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Documentation Index

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Advent of Code 2015 is a complete year — all 25 days solved, both parts. As the very first AoC event, 2015 established many of the series’ signature puzzle styles: navigating infinite grids, mining MD5 hashes, simulating circuit logic, parsing JSON, and running simple virtual machines. Every solution carries a passing test suite; Day 16 (“Aunt Sue”) has a question-mark on its test status because the answer depends on recognising specific input values. The entire year was solved retrospectively, with all submissions arriving long after the original competition.

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Progress

DayTitlePart 1Part 2Tests
01Not Quite Lisp⭐⭐✅
02I Was Told There Would Be No Math⭐⭐✅
03Perfectly Spherical Houses in a Vacuum⭐⭐✅
04The Ideal Stocking Stuffer⭐⭐✅
05Doesn’t He Have Intern-Elves For This?⭐⭐✅
06Probably a Fire Hazard⭐⭐✅
07Some Assembly Required⭐⭐✅
08Matchsticks⭐⭐✅
09All in a Single Night⭐⭐✅
10Elves Look, Elves Say⭐⭐✅
11Corporate Policy⭐⭐✅
12JSAbacusFramework.io⭐⭐✅
13Knights of the Dinner Table⭐⭐✅
14Reindeer Olympics⭐⭐✅
15Science for Hungry People⭐⭐✅
16Aunt Sue⭐⭐❓
17No Such Thing as Too Much⭐⭐✅
18Like a GIF For Your Yard⭐⭐✅
19Medicine for Rudolph⭐⭐✅
20Infinite Elves and Infinite Houses⭐⭐✅
21RPG Simulator 20XX⭐⭐✅
22Wizard Simulator 20XX⭐⭐✅
23Opening the Turing Lock⭐⭐✅
24It Hangs in the Balance⭐⭐✅
25Let It Snow⭐⭐✅

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Notable Solutions

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Day 4 — The Ideal Stocking Stuffer (MD5 Mining)

A brute-force MD5 hash search: find the lowest positive integer that, when appended to the puzzle input, produces a hex digest starting with five (Part 1) or six (Part 2) zeroes. The solution uses Python’s hashlib.md5 with itertools.count to iterate over candidates and returns the first match. 
def find_hash(secret_key: str, zeros: int = 5) -> int:
    zeros_str = "0" * zeros
    return next(
        i for i in itertools.count()
        if hashlib.md5(f"{secret_key}{i}".encode()).hexdigest().startswith(zeros_str)
    )

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Day 7 — Some Assembly Required (Logic Gates)

The wire network is parsed into a dict[str, str] mapping each wire name to its definition expression. A recursive signal_on function evaluates any wire by parsing its expression — handling AND, OR, LSHIFT, RSHIFT, NOT, literals, and wire references. Results are memoised with @functools.cache (using an immutable frozendict as the circuit argument) to avoid redundant recomputation. Part 2 overrides wire b with Part 1’s answer and re-evaluates. 
@functools.cache
def _signal_on(wire: str, wires: Mapping[str, str]) -> int:
    val = wires[wire]
    if val.isnumeric():
        return int(val)
    lhs, op, rhs = val.split()
    if op == "AND":
        return signal_on(lhs, wires) & signal_on(rhs, wires)
    if op == "OR":
        return signal_on(lhs, wires) | signal_on(rhs, wires)
    # ... LSHIFT, RSHIFT, NOT

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Day 3 — Perfectly Spherical Houses in a Vacuum

Santa follows a sequence of ^v<> movement instructions on an infinite grid, delivering presents to every house he visits. Part 1 tracks Santa’s single path with a set of visited coordinates. Part 2 splits the instruction stream between Santa and Robo-Santa using itertools.islice with a step of 2, then unions the two visited sets.
Puzzle inputs are personal and are not stored in this repository. Solutions fetch input at runtime using aocd (aocd.data).

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