_.withHeaders("etag" -> entityEtag)),
      requestEtag => Future.successful(notModified(requestEtag))
    )
  }

  protected def checkFresh[A, E: Etagable](entity: E)(onStale: => Result)(using request: Request[A]): Result = {
    compareEtag(request, entity).fold(
      entityEtag => onStale.withHeaders("etag" -> entityEtag),
      requestEtag => notModified(requestEtag)
    )
  }

  private def compareEtag[A, E: Etagable](request: Request[A], entity: E) = {
    val entityEtag = entity.etag
    request.headers.get("If-None-Match").filter(e => e == entityEtag) match {
      case Some(etag) => Right(etag)
      case None => Left(entityEtag)
    }
  }
  private def notModified(etag: String) = NotModified.withHeaders("etag" -> etag)
}