# SPDX-FileCopyrightText: 2021 ☭ Emery Hemingway
# SPDX-License-Identifier: Unlicense

## An unordered association of items to counts.
## An item count may be negative, unlike CountTable.

import tables

type
  ChangeDescription* = enum
    cdPresentToAbsent,
    cdAbsentToAbsent,
    cdAbsentToPresent,
    cdPresentToPresent

  Bag*[T] = Table[T, int]

proc change(count: var int; delta: int; clamp: bool): ChangeDescription =
  var
    oldCount = count
    newCount = oldCount + delta
  if clamp:
    newCount = max(0, newCount)
  if newCount == 0:
    result =
      if oldCount == 0: cdAbsentToAbsent
      else: cdPresentToAbsent
  else:
    result =
      if oldCount == 0: cdAbsentToPresent
      else: cdPresentToPresent
  count = newCount

proc change*[T](bag: var Bag[T]; key: T; delta: int; clamp = false): ChangeDescription =
  assert(delta != 0)
  result = change(bag.mGetOrPut(key, 0), delta, clamp)
  if result in {cdAbsentToAbsent, cdPresentToAbsent}:
    bag.del(key)