# nimbus-eth1
# Copyright (c) 2023-2024 Status Research & Development GmbH
# Licensed under either of
#  * Apache License, version 2.0, ([LICENSE-APACHE](LICENSE-APACHE) or
#    http://www.apache.org/licenses/LICENSE-2.0)
#  * MIT license ([LICENSE-MIT](LICENSE-MIT) or
#    http://opensource.org/licenses/MIT)
# at your option. This file may not be copied, modified, or distributed
# except according to those terms.

## Kvt DB -- Filter management
## ===========================
##

import
  std/[sequtils, tables],
  results,
  ./kvt_desc,
  ./kvt_desc/desc_backend,
  ./kvt_delta/[delta_merge, delta_reverse]

# ------------------------------------------------------------------------------
# Public functions
# ------------------------------------------------------------------------------

proc deltaPersistentOk*(db: KvtDbRef): bool =
  ## Check whether the balancer filter can be merged into the backend
  not db.backend.isNil and db.isCentre


proc deltaPersistent*(
    db: KvtDbRef;                      # Database
    reCentreOk = false;
      ): Result[void,KvtError] =
  ## Resolve (i.e. move) the backend filter into the physical backend database.
  ##
  ## This needs write permission on the backend DB for the argument `db`
  ## descriptor (see the function `aristo_desc.isCentre()`.) With the argument
  ## flag `reCentreOk` passed `true`, write permission will be temporarily
  ## acquired when needed.
  ##
  ## Other non-centre descriptors are updated so there is no visible database
  ## change for these descriptors.
  ##
  let be = db.backend
  if be.isNil:
    return err(FilBackendMissing)

  # Blind or missing filter
  if db.balancer.isNil:
    return ok()

  # Make sure that the argument `db` is at the centre so the backend is in
  # read-write mode for this peer.
  let parent = db.getCentre
  if db != parent:
    if not reCentreOk:
      return err(FilBackendRoMode)
    ? db.reCentre()
  # Always re-centre to `parent` (in case `reCentreOk` was set)
  defer: discard parent.reCentre()

  # Update forked balancers here do that errors are detected early (if any.)
  if 0 < db.nForked:
    let rev = db.revFilter(db.balancer).valueOr:
      return err(error[1])
    if 0  < rev.sTab.len: # Can an empty `rev` happen at all?
      var unsharedRevOk = true
      for w in db.forked:
        if not w.db.balancer.isValid:
          unsharedRevOk = false
        # The `rev` filter can be modified if one can make sure that it is
        # not shared (i.e. only previously merged into the w.db.balancer.)
        # Note that it is trivially true for a single fork.
        let modLowerOk = w.isLast and unsharedRevOk
        w.db.balancer = deltaMerge(
          w.db.balancer, modUpperOk=false, rev, modLowerOk=modLowerOk)

  # Store structural single trie entries
  let writeBatch = ? be.putBegFn()
  be.putKvpFn(writeBatch, db.balancer.sTab.pairs.toSeq)
  ? be.putEndFn writeBatch

  # Done with balancer, all saved to backend
  db.balancer = LayerRef(nil)

  ok()

# ------------------------------------------------------------------------------
# End
# ------------------------------------------------------------------------------