diff --git a/2018/d21/ex2/ex2.py b/2018/d21/ex2/ex2.py
new file mode 100755
index 0000000..e7a744d
--- /dev/null
+++ b/2018/d21/ex2/ex2.py
@@ -0,0 +1,138 @@
+#!/usr/bin/env python
+
+import copy
+import enum
+import sys
+from typing import NamedTuple
+
+
+class OpCode(enum.StrEnum):
+    ADDR = "addr"
+    ADDI = "addi"
+    MULR = "mulr"
+    MULI = "muli"
+    BANR = "banr"
+    BANI = "bani"
+    BORR = "borr"
+    BORI = "bori"
+    SETR = "setr"
+    SETI = "seti"
+    GTIR = "gtir"
+    GTRI = "gtri"
+    GTRR = "gtrr"
+    EQIR = "eqir"
+    EQRI = "eqri"
+    EQRR = "eqrr"
+
+    def apply(self, registers: list[int], a: int, b: int, c: int) -> list[int]:
+        registers = copy.deepcopy(registers)
+        if self == OpCode.ADDR:
+            registers[c] = registers[a] + registers[b]
+        if self == OpCode.ADDI:
+            registers[c] = registers[a] + b
+        if self == OpCode.MULR:
+            registers[c] = registers[a] * registers[b]
+        if self == OpCode.MULI:
+            registers[c] = registers[a] * b
+        if self == OpCode.BANR:
+            registers[c] = registers[a] & registers[b]
+        if self == OpCode.BANI:
+            registers[c] = registers[a] & b
+        if self == OpCode.BORR:
+            registers[c] = registers[a] | registers[b]
+        if self == OpCode.BORI:
+            registers[c] = registers[a] | b
+        if self == OpCode.SETR:
+            registers[c] = registers[a]
+        if self == OpCode.SETI:
+            registers[c] = a
+        if self == OpCode.GTIR:
+            registers[c] = a > registers[b]
+        if self == OpCode.GTRI:
+            registers[c] = registers[a] > b
+        if self == OpCode.GTRR:
+            registers[c] = registers[a] > registers[b]
+        if self == OpCode.EQIR:
+            registers[c] = a == registers[b]
+        if self == OpCode.EQRI:
+            registers[c] = registers[a] == b
+        if self == OpCode.EQRR:
+            registers[c] = registers[a] == registers[b]
+        return registers
+
+
+class Instruction(NamedTuple):
+    op: OpCode
+    a: int
+    b: int
+    c: int
+
+    def apply(self, registers: list[int]) -> list[int]:
+        return self.op.apply(registers, self.a, self.b, self.c)
+
+
+def solve(input: str) -> int:
+    def parse_instruction(input: str) -> Instruction:
+        op, *values = input.split()
+        return Instruction(OpCode(op), *map(int, values))
+
+    def parse(input: list[str]) -> tuple[int, list[Instruction]]:
+        ip = int(input[0].removeprefix("#ip "))
+        return ip, [parse_instruction(line) for line in input[1:]]
+
+    def hash_loop(n: int, seed: int, perturb: int) -> int:
+        n |= 0x10000  #
+        while n:
+            seed += n & 0xFF
+            seed &= 0xFFFFFF  # Keeps 24-bit
+            seed *= perturb
+            seed &= 0xFFFFFF  # Keeps 24-bit
+            n >>= 8
+        return seed
+
+    # Relies heavily on input having a specific shape
+    def hash_params(ip_reg: int, instructions: list[Instruction]) -> tuple[int, int]:
+        def seed_index() -> int:
+            for i, instr in enumerate(instructions):
+                if instr.op == OpCode.BORI and instr.b == 65536:
+                    return i + 1
+            assert False  # Sanity check
+
+        def perturb_index() -> int:
+            for i, instr in enumerate(instructions):
+                if instr.op == OpCode.BANI and instr.b == 16777215:
+                    return i + 1
+            assert False  # Sanity check
+
+        seed_instr = instructions[seed_index()]
+        perturb_instr = instructions[perturb_index()]
+
+        assert seed_instr.op == OpCode.SETI  # Sanity check
+        assert perturb_instr.op == OpCode.MULI  # Sanity check
+        assert perturb_instr.a == perturb_instr.c  # Sanity check
+        return seed_instr.a, perturb_instr.b
+
+    def find_comparison(ip_reg: int, instructions: list[Instruction]) -> int:
+        seed, perturb = hash_params(ip_reg, instructions)
+        value = 0
+        count = 0
+        seen: set[int] = set()
+        while True:
+            count += (value << 8) + (value << 16)
+            if (new_value := hash_loop(value, seed, perturb)) in seen:
+                return value
+            seen.add(new_value)
+            value = new_value
+        assert False  # Sanity check
+
+    ip_reg, instructions = parse(input.splitlines())
+    return find_comparison(ip_reg, instructions)
+
+
+def main() -> None:
+    input = sys.stdin.read()
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()