Day 21.

2022/day21.py

@@ -1,3 +1,109 @@
 # -*- encoding: utf-8 -*-
 
+import operator
 import sys
+from typing import Callable
+
+
+def compute(monkeys: dict[str, int | tuple[str, str, str]], monkey: str) -> int:
+    value = monkeys[monkey]
+    if isinstance(value, int):
+        return value
+    else:
+        op: dict[str, Callable[[int, int], int]] = {
+            "+": operator.add,
+            "-": operator.sub,
+            "*": operator.mul,
+            "/": operator.floordiv,
+        }
+        value = op[value[1]](compute(monkeys, value[0]), compute(monkeys, value[2]))
+        monkeys[monkey] = value
+        return value
+
+
+def invert(
+    monkeys: dict[str, int | tuple[str, str, str]], monkey: str, target: int
+) -> dict[str, int | tuple[str, str, str]]:
+    """
+    Revert the given mapping from monkey name to value or operation such that
+    the value from 'monkey' is computable by inverting operation until the root is
+    found.
+
+    Args:
+        monkeys: Dictionary of monkeys, that will be updated and returned.
+        monkey: Name of the monkey to start from.
+        target: Target value to set for the monkey that depends on root.
+
+    Returns:
+        The given dictionary of monkeys.
+    """
+
+    monkeys = monkeys.copy()
+
+    depends: dict[str, str] = {}
+    for m, v in monkeys.items():
+        if isinstance(v, int):
+            continue
+
+        op1, _, op2 = v
+
+        assert op1 not in depends
+        assert op2 not in depends
+        depends[op1] = m
+        depends[op2] = m
+
+    invert_op = {"+": "-", "-": "+", "*": "/", "/": "*"}
+
+    current = monkey
+    while True:
+        dep = depends[current]
+
+        if dep == "root":
+            monkeys[current] = target
+            break
+
+        val = monkeys[dep]
+        assert not isinstance(val, int)
+
+        op1, ope, op2 = val
+
+        if op1 == current:
+            monkeys[current] = (dep, invert_op[ope], op2)
+        elif ope in ("+", "*"):
+            monkeys[current] = (dep, invert_op[ope], op1)
+        else:
+            monkeys[current] = (op1, ope, dep)
+
+        current = dep
+
+    return monkeys
+
+
+lines = sys.stdin.read().splitlines()
+
+monkeys: dict[str, int | tuple[str, str, str]] = {}
+
+op_monkeys: set[str] = set()
+
+for line in lines:
+    parts = line.split(":")
+    name = parts[0].strip()
+
+    try:
+        value = int(parts[1].strip())
+        monkeys[name] = value
+    except ValueError:
+        op1, ope, op2 = parts[1].strip().split()
+        monkeys[name] = (op1, ope, op2)
+
+        op_monkeys.add(name)
+
+
+answer_1 = compute(monkeys.copy(), "root")
+print(f"answer 1 is {answer_1}")
+
+# assume the second operand of 'root' can be computed, and the first one depends on
+# humn, which is the case is my input and the test input
+p1, _, p2 = monkeys["root"]  # type: ignore
+answer_2 = compute(invert(monkeys, "humn", compute(monkeys.copy(), p2)), "humn")
+print(f"answer 2 is {answer_2}")

2022/tests/day21.txt

@@ -0,0 +1,15 @@
+root: pppw + sjmn
+dbpl: 5
+cczh: sllz + lgvd
+zczc: 2
+ptdq: humn - dvpt
+dvpt: 3
+lfqf: 4
+humn: 5
+ljgn: 2
+sjmn: drzm * dbpl
+sllz: 4
+pppw: cczh / lfqf
+lgvd: ljgn * ptdq
+drzm: hmdt - zczc
+hmdt: 32