# Nimbus
# Copyright (c) 2021-2022 Status Research & Development GmbH
# Licensed and distributed under either of
#   * MIT license (license terms in the root directory or at https://opensource.org/licenses/MIT).
#   * Apache v2 license (license terms in the root directory or at https://www.apache.org/licenses/LICENSE-2.0).
# at your option. This file may not be copied, modified, or distributed except according to those terms.

{.push raises: [Defect].}

import
  std/[options, heapqueue],
  eth/db/kvstore,
  eth/db/kvstore_sqlite3,
  stint,
  ./network/state/state_content

export kvstore_sqlite3

# This version of content db is the most basic, simple solution where data is
# stored no matter what content type or content network in the same kvstore with
# the content id as key. The content id is derived from the content key, and the
# deriviation is different depending on the content type. As we use content id,
# this part is currently out of the scope / API of the ContentDB.
# In the future it is likely that that either:
# 1. More kvstores are added per network, and thus depending on the network a
# different kvstore needs to be selected.
# 2. Or more kvstores are added per network and per content type, and thus
# content key fields are required to access the data.
# 3. Or databases are created per network (and kvstores pre content type) and
# thus depending on the network the right db needs to be selected.

const
  # Maximal number of ObjInfo objects held in memory per database scan. 100k
  # objects should result in memory usage of around 7mb which should be
  # appropriate for even low resource devices
  maxObjPerScan = 100000

type
  RowInfo = tuple
    contentId: array[32, byte]
    payloadLength: int64

  ObjInfo* = object
    contentId*: array[32, byte]
    payloadLength*: int64
    distFrom*: UInt256

  ContentDB* = ref object
    kv: KvStoreRef
    maxSize: uint32
    sizeStmt: SqliteStmt[NoParams, int64]
    unusedSizeStmt: SqliteStmt[NoParams, int64]
    vacStmt: SqliteStmt[NoParams, void]
    getAll: SqliteStmt[NoParams, RowInfo]

  PutResultType* = enum
    ContentStored, DbPruned

  PutResult* = object
    case kind*: PutResultType
    of ContentStored:
      discard
    of DbPruned:
      furthestStoredElementDistance*: UInt256
      fractionOfDeletedContent*: float64
      numOfDeletedElements*: int64

# Objects must be sorted from largest to closest distance
proc `<`(a, b: ObjInfo): bool =
  return a.distFrom < b.distFrom

template expectDb(x: auto): untyped =
  # There's no meaningful error handling implemented for a corrupt database or
  # full disk - this requires manual intervention, so we'll panic for now
  x.expect("working database (disk broken/full?)")

proc new*(T: type ContentDB, path: string, maxSize: uint32, inMemory = false): ContentDB =
  let db =
    if inMemory:
      SqStoreRef.init("", "fluffy-test", inMemory = true).expect(
        "working database (out of memory?)")
    else:
      SqStoreRef.init(path, "fluffy").expectDb()

  let getSizeStmt = db.prepareStmt(
    "SELECT page_count * page_size as size FROM pragma_page_count(), pragma_page_size();",
    NoParams, int64).get()

  let unusedSize = db.prepareStmt(
    "SELECT freelist_count * page_size as size FROM pragma_freelist_count(), pragma_page_size();",
    NoParams, int64).get()

  let vacStmt = db.prepareStmt(
    "VACUUM;",
    NoParams, void).get()

  let kvStore = kvStore db.openKvStore().expectDb()

  # This needs to go after `openKvStore`, as it checks whether the table name
  # kvstore already exists.
  let getKeysStmt = db.prepareStmt(
    "SELECT key, length(value) FROM kvstore",
    NoParams, RowInfo
  ).get()

  ContentDB(
    kv: kvStore,
    maxSize: maxSize,
    sizeStmt: getSizeStmt,
    vacStmt: vacStmt,
    getAll: getKeysStmt,
    unusedSizeStmt: unusedSize
  )

proc getNFurthestElements*(
    db: ContentDB, target: UInt256, n: uint64): (seq[ObjInfo], int64) =
  ## Get at most n furthest elements from db in order from furthest to closest.
  ## Payload lengths are also returned so the caller can decide how many of
  ## those elements need to be deleted.
  ##
  ## Currently it uses xor metric
  ##
  ## Currently works by querying for all elements in database and doing all
  ## necessary work on program level. This is mainly due to two facts:
  ## - sqlite does not have build xor function, also it does not handle bitwise
  ## operations on blobs as expected
  ## - our nim wrapper for sqlite does not support create_function api of sqlite
  ## so we cannot create custom function comparing blobs at sql level. If that
  ## would be possible we may be able to all this work by one sql query

  if n == 0:
    return (newSeq[ObjInfo](), 0'i64)

  var heap = initHeapQueue[ObjInfo]()
  var totalContentSize: int64 = 0

  var ri: RowInfo
  for e in db.getAll.exec(ri):
    let contentId = UInt256.fromBytesBE(ri.contentId)
    # TODO: Currently it assumes xor distance, but when we start testing
    # networks with other distance functions this needs to be adjusted to the
    # custom distance function
    let dist = contentId xor target
    let obj = ObjInfo(
      contentId: ri.contentId, payloadLength: ri.payloadLength, distFrom: dist)

    if (uint64(len(heap)) < n):
      heap.push(obj)
    else:
      if obj > heap[0]:
        discard heap.replace(obj)

    totalContentSize = totalContentSize + ri.payloadLength

  var res: seq[ObjInfo] = newSeq[ObjInfo](heap.len())

  var i = heap.len() - 1
  while heap.len() > 0:
    res[i] = heap.pop()
    dec i

  return (res, totalContentSize)

