# Nim-LibP2P
# Copyright (c) 2023 Status Research & Development GmbH
# Licensed under either of
#  * Apache License, version 2.0, ([LICENSE-APACHE](LICENSE-APACHE))
#  * MIT license ([LICENSE-MIT](LICENSE-MIT))
# at your option.
# This file may not be copied, modified, or distributed except according to
# those terms.

{.push raises: [].}

import std/[hashes, sets]
import chronos/timer, stew/results

import ../../utility

export results

const Timeout* = 10.seconds # default timeout in ms

type
  TimedEntry*[K] = ref object of RootObj
    key: K
    addedAt: Moment
    expiresAt: Moment
    next, prev: TimedEntry[K]

  TimedCache*[K] = object of RootObj
    head, tail: TimedEntry[K] # nim linked list doesn't allow inserting at pos
    entries: HashSet[TimedEntry[K]]
    timeout: Duration
    maxSize: int # Optional max size of the cache, 0 means unlimited

func `==`*[E](a, b: TimedEntry[E]): bool =
  if isNil(a) == isNil(b):
    isNil(a) or a.key == b.key
  else:
    false

func hash*(a: TimedEntry): Hash =
  if isNil(a):
    default(Hash)
  else:
    hash(a[].key)

func expire*(t: var TimedCache, now: Moment = Moment.now()) =
  while t.head != nil and t.head.expiresAt < now:
    t.entries.excl(t.head)
    t.head.prev = nil
    t.head = t.head.next
    if t.head == nil:
      t.tail = nil

func del*[K](t: var TimedCache[K], key: K): Opt[TimedEntry[K]] =
  # Removes existing key from cache, returning the previous value if present
  let tmp = TimedEntry[K](key: key)
  if tmp in t.entries:
    let item =
      try:
        t.entries[tmp] # use the shared instance in the set
      except KeyError:
        raiseAssert "just checked"
    t.entries.excl(item)

    if t.head == item:
      t.head = item.next
    if t.tail == item:
      t.tail = item.prev

    if item.next != nil:
      item.next.prev = item.prev
    if item.prev != nil:
      item.prev.next = item.next
    Opt.some(item)
  else:
    Opt.none(TimedEntry[K])

func put*[K](t: var TimedCache[K], k: K, now = Moment.now()): bool =
  # Puts k in cache, returning true if the item was already present and false
  # otherwise. If the item was already present, its expiry timer will be
  # refreshed.
  func ensureSizeBound(t: var TimedCache[K]) =
    if t.maxSize > 0 and t.entries.len() >= t.maxSize and k notin t:
      if t.head != nil:
        t.entries.excl(t.head)
        t.head = t.head.next
        if t.head != nil:
          t.head.prev = nil
        else:
          t.tail = nil

  t.expire(now)
  t.ensureSizeBound()

  let
    previous = t.del(k) # Refresh existing item
    addedAt =
      if previous.isSome():
        previous[].addedAt
      else:
        now

  let node = TimedEntry[K](key: k, addedAt: addedAt, expiresAt: now + t.timeout)
  if t.head == nil:
    t.tail = node
    t.head = t.tail
  else:
    # search from tail because typically that's where we add when now grows
    var cur = t.tail
    while cur != nil and node.expiresAt < cur.expiresAt:
      cur = cur.prev

    if cur == nil:
      node.next = t.head
      t.head.prev = node
      t.head = node
    else:
      node.prev = cur
      node.next = cur.next
      cur.next = node
      if cur == t.tail:
        t.tail = node

  t.entries.incl(node)

  previous.isSome()

func contains*[K](t: TimedCache[K], k: K): bool =
  let tmp = TimedEntry[K](key: k)
  tmp in t.entries

func addedAt*[K](t: var TimedCache[K], k: K): Moment =
  let tmp = TimedEntry[K](key: k)
  try:
    if tmp in t.entries: # raising is slow
      # Use shared instance from entries
      return t.entries[tmp][].addedAt
  except KeyError:
    raiseAssert "just checked"

  default(Moment)

func init*[K](T: type TimedCache[K], timeout: Duration = Timeout, maxSize: int = 0): T =
  T(timeout: timeout, maxSize: maxSize)