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bitwise swap

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This commit is contained in:
Joseph Montanaro 2021-07-15 21:27:04 -07:00
parent 2165511624
commit cd81150e6c
1 changed files with 26 additions and 4 deletions
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30
faststack.nim
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@ -68,7 +68,7 @@ proc `[]=`(s: var ColorStack, i: uint8 | BackwardsIndex, c: Color) =
iterator items(s: ColorStack): Color = iterator items(s: ColorStack): Color =
# s.len is unsigned so it will wrap around if it's 0 # s.len is unsigned so it will wrap around if it's 0
# but that's no problem, we can just no-op if len is 0 # so we will no-op in that case
if s.len != 0: if s.len != 0:
for i in countdown(s.len - 1, 0'u8): for i in countdown(s.len - 1, 0'u8):
yield cast[Color]((s.data shr (i * 3)) and masks[1]) yield cast[Color]((s.data shr (i * 3)) and masks[1])
@ -107,6 +107,26 @@ proc moveSubstackPre(src, dst: var ColorStack, nToMove: uint8) =
src.len -= nToMove src.len -= nToMove
proc swap(s: var ColorStack, i1, i2: uint8) =
# Swap the values at two indices in the stack
# further explanation to follow
if i1 == i2: return
let diff = if i1 > i2:
(i1 - i2) * 3
else:
(i2 - i1) * 3
let mask = (masks[1] shl s.offset(i1)) or (masks[1] shl s.offset(i2))
mask.show
let masked = s.data and mask
masked.show
let swapped = ((masked shl diff) and mask) or ((masked shr diff) and mask)
swapped.show
s.data = (s.data and bitnot(mask)) or swapped
s.data.show
proc `$`(s: ColorStack): string = proc `$`(s: ColorStack): string =
result = "St@[" result = "St@["
for c in s: for c in s:
@ -120,8 +140,8 @@ var one: ColorStack
one.add(cGreen) one.add(cGreen)
one.add(cBlue) one.add(cBlue)
one.add(cYellow) one.add(cYellow)
# one.add(cPurple) one.add(cPurple)
# one.add(cRed) one.add(cRed)
var two: ColorStack var two: ColorStack
# two.add(cPurple) # two.add(cPurple)
@ -131,8 +151,10 @@ two.add(cRed)
echo "one: ", one echo "one: ", one
echo "two: ", two echo "two: ", two
one[^2] = cPurple one.swap(0, 4)
echo "one: ", one echo "one: ", one
# one[^2] = cPurple
# echo "one: ", one
# echo "one 0: ", one[0] # echo "one 0: ", one[0]
# echo "one 2: ", one[2] # echo "one 2: ", one[2]