nim-eth/eth/p2p/discoveryv5/routing_table.nim

import
  std/[algorithm, times, sequtils, bitops, random, sets], stint, chronicles,
  types, node

type
  RoutingTable* = object
    thisNode: Node
    buckets: seq[KBucket]

  KBucket = ref object
    istart, iend: NodeId
    nodes: seq[Node]
    replacementCache: seq[Node]
    lastUpdated: float # epochTime

const
  BUCKET_SIZE* = 16
  BITS_PER_HOP = 8
  ID_SIZE = 256

proc distanceTo(n: Node, id: NodeId): UInt256 = n.id xor id
proc logDist*(a, b: NodeId): uint32 =
  let a = a.toBytes
  let b = b.toBytes
  var lz = 0
  for i in 0 ..< a.len:
    let x = a[i] xor b[i]
    if x == 0:
      result += 8
    else:
      result += bitops.countLeadingZeroBits(x).uint8
  uint32(a.len * 8 - lz)

proc newKBucket(istart, iend: NodeId): KBucket =
  result.new()
  result.istart = istart
  result.iend = iend
  result.nodes = @[]
  result.replacementCache = @[]

proc midpoint(k: KBucket): NodeId =
  k.istart + (k.iend - k.istart) div 2.u256

proc distanceTo(k: KBucket, id: NodeId): UInt256 = k.midpoint xor id
proc nodesByDistanceTo(k: KBucket, id: NodeId): seq[Node] =
  sortedByIt(k.nodes, it.distanceTo(id))

proc len(k: KBucket): int {.inline.} = k.nodes.len
proc head(k: KBucket): Node {.inline.} = k.nodes[0]

proc add(k: KBucket, n: Node): Node =
  ## Try to add the given node to this bucket.

  ## If the node is already present, it is moved to the tail of the list, and we return nil.

  ## If the node is not already present and the bucket has fewer than k entries, it is inserted
  ## at the tail of the list, and we return nil.

  ## If the bucket is full, we add the node to the bucket's replacement cache and return the
  ## node at the head of the list (i.e. the least recently seen), which should be evicted if it
  ## fails to respond to a ping.
  k.lastUpdated = epochTime()
  let nodeIdx = k.nodes.find(n)
  if nodeIdx != -1:
      k.nodes.delete(nodeIdx)
      k.nodes.add(n)
  elif k.len < BUCKET_SIZE:
      k.nodes.add(n)
  else:
      k.replacementCache.add(n)
      return k.head
  return nil

proc removeNode(k: KBucket, n: Node) =
  let i = k.nodes.find(n)
  if i != -1: k.nodes.delete(i)

proc split(k: KBucket): tuple[lower, upper: KBucket] =
  ## Split at the median id
  let splitid = k.midpoint
  result.lower = newKBucket(k.istart, splitid)
  result.upper = newKBucket(splitid + 1.u256, k.iend)
  for node in k.nodes:
    let bucket = if node.id <= splitid: result.lower else: result.upper
    discard bucket.add(node)
  for node in k.replacementCache:
    let bucket = if node.id <= splitid: result.lower else: result.upper
    bucket.replacementCache.add(node)

proc inRange(k: KBucket, n: Node): bool {.inline.} =
  k.istart <= n.id and n.id <= k.iend

proc isFull(k: KBucket): bool = k.len == BUCKET_SIZE

proc contains(k: KBucket, n: Node): bool = n in k.nodes

proc binaryGetBucketForNode(buckets: openarray[KBucket],
                            id: NodeId): KBucket {.inline.} =
  ## Given a list of ordered buckets, returns the bucket for a given node.
  let bucketPos = lowerBound(buckets, id) do(a: KBucket, b: NodeId) -> int:
    cmp(a.iend, b)
  # Prevents edge cases where bisect_left returns an out of range index
  if bucketPos < buckets.len:
    let bucket = buckets[bucketPos]
    if bucket.istart <= id and id <= bucket.iend:
      result = bucket

  if result.isNil:
    raise newException(ValueError, "No bucket found for node with id " & $id)

proc computeSharedPrefixBits(nodes: openarray[Node]): int =
  ## Count the number of prefix bits shared by all nodes.
  if nodes.len < 2:
    return ID_SIZE

  var mask = zero(UInt256)
  let one = one(UInt256)

  for i in 1 .. ID_SIZE:
    mask = mask or (one shl (ID_SIZE - i))
    let reference = nodes[0].id and mask
    for j in 1 .. nodes.high:
      if (nodes[j].id and mask) != reference: return i - 1

  for n in nodes:
    echo n.id.toHex()

  doAssert(false, "Unable to calculate number of shared prefix bits")

