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This puzzle challenge brings joy to the world of code

Thousands worldwide, from San Francisco to Slovenia, are counting down to Christmas with a puzzle-a-day programming challenge called Advent of Code.

December 17, 2021
Ms Tech | Advent of Code

By midnight on December 1, 2015, when Eric Wastl first launched his annual Santa-themed puzzle-a-day programming challenge Advent of Code, 81 people had signed up. That pretty much matched his capacity planning for 70 participants. Wastl figured this amusement might be of interest to a few friends, friends of friends, and maybe some of their friends as well.

But Wastl, a software engineer who works as a senior architect for TCGPlayer, an online marketplace for trading card games, had failed to anticipate how social media’s recursive contagion might overwhelm these modest expectations. He jokes that the technical term for what happened next is: “OH NO!” Within 12 hours there were about 4,000 participants. The server nearly crashed. At 48 hours, there were 15,000 people, and by the end of the event, on December 25, the grand total was 52,000. The following year, he moved the operation to Amazon Web Services, and numbers have since continued to grow.

Last year, perhaps due to the pandemic, the event saw a 50% spike in traffic, with more than 180,000 participants worldwide.

And now again this year, thousands of coders from San Francisco to Slovenia—students and software engineers and competitive programmers alike—are counting down to Christmas with Advent of Code (AoC). While traditional advent calendars deliver daily gifts of chocolate or toys (and some alternative versions deliver dog treats, Jack Daniel’s, Lego figures, or even digital delights via apps), Advent of Coders unwrap playfully mathy problems and then write computer mini-programs that do the solving.

The fun of it, partly, is simply in the time-honored magic of a holiday ritual. But it’s also in submitting to pleasurable puzzlement. Peter Norvig, a research director at Google, finds it fun because he trusts the creator, Wastl, “to make it worth my time”—in a similar way, Norvig says, to how New York Times crossword puzzlers trust Will Shortz to do right by them. “There will be some tricks that make it interesting,” says Norvig, “but there are bounds on how tricky.” 

The joy of coding

At midnight US Eastern time (Wastl is based in Buffalo, New York), every night from December 1 to 25, a new puzzle lights up at, embedded within a cleverly composed Christmas-caper narrative—one player described the story as “an Excuse Plot if there ever was such a thing.”

This year’s event got off to a fine start when Santa’s elves lost the keys to the sleigh. The first problem set the scene as follows: “You’re minding your own business on a ship at sea when the overboard alarm goes off! You rush to see if you can help. Apparently one of the Elves tripped and accidentally sent the sleigh keys flying into the ocean!”

Luckily, the Elves had a submarine handy for just such emergencies, and from there participants set off on a 25-day underwater quest. They try to solve two puzzles daily (the second adding a twist, or more difficulty), each worth a star and some praise: “That’s the right answer! You are one gold star closer to finding the sleigh keys.”

Every player earns a star for solving a problem, but if you’re the first to get a star, you receive 100 points; if you’re second, you receive 99 points; and so on, with the 100th place earning one point.

“In order to save Christmas,” the puzzle master explains, “you’ll need to get all fifty stars by December 25th.”

ascii santa hat

The object of Advent of Code is to solve the puzzles using your programming language of choice (Python is the most popular). Participants also use by-hook-or-by-crook strategies—such as “Excel madness,” as Wastl describes it, or reams of graph paper, and a surprising number solve the puzzles in Minecraft.

But the broader motivation varies from player to player. Some treat it as an annual tune-up for their programming skills; others see it as the perfect opportunity to learn to code or try a new language. José Valim, creator of the Elixir programming language, is live-streaming his AoC solutions on Twitch.

At the top of the global leaderboard, which ranks the 100 players with the highest total score, competitive programmers like Brian Chen (his handle is “betaveros”) and Andrew He (“ecnerwala”) are out for speed. A security software engineer working on end-to-end encryption at Zoom, Chen placed first last year (and the year before), while He came a close second.

“Going fast is fun,” Chen says, “just like optimizing anything where you can get fairly immediate feedback. There are lots of little knobs to tweak, and lots of little moments to be proud of where you made the right choice or prepared something that came in useful.”

