Day 21 - decompiled.

README.md

@@ -32,7 +32,7 @@ Now located in this repository's wiki.
 | 18  | ~ 6         |
 | 19  | ~ 3         |
 | 20  | ~ 40        |
-| 21  |             |
+| 21  | ~ 1         |
 | 22  |             |
 | 23  |             |
 | 24  |             |

app/Main.hs

@@ -30,4 +30,4 @@ import qualified Day24
 import qualified Day25
 
 main :: IO ()
-main = Day20.main
+main = Day21.main

input/21.txt

@@ -0,0 +1,32 @@
+#ip 1
+seti 123 0 3
+bani 3 456 3
+eqri 3 72 3
+addr 3 1 1
+seti 0 0 1
+seti 0 9 3
+bori 3 65536 5
+seti 15028787 4 3
+bani 5 255 2
+addr 3 2 3
+bani 3 16777215 3
+muli 3 65899 3
+bani 3 16777215 3
+gtir 256 5 2
+addr 2 1 1
+addi 1 1 1
+seti 27 3 1
+seti 0 9 2
+addi 2 1 4
+muli 4 256 4
+gtrr 4 5 4
+addr 4 1 1
+addi 1 1 1
+seti 25 1 1
+addi 2 1 2
+seti 17 8 1
+setr 2 4 5
+seti 7 3 1
+eqrr 3 0 2
+addr 2 1 1
+seti 5 3 1

input/21_notes.txt

@@ -0,0 +1,36 @@
+#ip 1
+
+0  1 2 3 4 5 6
+a ip c d e f g
+
+ 0  seti 123 0 3            d  = 123
+ 1  bani 3 456 3        L1: d  = 123 & 456
+ 2  eqri 3 72 3             if (123 & 456 != 72)
+ 3  addr 3 1 1              then
+ 4  seti 0 0 1              goto L1                 int f; int d = 0;
+ 5  seti 0 9 3              else d = 0              while (true) {
+ 6  bori 3 65536 5      L0: f  = d | 65536              f = d | 65536;
+ 7  seti 15028787 4 3       d  = 15028787               d = 15028787;
+ 8  bani 5 255 2        L8: c  = f & 255
+ 9  addr 3 2 3              d += c
+10  bani 3 16777215 3       d &= 16777215
+11  muli 3 65899 3          d *= 65899
+12  bani 3 16777215 3       d &= 16777215               d = (d + f & 255) & 16777215 * 65899 & 16777215;
+13  gtir 256 5 2            if (256 > f)                while (f >= 256) {
+14  addr 2 1 1              then goto L2                
+15  addi 1 1 1              else goto L3
+16  seti 27 3 1         L2: goto L4
+17  seti 0 9 2          L3: c  = 0
+18  addi 2 1 4          L9: e  = c + 1  
+19  muli 4 256 4            e *= 256
+20  gtrr 4 5 4              if (e > f)
+21  addr 4 1 1              then goto L5
+22  addi 1 1 1              else goto L6
+23  seti 25 1 1         L5: goto L7
+24  addi 2 1 2          L6: c += 1
+25  seti 17 8 1             goto L9
+26  setr 2 4 5          L7: f = c                           f /= 256;
+27  seti 7 3 1              goto L8                         d = (d + f & 255) & 16777215 * 65899 & 16777215;
+28  eqrr 3 0 2          L4: if (a == d)                 }
+29  addr 2 1 1              then return                 if (a == d) return;
+30  seti 5 3 1              goto L0                 }

src/Day21.hs

@@ -1,6 +1,30 @@
 module Day21 (main) where
 
+import Data.Bits ((.|.), (.&.), shiftR)
+import Data.Set (empty, member, insert)
+
+loop :: Int -> [Int]
+loop d =
+    let d' = innerLoop (d .|. 0x10000) 15028787
+    in  d':(loop d')
+    where
+        innerLoop f d =
+            let d' = math f d
+                f' = shiftR f 8
+            in  if f < 0x100 then d' else innerLoop f' d'
+        math f d = (d + f .&. 0xFF) .&. 0xFFFFFF * 65899 .&. 0xFFFFFF
+
+part1 :: Int
+part1 = head $ loop 0
+
+part2 :: Int
+part2 =
+    let vals = loop 0
+    in  prevRepeat (head vals) (tail vals) empty
+    where
+        prevRepeat p (v:vals) set = if member v set then p else prevRepeat v vals (insert v set)
+
 main :: IO ()
 main = do
-    input <- readFile "input/21.txt"
+    print $ part1
-    print input
+    print $ part2