From 8d889170dfeaf3029bb4ea4073b3b071e9151736 Mon Sep 17 00:00:00 2001
From: Joseph Montanaro <jfmonty2@gmail.com>
Date: Fri, 16 Jul 2021 22:43:45 -0700
Subject: [PATCH] more procs for working on faststacks

---
 faststack.nim | 53 ++++++++++++++++++++++++++++++++++++++-------------
 1 file changed, 40 insertions(+), 13 deletions(-)

diff --git a/faststack.nim b/faststack.nim
index a5949fc..e6d5c63 100644
--- a/faststack.nim
+++ b/faststack.nim
@@ -1,4 +1,4 @@
-import bitops, strutils
+import bitops, strutils, random
 
 
 proc show(i: SomeInteger, bitlength = 16) =
@@ -67,8 +67,7 @@ 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
-  # so we will no-op in that case
+  # 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])
@@ -81,6 +80,12 @@ iterator pairs(s: ColorStack): (int, 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
@@ -109,22 +114,44 @@ proc moveSubstackPre(src, dst: var ColorStack, nToMove: uint8) =
 
 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
 
+  # 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))
-  mask.show
+  # get rid of everything but the two values we're swapping
   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
+  # 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 `$`(s: ColorStack): string =
@@ -137,11 +164,11 @@ proc `$`(s: ColorStack): string =
 
 
 var one: ColorStack
+one.add(cRed)
 one.add(cGreen)
 one.add(cBlue)
 one.add(cYellow)
 one.add(cPurple)
-one.add(cRed)
 
 var two: ColorStack
 # two.add(cPurple)
@@ -149,9 +176,9 @@ two.add(cRed)
 # two.add(cYellow)
 
 echo "one: ", one
-echo "two: ", two
+# echo "two: ", two
 
-one.swap(0, 4)
+one.reverse(0, 4)
 echo "one: ", one
 # one[^2] = cPurple
 # echo "one: ", one