Both MIT computer science alums who live in the Bay Area, Chen and He are friendly rivals who’ve competed together in programming challenges over the years—on the same team at the International Collegiate Programming Contest (ICPC) and as competitors at Codeforces and Google’s Code Jam. This year again, Chen is beating He. “To be honest, it’s ’cause he’s a little better than me”—better at various tricks and implementations that optimize speed—“but I don’t like admitting that,” says He, a founding engineer at the startup Modal, which builds infrastructure and tooling for data teams.

The leaderboard is out of reach for the majority of participants—especially as puzzles get harder by the day. Kathryn Tang, who runs an engineering operations team at Shopify, placed 36th on day one and was still hanging on to 81st by day three, but she knew her leaderboard status wouldn’t last long. “I’m doing this for fun using Google sheets,” she says.

The element of contest, however, is replicated—at Shopify and Google and many companies big and small—with private leaderboards, as well as dedicated chat channels where players share solutions and kvetch about the problems in post-mortems.

“The competitiveness helps commitment,” said the engineer Alec Brickner, commenting in a Slack channel at, a natural-language-processing startup in San Francisco (Brickner has made the leaderboard on a couple of days so far).

“Meh,” replied his colleague Michael Leikam. “The payoff for me is the joy of coding.”

John Bohannon, Primer’s director of science, seconded that with an emoji: “SAME.”

Bohannon also loves the silly story that sets up the problems, but the plot has little to zero utility. “The speed-demon solvers completely ignore the story, focusing on the variables of the problem to solve and just getting to it,” he says.

Nora Petrova, a data scientist and engineer at Primer’s office in London, UK, is there for the beauty, not the sport: “I love the drama that’s unfolding in every puzzle,” she says. For instance, on day four, a giant squid attached itself to the submarine—it wanted to play bingo, of course. The puzzle input was a random set of 100 bingo boards, and the challenge was to predict the winning board and give it to the squid.

Love it XOR Hate it

Wastl’s main motivation in creating Advent of Code was to help people become better programmers. “Beginners who are just getting into programming are the people I want to get the most out of this,” he says. “The success metric for most people should be ‘How many new things did I learn?’—not ‘Was I one of the very, very fastest people in the world to solve this puzzle?’”

Russell Helmstedter, a middle school teacher at the De Anza Academy of Technology and the Arts, in Ventura, California, is using Advent of Code to teach Python to his students in sixth, seventh, and eighth grades. They tackled the first two problems together as a class. From a teaching perspective, the problems are effective exercises because if you fail, you can simply try again—very much in the spirit of test-driven software development.

Helmstedter found that some of his students were a bit overwhelmed with the two-pronged challenge—deciphering the problem and coding a machine to solve it—but most embraced the struggle. “I like that it is hard to do,” one student said on a survey. And another said, “There is honestly no downside. I really like how you start working progressively toward a goal.” Although the survey’s multiple-choice question ranking “feels” elicited one “Hate it,” 41 respondents chose “Like it” (to varying degrees) and eight “Love it.”

ascii reindeer

At the University of Ljubljana, in Slovenia, the computer scientist Janez Demšar uses the AoC problems both as a professor and to hone his own skills (he’s on the core team of Orange, an open-source machine learning and data visualization toolbox). “I need to have some regular practice, like a violinist who plays in an orchestra and does some teaching but still needs some small pieces to practice,” he says. “So these are my etudes.” Demšar teaches Programming 101 to a heterogenous group of more than 200 students. “My greatest concern,” he says, “is how to keep those who already know some (or a lot) of programming interested and occupied. AoC tasks are great because they require various skills”—from pure coding to algorithms.

Gregor Kikelj, a third-year mathematics undergraduate at the university, first tried Advent of Code in 2019. He did well enough to land himself an internship at (working on Openpilot, its software for semi-automated driving systems), since the founder of the company was also competing. And Kikelj boosted his grade in the programming course (with another professor), since every problem solved was worth extra points on the final exam—plus bonus points for placing on the leaderboard.

