2024 day 17 kind-of-generic.

src/holt59/aoc/2024/day17.py

@@ -101,17 +101,17 @@ class Solver(BaseSolver):
 
         yield ",".join(map(str, run(registers.copy(), program)))
 
-        # out_index = next(i for i in range(0, len(program), 2) if program[i] == 5)
+        # last instruction is JNZ 0 (jump at the beginning), and it is the only jump
-        # out_register = "ABC"[program[out_index + 1] - 4]
+        # in the program
-
-        # there is only one jump instruction and it goes back to the beginning of the
-        # program
         jnz_indices = [i for i in range(0, len(program), 2) if program[i] == 3]
-        assert (
+        assert jnz_indices == [len(program) - 2] and program[-1] == 0
-            len(jnz_indices) == 1
+
-            and jnz_indices[0] == len(program) - 2
+        # previous instruction is dividing A by 8, or A = A >> 3
-            and program[-1] == 0
+        assert program[-4:-2] == [0, 3]
-        )
+
+        # previous instruction is a OUT B % 8, and it is the only OUT in the program
+        out_indices = [i for i in range(0, len(program), 2) if program[i] == 5]
+        assert out_indices == [len(program) - 6] and program[len(program) - 5] == 5
 
         valid: list[int] = [0]
         for p in reversed(program):
@@ -119,10 +119,10 @@ class Solver(BaseSolver):
             for v in valid:
                 a_high = v << 3
                 for a_low in range(0, 2**3):
-                    a = a_high | a_low
+                    registers["A"] = a_high | a_low
-                    b = (((a % 8) ^ 7) ^ (a >> ((a % 8) ^ 7))) ^ 4
+                    run(registers, program[:-6])
-                    if b % 8 == p:
+                    if registers["B"] % 8 == p:
-                        new_valid.append(a)
+                        new_valid.append(a_high | a_low)
             valid = new_valid
 
         assert run(registers | {"A": min(valid)}, program) == program