# nimbus-eth1
# Copyright (c) 2021 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.

## Aristo DB -- Patricia Trie Merkleisation
## ========================================
##
## For the current state of the `Patricia Trie`, keys (equivalent to hashes)
## are associated with the vertex IDs. Existing key associations are checked
## (i.e. recalculated and compared) unless the ID is locked. In the latter
## case, the key is assumed to be correct without checking.
##
## The association algorithm is an optimised version of:
##
## * For all leaf vertices, label them with parent vertex so that there are
##   chains from the leafs to the root vertex.
##
## * Apply a width-first traversal starting with the set of leafs vertices
##   compiling the keys to associate with by hashing the current vertex.
##
##   Apperently, keys (aka hashes) can be compiled for leaf vertices. For the
##   other vertices, the keys can be compiled if all the children keys are
##   known which is assured by the nature of the width-first traversal method.
##
## For production, this algorithm is slightly optimised:
##
## * For each leaf vertex, calculate the chain from the leaf to the root vertex.
##   + Starting at the leaf, calculate the key for each vertex towards the root
##     vertex as long as possible.
##   + Stash the rest of the partial chain to be completed later
##
## * While there is a partial chain left, use the ends towards the leaf
##   vertices and calculate the remaining keys (which results in a width-first
##   traversal, again.)

{.push raises: [].}

import
  std/[algorithm, sequtils, sets, strutils, tables],
  chronicles,
  eth/common,
  stew/[interval_set, results],
  "."/[aristo_desc, aristo_get, aristo_hike, aristo_transcode, aristo_utils,
       aristo_vid]

type
  BackVidValRef = ref object
    root: VertexID                      ## Root vertex
    onBe: bool                          ## Table key vid refers to backend
    toVid: VertexID                     ## Next/follow up vertex

  BackVidTab =
    Table[VertexID,BackVidValRef]

logScope:
  topics = "aristo-hashify"

# ------------------------------------------------------------------------------
# Private helpers
# ------------------------------------------------------------------------------

template logTxt(info: static[string]): static[string] =
  "Hashify " & info

func getOrVoid(tab: BackVidTab; vid: VertexID): BackVidValRef =
  tab.getOrDefault(vid, BackVidValRef(nil))

func isValid(brv: BackVidValRef): bool =
  brv != BackVidValRef(nil)

# ------------------------------------------------------------------------------
# Private functions
# ------------------------------------------------------------------------------

proc updateHashKey(
    db: AristoDbRef;                   # Database, top layer
    root: VertexID;                    # Root ID
    vid: VertexID;                     # Vertex ID to check for
    expected: HashKey;                 # Hash key for vertex address by `vid`
    backend: bool;                     # Set `true` id vertex is on backend
      ): Result[void,AristoError] =
  ## Update the argument hash key `expected` for the vertex addressed by `vid`.
  ##
  # If the Merkle hash has been cached locally, already it must match.
  block:
    let key = db.top.kMap.getOrVoid(vid).key
    if key.isValid:
      if key != expected:
        let error = HashifyExistingHashMismatch
        debug logTxt "hash update failed", vid, key, expected, error
        return err(error)
      return ok()

  # If the vertex had been cached locally, there would be no locally cached
  # Merkle hash key. It will be created at the bottom end of the function.
  #
  # So there remains tha case when vertex is available on the backend only.
  # The Merkle hash not cached locally. It might be overloaded (and eventually
  # overwitten.)
  if backend:
    # Ok, vertex is on the backend.
    let rc = db.getKeyBackend vid
    if rc.isOk:
      let key = rc.value
      if key == expected:
        return ok()

      # This step is a error in the sense that something the on the backend
      # is fishy. There should not be contradicting Merkle hashes. Throwing
      # an error heres would lead to a deadlock so we correct it.
      debug "correcting backend hash key mismatch", vid, key, expected
      # Proceed `vidAttach()`, below

    elif rc.error != GetKeyNotFound:
      debug logTxt "backend key fetch failed", vid, expected, error=rc.error
      return err(rc.error)

    else:
      discard
      # Proceed `vidAttach()`, below

  # Othwise there is no Merkle hash, so create one with the `expected` key
  db.vidAttach(HashLabel(root: root, key: expected), vid)
  ok()


proc leafToRootHasher(
    db: AristoDbRef;                   # Database, top layer
    hike: Hike;                        # Hike for labelling leaf..root
      ): Result[int,(VertexID,AristoError)] =
  ## Returns the index of the first node that could not be hashed
  for n in (hike.legs.len-1).countDown(0):
    let
      wp = hike.legs[n].wp
      bg = hike.legs[n].backend
      rc = wp.vtx.toNode db
    if rc.isErr:
      return ok n

