From c81bc2c5d8e81c3d27914c9fba9b4f60de676d1c Mon Sep 17 00:00:00 2001
From: Bruno BELANYI <bruno@belanyi.fr>
Date: Tue, 8 Dec 2020 08:24:09 +0100
Subject: [PATCH] 2020: d08: ex2: add solution

---
 2020/d08/ex2/ex2.py | 62 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 62 insertions(+)
 create mode 100755 2020/d08/ex2/ex2.py

diff --git a/2020/d08/ex2/ex2.py b/2020/d08/ex2/ex2.py
new file mode 100755
index 0000000..bba650d
--- /dev/null
+++ b/2020/d08/ex2/ex2.py
@@ -0,0 +1,62 @@
+#!/usr/bin/env python
+
+
+import sys
+from copy import deepcopy
+from typing import List, Tuple
+
+
+def run(code: List[Tuple[str, int]]) -> Tuple[int, bool]:
+    accumulator = 0
+    rip = 0
+
+    def acc(val: int) -> None:
+        nonlocal accumulator
+        nonlocal rip
+        accumulator += val
+        rip += 1
+
+    def nop(val: int) -> None:
+        nonlocal rip
+        rip += 1
+
+    def jmp(val: int) -> None:
+        nonlocal rip
+        rip += val
+
+    instrs = {
+        "acc": acc,
+        "jmp": jmp,
+        "nop": nop,
+    }
+    seen = set()
+
+    while rip not in seen and rip < len(code):
+        seen |= {rip}
+        func = instrs[code[rip][0]]
+        func(code[rip][1])
+
+    return accumulator, rip == len(code)
+
+
+def solve(raw: List[str]) -> int:
+    code = [(line[:3], int(line[3:])) for line in raw]
+    lut = {"jmp": "nop", "nop": "jmp"}
+    for i in range(len(code)):
+        if code[i][0] not in lut:
+            continue
+        new_code = deepcopy(code)
+        new_code[i] = lut[new_code[i][0]], new_code[i][1]
+        val, halted = run(new_code)
+        if halted:
+            return val
+    assert False  # Sanity check
+
+
+def main() -> None:
+    input = [line.strip() for line in sys.stdin.readlines()]
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()