From 14520a4625ec40ad6bfd7490105989bb38250bf5 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Anders=20Engl=C3=B6f=20Ytterstr=C3=B6m?= <yttan@fastmail.se>
Date: Sat, 21 Dec 2024 01:20:13 +0100
Subject: [PATCH] Solve 2024:17 p1-2 "Chronospatial Computer"

Slow burner.

First part was a straightforward implementation
of a runner, the code is extracted to `org_version()`.

Part 2 was way more tricky. Like many others, trying
to understand the program felt necessary. This
intepretation is this:

    """Python version of puzzle input program."""
    a = <unknown>
    b, c = 0, 0
    while a:
        b = (a % 8) ^ 1
        c = a // (2**b)
        b = (b ^ 5) ^ c
        a = a // (2**3)
        print(b % 8)

Some clarification here:

- the value is octagonal. 1-8 is the key.
- The value of reg a is _high_. It is not meant to
be brute forced.

The idea, which is not originally mine, is to go
from right to left. This code can be used to try
out some patterns:

    while True:
        python_version(input("Provide A:"))

Here, it was apparent a=4 gives the last digit of
my puzzle input. a=37 (4 * 8 + 3) gives the last
2 digits. a=222 (37 * 8 + 6) gives the last 3
digits, and so on.

Knowing the program could be reconstructed like
this, the first code halted at wrong values. Turns
out some steps give more than 1 possible scenario.

The code was therefore in need of a queue.
---
 2024-python/output/day_17.py | 75 ++++++++++++++++++++++++++++++++++++
 1 file changed, 75 insertions(+)
 create mode 100644 2024-python/output/day_17.py

diff --git a/2024-python/output/day_17.py b/2024-python/output/day_17.py
new file mode 100644
index 0000000..bbd5c0d
--- /dev/null
+++ b/2024-python/output/day_17.py
@@ -0,0 +1,75 @@
+from collections import deque
+
+from output import ints
+
+
+def solve(data):
+    R, program = data.split("\n\n")
+    R = {k: int(v) for k, v in zip("abc", ints(R))}
+    program = ints(program)
+    P = ",".join(map(str, program))
+
+    p1 = org_version(R["a"], program)
+    p2 = float("inf")
+
+    Q = deque([1])
+    while Q:
+        x = Q.popleft()
+        for i in range(8):
+            o = org_version(x + i, program)
+            if P.endswith(o):
+                if o == P:
+                    p2 = min(p2, x + i)
+                else:
+                    Q.append((x + i) * 8)
+
+    return p1, p2
+
+
+def org_version(A, program):
+    R = {"a": A, "b": 0, "c": 0}
+
+    def combo(v):
+        return v if v < 4 else R[chr(93 + v)]
+
+    i = 0
+    out = []
+    while i < len(program):
+        cl = program[i + 1]
+        match program[i]:
+            case 0:
+                R["a"] = R["a"] // (2 ** combo(cl))
+                i += 2
+            case 1:
+                a, b = R["b"], cl
+                R["b"] = a ^ b
+                i += 2
+            case 2:
+                R["b"] = combo(cl) % 8
+                i += 2
+            case 3:
+                i = (i + 2) if R["a"] == 0 else cl
+            case 4:
+                a, b = R["b"], R["c"]
+                R["b"] = a ^ b
+                i += 2
+            case 5:
+                out.append(combo(cl) % 8)
+                i += 2
+            case 6:
+                R["b"] = R["a"] // (2 ** combo(cl))
+                i += 2
+            case 7:
+                R["c"] = R["a"] // (2 ** combo(cl))
+                i += 2
+    return ",".join(map(str, out))
+
+
+if __name__ == "__main__":
+    with open("./input/17.txt", "r") as f:
+        inp = f.read().strip()
+
+    p1, p2 = solve(inp)
+
+    print(p1)
+    print(p2)