    # Vertices marked proof nodes need not be checked
    if wp.vid in db.top.pPrf:
      continue

    # Check against existing key, or store new key
    let
      key = rc.value.to(HashKey)
      rx = db.updateHashKey(hike.root, wp.vid, key, bg)
    if rx.isErr:
      return err((wp.vid,rx.error))

  ok -1 # all could be hashed

# ------------------

proc deletedLeafHasher(
    db: AristoDbRef;                   # Database, top layer
    hike: Hike;                        # Hike for labelling leaf..root
      ): Result[void,(VertexID,AristoError)] =
  var
    todo = hike.legs.reversed.mapIt(it.wp)
    solved: HashSet[VertexID]
  # Edge case for empty `hike`
  if todo.len == 0:
    let vtx = db.getVtx hike.root
    if not vtx.isValid:
      return err((hike.root,HashifyVtxMissing))
    todo = @[VidVtxPair(vid: hike.root, vtx: vtx)]
  while 0 < todo.len:
    var
      delayed: seq[VidVtxPair]
      didHere: HashSet[VertexID] # avoid duplicates
    for wp in todo:
      let rc = wp.vtx.toNode(db, stopEarly=false)
      if rc.isOk:
        let
          expected = rc.value.to(HashKey)
          key = db.getKey wp.vid
        if key.isValid:
          if key != expected:
            return err((wp.vid,HashifyExistingHashMismatch))
        else:
          db.vidAttach(HashLabel(root: hike.root, key: expected), wp.vid)
        solved.incl wp.vid
      else:
        # Resolve follow up vertices first
        for vid in rc.error:
          let vtx = db.getVtx vid
          if not vtx.isValid:
            return err((vid,HashifyVtxMissing))
          if vid in solved:
            discard wp.vtx.toNode(db, stopEarly=false)
            return err((vid,HashifyVidCircularDependence))
          if vid notin didHere:
            didHere.incl vid
            delayed.add VidVtxPair(vid: vid, vtx: vtx)

        # Followed by this vertex which relies on the ones registered above.
        if wp.vid notin didHere:
          didHere.incl wp.vid
          delayed.add wp

    todo = delayed

  ok()

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

proc hashifyClear*(
    db: AristoDbRef;                   # Database, top layer
    locksOnly = false;                 # If `true`, then clear only proof locks
      ) =
  ## Clear all `Merkle` hashes from the  `db` argument database top layer.
  if not locksOnly:
    db.top.pAmk.clear
    db.top.kMap.clear
  db.top.pPrf.clear


proc hashify*(
    db: AristoDbRef;                   # Database, top layer
      ): Result[HashSet[VertexID],(VertexID,AristoError)] =
  ## Add keys to the  `Patricia Trie` so that it becomes a `Merkle Patricia
  ## Tree`. If successful, the function returns the keys (aka Merkle hash) of
  ## the root vertices.
  var
    roots: HashSet[VertexID]
    completed: HashSet[VertexID]

    # Width-first leaf-to-root traversal structure
    backLink: BackVidTab
    downMost: BackVidTab

  for (lky,vid) in db.top.lTab.pairs:
    let hike = lky.hikeUp(db)

    # There might be deleted entries on the leaf table. If this is tha case,
    # the Merkle hashes for the vertices in the `hike` can all be compiled.
    if not vid.isValid:
      let rc = db.deletedLeafHasher hike
      if rc.isErr:
        return err(rc.error)

    elif hike.error != AristoError(0):
      return err((vid,hike.error))

    else:
      # Hash as much of the `hike` as possible
      let n = block:
        let rc = db.leafToRootHasher hike
        if rc.isErr:
          return err(rc.error)
        rc.value

      roots.incl hike.root

      if 0 < n:
        # Backtrack and register remaining nodes. Note that in case *n == 0*,
        # the root vertex has not been fully resolved yet.
        #
        # hike.legs: (leg[0], leg[1], .., leg[n-1], leg[n], ..)
        #               |       |           |          |
        #               | <---- |     <---- |   <----  |
        #               |                   |          |
        #               |     backLink[]    | downMost |
        #
        downMost[hike.legs[n].wp.vid] = BackVidValRef(
          root:  hike.root,
          onBe:  hike.legs[n].backend,
          toVid: hike.legs[n-1].wp.vid)
        for u in (n-1).countDown(1):
          backLink[hike.legs[u].wp.vid] = BackVidValRef(
            root:  hike.root,
            onBe:  hike.legs[u].backend,
            toVid: hike.legs[u-1].wp.vid)
      elif n < 0:
        completed.incl hike.root

  # At least one full path leaf..root should have succeeded with labelling
  # for each root.
  if completed.len < roots.len:
    return err((VertexID(0),HashifyLeafToRootAllFailed))

  # Update remaining hashes
  while 0 < downMost.len:
    var
      redo: BackVidTab
      done: HashSet[VertexID]

    for (vid,val) in downMost.pairs:
      # Try to convert vertex to a node. This is possible only if all link
      # references have Merkle hashes.
      #
      # Also `db.getVtx(vid)` => not nil as it was fetched earlier, already
      let rc = db.getVtx(vid).toNode db
      if rc.isErr:
        # Cannot complete with this vertex, so do it later
        redo[vid] = val

      else:
        # Update Merkle hash
        let
          key = rc.value.to(HashKey)
          rx = db.updateHashKey(val.root, vid, key, val.onBe)
        if rx.isErr:
          return err((vid,rx.error))

        done.incl vid

        # Proceed with back link
        let nextItem = backLink.getOrVoid val.toVid
        if nextItem.isValid:
          redo[val.toVid] = nextItem

    # Make sure that the algorithm proceeds
    if done.len == 0:
      let error = HashifyCannotComplete
      return err((VertexID(0),error))

    # Clean up dups from `backLink` and restart `downMost`
    for vid in done.items:
      backLink.del vid
    downMost = redo

  ok completed

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