block-explorer/server/app/com/xsn/explorer/services/LedgerSynchronizerService.scala

package com.xsn.explorer.services

import javax.inject.Inject

import com.alexitc.playsonify.core.FutureApplicationResult
import com.alexitc.playsonify.core.FutureOr.Implicits.{FutureListOps, FutureOps, OptionOps}
import com.xsn.explorer.data.async.{BlockFutureDataHandler, LedgerFutureDataHandler}
import com.xsn.explorer.errors.BlockNotFoundError
import com.xsn.explorer.models.rpc.Block
import com.xsn.explorer.models.{Blockhash, Height, Transaction}
import com.xsn.explorer.util.Extensions.FutureOrExt
import org.scalactic.Good
import org.slf4j.LoggerFactory

import scala.concurrent.{ExecutionContext, Future}

class LedgerSynchronizerService @Inject() (
    xsnService: XSNService,
    transactionService: TransactionService,
    ledgerDataHandler: LedgerFutureDataHandler,
    blockDataHandler: BlockFutureDataHandler)(
    implicit ec: ExecutionContext) {

  private val logger = LoggerFactory.getLogger(this.getClass)

  /**
   * Synchronize the given block with our ledger database.
   *
   * The synchronization involves a very complex logic in order to handle
   * several corner cases, be sure to not call this method concurrently
   * because the behavior is undefined.
   */
  def synchronize(blockhash: Blockhash): FutureApplicationResult[Unit] = {
    val result = for {
      block <- xsnService.getBlock(blockhash).toFutureOr
      transactions <- block.transactions.map(transactionService.getTransaction).toFutureOr
      _ <- synchronize(block, transactions).toFutureOr
    } yield ()

    result.toFuture
  }

  private def synchronize(block: Block, transactions: List[Transaction]): FutureApplicationResult[Unit] = {
    logger.info(s"Synchronize block ${block.height}, hash = ${block.hash}")

    val result = for {
      latestBlockMaybe <- blockDataHandler
          .getLatestBlock()
          .toFutureOr
          .map(Option.apply)
          .recoverFrom(BlockNotFoundError)(None)

      _ <- latestBlockMaybe
          .map { latestBlock => onLatestBlock(latestBlock, block, transactions) }
          .getOrElse { onEmptyLedger(block, transactions) }
          .toFutureOr
    } yield ()

    result.toFuture
  }

  /**
   * 1. current ledger is empty:
   * 1.1. the given block is the genensis block, it is added.
   * 1.2. the given block is not the genesis block, sync everything until the given block.
   */
  private def onEmptyLedger(block: Block, transactions: List[Transaction]): FutureApplicationResult[Unit] = {
    if (block.height.int == 0) {
      logger.info(s"Synchronize genesis block on empty ledger, hash = ${block.hash}")
      ledgerDataHandler.push(block, transactions)
    } else {
      logger.info(s"Synchronize block ${block.height} on empty ledger, hash = ${block.hash}")
      val result = for {
        _ <- sync(0 until block.height.int).toFutureOr
        _ <- synchronize(block, transactions).toFutureOr
      } yield ()

      result.toFuture
    }
  }

  /**
   * 2. current ledger has blocks until N, given block height H:
   * 2.1. if N+1 == H and its previous blockhash is N, it is added.
   * 2.2. if N+1 == H and its previous blockhash isn't N, pick the expected block N from H and apply the whole process with it, then, apply H.
   * 2.3. if H > N+1, sync everything until H.
   * 2.4. if H <= N, if the hash already exists, it is ignored.
   * 2.5. if H <= N, if the hash doesn't exists, remove blocks from N to H (included), then, add the new H.
   */
  private def onLatestBlock(ledgerBlock: Block, newBlock: Block, newTransactions: List[Transaction]): FutureApplicationResult[Unit] = {
    if (ledgerBlock.height.int + 1 == newBlock.height.int &&
        newBlock.previousBlockhash.contains(ledgerBlock.hash)) {

      logger.info(s"Appending block ${newBlock.height}, hash = ${newBlock.hash}")
      ledgerDataHandler.push(newBlock, newTransactions)
    } else if (ledgerBlock.height.int + 1 == newBlock.height.int) {
      logger.info(s"Reorganization to push block ${newBlock.height}, hash = ${newBlock.hash}")
      val result = for {
        blockhash <- newBlock.previousBlockhash.toFutureOr(BlockNotFoundError)
        previousBlock <- xsnService.getBlock(blockhash).toFutureOr
        previousTransactions <- previousBlock.transactions.map(transactionService.getTransaction).toFutureOr
        _ <- synchronize(previousBlock, previousTransactions).toFutureOr
        _ <- synchronize(newBlock, newTransactions).toFutureOr
      } yield ()

      result.toFuture
    } else if (newBlock.height.int > ledgerBlock.height.int) {
      logger.info(s"Filling holes to push block ${newBlock.height}, hash = ${newBlock.hash}")
      val result = for {
        _ <- sync(ledgerBlock.height.int + 1 until newBlock.height.int).toFutureOr
        _ <- synchronize(newBlock, newTransactions).toFutureOr
      } yield ()

      result.toFuture
    } else {
      val result = for {
        expectedBlockMaybe <- blockDataHandler
            .getBy(newBlock.hash)
            .toFutureOr
            .map(Option.apply)
            .recoverFrom(BlockNotFoundError)(None)

        _ = logger.info(s"Checking possible existing block ${newBlock.height}, hash = ${newBlock.hash}, exists = ${expectedBlockMaybe.isDefined}")
        _ <- expectedBlockMaybe
            .map { _ => Future.successful(Good(())) }
            .getOrElse {
              val x = for {
                _ <- trimTo(newBlock.height).toFutureOr
                _ <- synchronize(newBlock, newTransactions).toFutureOr
              } yield ()
              x.toFuture
            }
            .toFutureOr
      } yield ()

      result.toFuture
    }
  }

  /**
   * Sync the given range to our ledger.
   */
  private def sync(range: Range): FutureApplicationResult[Unit] = {
    logger.info(s"Syncing block range = $range")

    // TODO: check, it might be safer to use the nextBlockhash instead of the height
    range.foldLeft[FutureApplicationResult[Unit]](Future.successful(Good(()))) { case (previous, height) =>
      val result = for {
        _ <- previous.toFutureOr
        blockhash <- xsnService.getBlockhash(Height(height)).toFutureOr
        block <- xsnService.getBlock(blockhash).toFutureOr
        transactions <- block.transactions.map(transactionService.getTransaction).toFutureOr
        _ <- synchronize(block, transactions).toFutureOr
      } yield ()

      result.toFuture
    }
  }

  /**
   * Trim the ledger until the given block height, if the height is 4,
   * the last stored block will be 3.
   */
  private def trimTo(height: Height): FutureApplicationResult[Unit] = {
    val result = ledgerDataHandler
        .pop()
        .toFutureOr
        .flatMap { block =>
          logger.info(s"Trimmed block ${block.height} from the ledger")
          val result = if (block.height == height) {
            Future.successful(Good(()))
          } else {
            trimTo(height)
          }

          result.toFutureOr
        }

    result.toFuture
  }
}