Kikelj (“grekiki”) got up every morning for the puzzle drop—6 a.m. in Slovenia—and ranked 52 overall on the leaderboard, accumulating a total of 23 extra exam points. “After that year, they put the cap on the amount of points you can receive to 5,” he recalls. But he’s still rising with the sun to pounce on the puzzle. This year his best ranking, on day five, was 25th—he’s aiming to stay in the top 100. “We’ll see how it goes as the problems get harder,” Kikelj says.

How to leaderboard

If the leaderboard is your game, competition is fierce and the daily countdown is key—players wait like a hawk for the puzzle to drop, and then click lickety-split to download. Last year, this “giant burst of traffic synchronized to a single second” (as Wastl describes it) troubled even Amazon’s load balancers.

The AoC Subreddit—one of many communities around the internet—is full of inside-baseball banter about how to prevail (with solutions and help threads, as well as self-satire and memes). But the best resource is perhaps Brian Chen’s blog post on “how to leaderboard.”

Chen goes deep on such general tips as “Don’t write bugs.” And “If you have bugs, catch them early.” And “Go fast”—“Cut whatever corners you need to get your code out faster. Write hacks. Copy-paste code. Hardcode constants. Mutate data unreservedly. Use exceptions for control flow. Coerce booleans to integers and use them as indexes. Represent data as strings even when you definitely shouldn’t. Be proud.”

Chen wants to emphasize that “you should normally never do this in production code”—i.e., code meant to power a product and end up in front of a customer. But for AoC, the bottom line is that you only need code that is good enough. It doesn’t have to be robust or beautiful: brilliant hacky shortcuts do the job.

When writing his code, Peter Norvig bears in mind the “YAGNI” principle—“You aren’t gonna need it.”

As in years past, Norvig is posting all his solutions to part one and part two of the puzzles on GitHub (he’s not competing for points, just for fun). These posts inevitably elicit applause on Hacker News and the like: “Norvig’s code is gold,” said one admirer. Another commented: “His code is really, really pleasant to read in the same way you can almost taste some proofs in mathematics or enjoy a brilliant novelist’s prose.”

“Part of the idea of AoC,” Norvig explains, “is that you have to make some design choices to solve part 1 before you get to see the description of part 2. So there is a tension of wanting the solution to part 1 to provide general components that might be reused in part 2, without falling victim to YAGNI.”

Aside from doing everything you can to program fast and clean, Chen’s most important piece of advice is basic if non-trivial: “Be free when puzzles are released.” In 2018, he solved one puzzle while at karaoke with friends. “I paused, opened my laptop, and did it in the corner, and went back to karaoke,” he says. “Of course, it helped that I was fast”—though in the end he placed second that year. In 2019, he did day 17 with fragile Wi-Fi during a flight from San Francisco to Taipei; and then he banged out day 22 on his phone (discreetly) while attending a relative’s wedding.

“What a monster!” says rival Geoffry Song (“goffrie”), a software engineer at Dropbox who’s also vying for the top spot.

In pursuit of an edge, Song sometimes uses a typing speed contest (TypeRacer) to warm up his finger muscles beforehand. And all serious players set a daily alarm for a few minutes before the puzzle drops—giving themselves time to get in the zone and get settled at the keyboard. Even then, though, things can go sideways. This year on day two, Andrew He, who is based in the Bay Area, jumped to when his alarm went off at 8:55 p.m. PST. Already at his computer, he got his template ready on the screen and then still essentially had five minutes to kill. But before he knew it, he’d killed 10 minutes. “It was just an accident,” says He. “I got distracted and started at 9:05.”

By then it was all over. Chen solved the day’s puzzles in 66 seconds and 39 seconds, respectively. Meanwhile, He missed out on a lot of points: “It’s definitely possible to bring it back, but I also don’t know if it’s, uh, you know—I’ll try, but we’ll see if it happens.” (He notes that Chen missed a day last year and still won handily.)

He was back in contention by mid-December—which is also when players start to drop off. Sometimes that’s because the puzzles are getting harder to wrangle, but it’s also because, as noted on a Hacker News thread, real-life holiday festivities are encroaching on people’s free time. “I’m certainly not going to miss out on eggnog with friends and family so I can solve a few extra AoC problems,” said one commentator. To which came the reply: “You only have yourself to blame if you stop early due to misplaced priorities.”

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Illustration by Rose WongIllustration by Rose Wong

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