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Documentation Index

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Advent of Code 2017 is a complete year — all 25 days solved, both parts. The year has a playful computer-science theme: from circular buffers and hash functions to a Turing machine finale. Standout puzzles include the reusable Knot Hash algorithm (Days 10 and 14), the Spinlock circular buffer (Day 17), the Duet dual-program VM (Day 18), and the recursive tower balancing puzzle (Day 7). All solutions carry tests; Day 23 has a question-mark on its test status due to input-dependent behaviour.

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Progress

DayTitlePart 1Part 2Tests
01Inverse Captcha⭐⭐✅
02Corruption Checksum⭐⭐✅
03Spiral Memory⭐⭐✅
04High-Entropy Passphrases⭐⭐✅
05A Maze of Twisty Trampolines, All Alike⭐⭐✅
06Memory Reallocation⭐⭐✅
07Recursive Circus⭐⭐✅
08I Heard You Like Registers⭐⭐✅
09Stream Processing⭐⭐✅
10Knot Hash⭐⭐✅
11Hex Ed⭐⭐✅
12Digital Plumber⭐⭐✅
13Packet Scanners⭐⭐✅
14Disk Defragmentation⭐⭐✅
15Dueling Generators⭐⭐✅
16Permutation Promenade⭐⭐✅
17Spinlock⭐⭐✅
18Duet⭐⭐✅
19A Series of Tubes⭐⭐✅
20Particle Swarm⭐⭐✅
21Fractal Art⭐⭐✅
22Sporifica Virus⭐⭐✅
23Coprocessor Conflagration⭐⭐❓
24Electromagnetic Moat⭐⭐✅
25The Halting Problem⭐⭐✅

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Notable Solutions

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Day 10 — Knot Hash

The Knot Hash is a custom hash algorithm used as a primitive in both Day 10 and Day 14. Part 1 is a single round of in-place circular-list reversal. Part 2 implements the full hash: the input string is converted to ASCII codes, a fixed suffix [17, 31, 73, 47, 23] is appended, and 64 rounds of reversal are run. The 256-byte sparse hash is condensed to a 16-byte dense hash by XOR-ing each 16-element chunk, then hex-encoded. 
def knot_hash(txt: str) -> str:
    ascii_codes = list(map(ord, txt))
    lengths = [*ascii_codes, *SUFFIX]
    nums = [*range(DEFAULT_LIST_SIZE)]
    pos = skip_size = 0
    for _ in range(64):
        pos, skip_size = do_round(nums, lengths, pos=pos, skip_size=skip_size)
    sparse_hash = tuple(nums)
    return dense_hash(sparse_hash)

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Day 7 — Recursive Circus

Part 1 finds the root of a weighted tree by identifying the one program that is never listed as a child. Part 2 detects the single unbalanced node — the one whose subtree weight differs from all its siblings — and computes what its weight should be. The solution walks the tree recursively, propagating subtree weights upward.

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Day 18 — Duet

Part 1 interprets a register-based assembly language where the snd instruction plays a “sound” and rcv recovers it. Part 2 reinterprets snd/rcv as message-passing channels between two programs running concurrently. The two Program instances share queues and alternate execution until both are blocked, then the number of values sent by Program 1 is the answer.
Puzzle inputs are personal and are not stored in this repository. Solutions fetch input at runtime using aocd (aocd.data).

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