cup/faststack.nim

import bitops, strutils, random


proc show(i: SomeInteger, bitlength = 16) =
  echo BiggestInt(i).toBin(bitlength)


type
  Color = enum
    cRed, cGreen, cBlue, cYellow, cPurple

  ColorStack = object
    data: uint16
    len: uint8


const
  masks = [
    0'u16, # dummy value just to get the indices right
    0b0_000_000_000_000_111,
    0b0_000_000_000_111_111,
    0b0_000_000_111_111_111,
    0b0_000_111_111_111_111,
    0b0_111_111_111_111_111,
  ]


template offset(s: ColorStack, idx: Natural): uint8 =
  # Compute the bit offset for a given index. 
  # Dependent on the stack's length.
  (s.len - 1 - cast[uint8](idx)) * 3
  # items are stored from left to right but right-aligned, so we
  # need to shift right to access anything other than the last


template offset(s: ColorStack, idx: BackwardsIndex): uint8 =
  # for backwards index, we are still shifting right but
  # the lower the index the less we have to shift
  (cast[uint8](idx) - 1) * 3


proc add(s: var ColorStack, c: Color) =
  # e.g. if stack is 0b0000000000000100:
  # and color is 0b00000011
  # shift: 0b0000000000100000
  # bitor: 0b0000000000100000 and 0b00000011
  # results in 0b0000000000100011
  s.data = (s.data shl 3).bitor(cast[uint8](c))
  inc s.len


proc high(s: ColorStack): uint8 = s.len - 1

proc low(s: ColorStack): uint8 = 0 # just... always 0, I guess


proc `[]`(s: ColorStack, i: uint8 | BackwardsIndex): Color =
  # shift, then mask everything but the three rightmost bits
  result = cast[Color](
    (s.data shr s.offset(i)) and masks[1]
  )


proc `[]=`(s: var ColorStack, i: uint8 | BackwardsIndex, c: Color) =
  let offset = s.offset(i)
  s.data = (s.data and bitnot(masks[1] shl offset)) or (cast[uint16](c) shl offset)


iterator items(s: ColorStack): Color =
  # s.len is unsigned so it will wrap around if we do s.len - 1 in that case
  if s.len != 0:
    for i in countdown(s.len - 1, 0'u8):
      yield cast[Color]((s.data shr (i * 3)) and masks[1])


iterator pairs(s: ColorStack): (int, Color) =
  var count = 0
  for color in s:
    yield (count, color)
    inc count


proc find(s: ColorStack, needle: Color): uint8 =
  for i in 0'u8 .. s.high:
    if s[i] == needle:
      return i


proc moveSubstack(src, dst: var ColorStack, nToMove: uint8) =
  # Moves a sub-stack from the top of src to the top of dst
  # shift the dst stack by the length of the substack to make room
  let shift = nToMove * 3
  dst.data = dst.data shl shift
  # then we mask the source data to present only the items 
  # being moved, and OR that with the shifted dst data
  dst.data = dst.data or (src.data and masks[src.len])
  dst.len += nToMove
  # then we shift the source to get rid of the moved items
  src.data = src.data shr shift
  src.len -= nToMove


proc moveSubstackPre(src, dst: var ColorStack, nToMove: uint8) =
  # Moves a sub-stack from the top of src to the bottom of dst
  # shift src stack to position substack above its destination,
  # get rid of everything to the left of the substack,
  # and OR that with the existing dst data
  dst.data = dst.data or ( (src.data shl (dst.len * 3)) and masks[src.len])
  dst.len += nToMove
  # get rid of the substack we just moved
  src.data = src.data shr (nToMove * 3)
  src.len -= nToMove


proc swap(s: var ColorStack, i1, i2: uint8) =
  # Swap the values at two indices in the stack
  if i1 == i2: return

  # i1 and i2 are unsigned, so we have to watch out for underflows
  let diff = if i1 > i2:
    (i1 - i2) * 3
  else:
    (i2 - i1) * 3

  # take masks[1] from above (rightmost position) and shift to position of i1.
  # then do the same for i2, and OR them together.
  let mask = (masks[1] shl s.offset(i1)) or (masks[1] shl s.offset(i2))
  # get rid of everything but the two values we're swapping
  let masked = s.data and mask
  # shift by the distance between values in both directions, combine, then mask
  let swapped = ((masked shl diff) or (masked shr diff)) and mask
  # finally, AND with the inverse of mask so that only the values being 
  # swapped are erased, and combine that with the swapped values
  s.data = (s.data and mask.bitnot) or swapped


proc shuffle(r: var Rand, s: var ColorStack) =
  # Fisher-Yates shuffle
  for i in countdown(s.high, 1'u8):
    let j = cast[uint8](r.rand(i))
    if j != i:
      s.swap(i, j)


proc reverse(s: var ColorStack, first, last: uint8) =
  var x = first
  var y = last
  while x < y:
    s.swap(x, y)
    inc x
    dec y


proc asInt(s: ColorStack): uint16 = s.data


proc `$`(s: ColorStack): string =
  result = "St@["
  for c in s:
    if result[^1] != '[':
      result.add(", ")
    result.add($c)
  result.add("]")


var one: ColorStack
one.add(cRed)
one.add(cGreen)
one.add(cBlue)
one.add(cYellow)
one.add(cPurple)

var two: ColorStack
# two.add(cPurple)
two.add(cRed)
# two.add(cYellow)

echo "one: ", one
# echo "two: ", two

one.reverse(0, 4)
echo "one: ", one
# one[^2] = cPurple
# echo "one: ", one

# echo "one 0: ", one[0]
# echo "one 2: ", one[2]
# echo "one 3: ", one[3]

# echo "one ^1: ", one[^1]
# echo "one ^2: ", one[^2]
# echo "one ^4: ", one[^4]

# echo "<move>"
# moveSubstack(one, two, 2)
# echo "one: ", one
# echo "two: ", two