07

2015, Day 7: Some Assembly Required

This year, Santa brought little Bobby Tables a set of wires and bitwise logic gates! Unfortunately, little Bobby is a little under the recommended age range, and he needs help assembling the circuit.

Part 1

Each wire has an identifier (some lowercase letters) and can carry a 16-bit signal (a number from 0 to 65535). A signal is provided to each wire by a gate, another wire, or some specific value. Each wire can only get a signal from one source, but can provide its signal to multiple destinations. A gate provides no signal until all of its inputs have a signal.

The included instructions booklet describes how to connect the parts together: x AND y -> z means to connect wires x and y to an AND gate, and then connect its output to wire z.

For example:

• 123 -> x means that the signal 123 is provided to wire x.
• x AND y -> z means that the bitwise AND of wire x and wire y is provided to wire z.
• p LSHIFT 2 -> q means that the value from wire p is left-shifted by 2 and then provided to wire q.
• NOT e -> f means that the bitwise complement of the value from wire e is provided to wire f.

Other possible gates include OR (bitwise OR) and RSHIFT (right-shift). If, for some reason, you'd like to emulate the circuit instead, almost all programming languages (for example, C, JavaScript, or Python) provide operators for these gates.

For example, here is a simple circuit:

123 -> x
456 -> y
x AND y -> d
x OR y -> e
x LSHIFT 2 -> f
y RSHIFT 2 -> g
NOT x -> h
NOT y -> i

After it is run, these are the signals on the wires:

d: 72
e: 507
f: 492
g: 114
h: 65412
i: 65079
x: 123
y: 456

In little Bobby's kit's instructions booklet (provided as your puzzle input), what signal is ultimately provided to wire a?

Your puzzle answer was 16076.

Part 2

Now, take the signal you got on wire a, override wire b to that signal, and reset the other wires (including wire a). What new signal is ultimately provided to wire a?

Your puzzle answer was 2797.

Solution Notes

A nice straightforward simulation evaluation puzzle, the only caveat of which is that a cache for already evaluated wires is required to get acceptable performance.

In the golf version, I tried to define the operators as lambdas in a dictionary, but a plain if cascade turned out to be smaller than that. It also doesn't make much sense to split the implementation into two parts, as the only major difference is an additional line for part 2.

• Parts 1+2, Python: 419 bytes, <100 ms