proc reclaimSpace*(db: ContentDB): void =
  ## Runs sqlite VACUUM commands which rebuilds the db, repacking it into a
  ## minimal amount of disk space.
  ## Ideal mode of operation, is to run it after several deletes.
  ## Another options would be to run 'PRAGMA auto_vacuum = FULL;' statement at
  ## the start of db to leave it up to sqlite to clean up
  db.vacStmt.exec().expectDb()

proc size*(db: ContentDB): int64 =
  ## Retrun current size of DB as product of sqlite page_count and page_size
  ## https://www.sqlite.org/pragma.html#pragma_page_count
  ## https://www.sqlite.org/pragma.html#pragma_page_size
  ## It returns total size of db i.e both data and metadata used to store content
  ## also it is worth noting that when deleting content, size may lags behind due
  ## to the way how deleting works in sqlite.
  ## Good description can be found in: https://www.sqlite.org/lang_vacuum.html

  var size: int64 = 0
  discard (db.sizeStmt.exec do(res: int64):
    size = res).expectDb()
  return size

proc unusedSize(db: ContentDB): int64 =
  ## Returns the total size of the pages which are unused by the database,
  ## i.e they can be re-used for new content.

  var size: int64 = 0
  discard (db.unusedSizeStmt.exec do(res: int64):
    size = res).expectDb()
  return size

proc realSize*(db: ContentDB): int64 =
  db.size() - db.unusedSize()

proc get*(db: ContentDB, key: openArray[byte]): Option[seq[byte]] =
  var res: Option[seq[byte]]
  proc onData(data: openArray[byte]) = res = some(@data)

  discard db.kv.get(key, onData).expectDb()

  return res

proc put(db: ContentDB, key, value: openArray[byte]) =
  db.kv.put(key, value).expectDb()

proc contains*(db: ContentDB, key: openArray[byte]): bool =
  db.kv.contains(key).expectDb()

proc del*(db: ContentDB, key: openArray[byte]) =
  db.kv.del(key).expectDb()

# TODO: Could also decide to use the ContentKey SSZ bytestring, as this is what
# gets send over the network in requests, but that would be a bigger key. Or the
# same hashing could be done on it here.
# However ContentId itself is already derived through different digests
# depending on the content type, and this ContentId typically needs to be
# checked with the Radius/distance of the node anyhow. So lets see how we end up
# using this mostly in the code.

proc get*(db: ContentDB, key: ContentId): Option[seq[byte]] =
  # TODO: Here it is unfortunate that ContentId is a uint256 instead of Digest256.
  db.get(key.toByteArrayBE())

proc put*(db: ContentDB, key: ContentId, value: openArray[byte]) =
  db.put(key.toByteArrayBE(), value)

proc contains*(db: ContentDB, key: ContentId): bool =
  db.contains(key.toByteArrayBE())

proc del*(db: ContentDB, key: ContentId) =
  db.del(key.toByteArrayBE())

proc deleteFractionOfContent*(
    db: ContentDB,
    target: Uint256,
    targetFraction: float64): (UInt256, int64, int64, int64) =
  ## Procedure which tries to delete fraction of database by scanning maxObjPerScan
  ## furthest elements.
  ## If the maxObjPerScan furthest elements, is not enough to attain required fraction
  ## procedure deletes all but one element and report how many bytes have been
  ## deleted
  ## Procedure do not call reclaim space, it is left to the caller.

  let (furthestElements, totalContentSize) = db.getNFurthestElements(target, maxObjPerScan)
  var bytesDeleted: int64 = 0
  let bytesToDelete = int64(targetFraction * float64(totalContentSize))
  let numOfElements = len(furthestElements)
  var numOfDeletedElements: int64 = 0

  if numOfElements == 0:
    # no elements in database, return some zero value
    return (UInt256.zero, 0'i64, 0'i64, 0'i64)

  let lastIdx = len(furthestElements) - 1

  for i, elem in furthestElements:
    if i == lastIdx:
      # this is our last element, do not delete it and report it as last non deleted
      # element
      return (elem.distFrom, bytesDeleted, totalContentSize, numOfDeletedElements)

    if bytesDeleted + elem.payloadLength < bytesToDelete:
      db.del(elem.contentId)
      bytesDeleted = bytesDeleted + elem.payloadLength
      inc numOfDeletedElements
    else:
      return (elem.distFrom, bytesDeleted, totalContentSize, numOfDeletedElements)

proc put*(
    db: ContentDB,
    key: ContentId,
    value: openArray[byte],
    target: UInt256): PutResult =

  db.put(key, value)
  
  # We use real size for our pruning threshold, which means that database file
  # will reach size specified in db.maxSize, and will stay that size thorough
  # node life time, as after content deletion free pages will be re used.
  # TODO:
  # 1. Devise vacuum strategy - after few pruning cycles database can become
  # fragmented which may impact performance, so at some point in time `VACUUM`
  # will need to be run to defragment the db.
  # 2. Deal with the edge case where a user configures max db size lower than
  # current db.size(). With such config the database would try to prune itself with
  # each addition.
  let dbSize = db.realSize()

  if dbSize < int64(db.maxSize):
    return PutResult(kind: ContentStored)
  else:
    # TODO Add some configuration for this magic number
    let (
      furthestNonDeletedElement,
      deletedBytes,
      totalContentSize,
      deletedElements
    ) =
      db.deleteFractionOfContent(target, 0.25)

    let deletedFraction = float64(deletedBytes) / float64(totalContentSize)

    return PutResult(
      kind: DbPruned,
      furthestStoredElementDistance: furthestNonDeletedElement,
      fractionOfDeletedContent: deletedFraction,
      numOfDeletedElements: deletedElements)