From cd81150e6cbed07960e880214cadad7f8105ce74 Mon Sep 17 00:00:00 2001
From: Joseph Montanaro <jfmonty2@gmail.com>
Date: Thu, 15 Jul 2021 21:27:04 -0700
Subject: [PATCH] bitwise swap

---
 faststack.nim | 30 ++++++++++++++++++++++++++----
 1 file changed, 26 insertions(+), 4 deletions(-)

diff --git a/faststack.nim b/faststack.nim
index e2c7b38..a5949fc 100644
--- a/faststack.nim
+++ b/faststack.nim
@@ -68,7 +68,7 @@ proc `[]=`(s: var ColorStack, i: uint8 | BackwardsIndex, c: Color) =
 
 iterator items(s: ColorStack): Color =
   # 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:
     for i in countdown(s.len - 1, 0'u8):
       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
 
 
+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 =
   result = "St@["
   for c in s:
@@ -120,8 +140,8 @@ var one: ColorStack
 one.add(cGreen)
 one.add(cBlue)
 one.add(cYellow)
-# one.add(cPurple)
-# one.add(cRed)
+one.add(cPurple)
+one.add(cRed)
 
 var two: ColorStack
 # two.add(cPurple)
@@ -131,8 +151,10 @@ two.add(cRed)
 echo "one: ", one
 echo "two: ", two
 
-one[^2] = cPurple
+one.swap(0, 4)
 echo "one: ", one
+# one[^2] = cPurple
+# echo "one: ", one
 
 # echo "one 0: ", one[0]
 # echo "one 2: ", one[2]