proc init*(r: var RoutingTable, thisNode: Node) {.inline.} =
  r.thisNode = thisNode
  r.buckets = @[newKBucket(0.u256, high(Uint256))]

proc splitBucket(r: var RoutingTable, index: int) =
  let bucket = r.buckets[index]
  let (a, b) = bucket.split()
  r.buckets[index] = a
  r.buckets.insert(b, index + 1)

proc bucketForNode(r: RoutingTable, id: NodeId): KBucket =
  binaryGetBucketForNode(r.buckets, id)

proc removeNode*(r: var RoutingTable, n: Node) =
  r.bucketForNode(n.id).removeNode(n)

proc addNode*(r: var RoutingTable, n: Node): Node =
  if n == r.thisNode:
    # warn "Trying to add ourselves to the routing table", node = n
    return
  let bucket = r.bucketForNode(n.id)
  let evictionCandidate = bucket.add(n)
  if not evictionCandidate.isNil:
    # Split if the bucket has the local node in its range or if the depth is not congruent
    # to 0 mod BITS_PER_HOP

    let depth = computeSharedPrefixBits(bucket.nodes)
    if bucket.inRange(r.thisNode) or (depth mod BITS_PER_HOP != 0 and depth != ID_SIZE):
      r.splitBucket(r.buckets.find(bucket))
      return r.addNode(n) # retry

    # Nothing added, ping evictionCandidate
    return evictionCandidate

proc getNode*(r: RoutingTable, id: NodeId): Node =
  let b = r.bucketForNode(id)
  for n in b.nodes:
    if n.id == id:
      return n

proc contains*(r: RoutingTable, n: Node): bool = n in r.bucketForNode(n.id)

proc bucketsByDistanceTo(r: RoutingTable, id: NodeId): seq[KBucket] =
  sortedByIt(r.buckets, it.distanceTo(id))

proc notFullBuckets(r: RoutingTable): seq[KBucket] =
  r.buckets.filterIt(not it.isFull)

proc neighbours*(r: RoutingTable, id: NodeId, k: int = BUCKET_SIZE): seq[Node] =
  ## Return up to k neighbours of the given node.
  result = newSeqOfCap[Node](k * 2)
  for bucket in r.bucketsByDistanceTo(id):
    for n in bucket.nodesByDistanceTo(id):
      if n.id != id:
        result.add(n)
        if result.len == k * 2:
          break
  result = sortedByIt(result, it.distanceTo(id))
  if result.len > k:
    result.setLen(k)

proc idAtDistance(id: NodeId, dist: uint32): NodeId =
  id and (Uint256.high shl dist.int)

proc neighboursAtDistance*(r: RoutingTable, distance: uint32, k: int = BUCKET_SIZE): seq[Node] =
  r.neighbours(idAtDistance(r.thisNode.id, distance), k)

proc len*(r: RoutingTable): int =
  for b in r.buckets: result += b.len

proc moveRight[T](arr: var openarray[T], a, b: int) {.inline.} =
  ## In `arr` move elements in range [a, b] right by 1.
  var t: T
  shallowCopy(t, arr[b + 1])
  for i in countdown(b, a):
    shallowCopy(arr[i + 1], arr[i])
  shallowCopy(arr[a], t)

proc setJustSeen*(r: RoutingTable, n: Node) =
  # Move `n` to front of its bucket
  let b = r.bucketForNode(n.id)
  let idx = b.nodes.find(n)
  doAssert(idx >= 0)
  if idx != 0:
    b.nodes.moveRight(0, idx - 1)
  b.nodes[0] = n
  b.lastUpdated = epochTime()

proc nodeToRevalidate*(r: RoutingTable): Node =
  var buckets = r.buckets
  shuffle(buckets)
  # TODO: Should we prioritize less-recently-updated buckets instead?
  for b in buckets:
    if b.len > 0:
      return b.nodes[^1]

proc randomNodes*(r: RoutingTable, count: int): seq[Node] =
  var count = count
  let sz = r.len
  if count > sz:
    debug  "Looking for peers", requested = count, present = sz
    count = sz

  result = newSeqOfCap[Node](count)
  var seen = initHashSet[Node]()

  # This is a rather inneficient way of randomizing nodes from all buckets, but even if we
  # iterate over all nodes in the routing table, the time it takes would still be
  # insignificant compared to the time it takes for the network roundtrips when connecting
  # to nodes.
  while len(seen) < count:
    let bucket = sample(r.buckets)
    if bucket.nodes.len != 0:
      let node = sample(bucket.nodes)
      if node notin seen:
        result.add(node)
        seen.incl(node)