# Copyright (c) 2021 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.

import
  std/[options, math, sugar]

export options

# Buffer implementation similar to the one in in reference implementation.
# Main rationale for it, is to refer to items in buffer by their sequence number,
# and support out of order packets.
# Therefore it is super specific data structure, and it mostly usefull for
# utp implementation.
# Another alternative would be to use standard deque from deques module, and caluclate
# item indexes from their sequence numbers.
type
  GrowableCircularBuffer*[A] = object
    items: seq[Option[A]]
    mask: uint32

# provided size will always be adjusted to next power of two
proc init*[A](T: type GrowableCircularBuffer[A], size: uint32 = 16): T =
  let powOfTwoSize = nextPowerOfTwo(int(size))
  T(
    items: newSeq[Option[A]](powOfTwoSize),
    mask: uint32(powOfTwoSize - 1)
  )

proc get*[A](buff: GrowableCircularBuffer[A], i: uint32): Option[A] =
  buff.items[i and buff.mask]

proc putImpl[A](buff: var GrowableCircularBuffer[A], i: uint32, elem: Option[A]) =
  buff.items[i and buff.mask] = elem

proc put*[A](buff: var GrowableCircularBuffer[A], i: uint32, elem: A) =
  buff.putImpl(i, some(elem))

proc delete*[A](buff: var GrowableCircularBuffer[A], i: uint32) =
  buff.putImpl(i, none[A]())

proc hasKey*[A](buff: GrowableCircularBuffer[A], i: uint32): bool =
  buff.get(i).isSome()

proc exists*[A](buff: GrowableCircularBuffer[A], i: uint32, check: proc (x: A): bool): bool =
  let maybeElem = buff.get(i)
  if (maybeElem.isSome()):
    let elem = maybeElem.unsafeGet()
    check(elem)
  else:
    false

proc `[]`*[A](buff: var GrowableCircularBuffer[A], i: uint32): var A =
  ## Returns contents of the `var GrowableCircularBuffer`. If it is not set, then an exception
  ## is thrown.
  buff.items[i and buff.mask].get()

proc len*[A](buff: GrowableCircularBuffer[A]): int =
  int(buff.mask) + 1

proc calculateNextMask(currentMask: uint32, index:uint32): uint32 =
  # Increase mask,so that index will fit in buffer size i.e mask + 1
  if currentMask == uint32.high:
    return currentMask
  
  var newSize = currentMask + 1 
  while true:
    newSize = newSize * 2
    if newSize == 0 or index < newSize:
      break
  return newSize - 1

# Item contains the element we want to make space for
# index is the index in the list.
proc ensureSize*[A](buff: var GrowableCircularBuffer[A], item: uint32, index: uint32) =
  if (index > buff.mask):
    let newMask = calculateNextMask(buff.mask, index)
    var newSeq = newSeq[Option[A]](int(newMask) + 1)
    var i = 0'u32
    while i <= buff.mask:
      let idx = item - index + i
      newSeq[idx and newMask] = buff.get(idx)
      inc i
    buff.items = move(newSeq)
    buff.mask = newMask

iterator items*[A](buff: GrowableCircularBuffer[A]): Option[A] =
  for e in buff.items:
    yield e