Nov 12 2025

Test-Driven Christmas — A Guide to the Advent of Code Mastery

It’s 6:00 AM on a dark, cold December morning. For many, the only motivation is the first cup of coffee. For me, it’s the quiet, thrilling anticipation of a gift about to arrive. My screen glows in the dark, not with work emails, but with the day’s new puzzle from the Advent of Code. As a Senior Test Automation Engineer, my career is built on a foundation of structure, quality, and a healthy obsession with finding flaws before they become problems. As a Christmas enthusiast, I believe the season is about joy, tradition, and persistence. Most people would see these two worlds as separate. I discovered they are the secret ingredients to a top-10 finish in the world’s most beloved coding challenge. My 7th place finish in 2024 wasn’t a holiday miracle — it was the result of a “Test-Driven Christmas”, a methodology that fuses professional discipline with festive passion. This guide will break down that approach and show you how a tester’s mindset can help you master the daily puzzles and embrace the true spirit of the event.

It’s The Most Wonderful Time Of The Year… for Coders #

As winter’s chill settles in and festive lights begin to twinkle, a special kind of magic awakens in the world of programming. It’s a tradition whispered among coders, an enchanted Advent calendar known as Advent of Code (AoC). Each morning (6:00 AM for us here in Poland) from December 1st to the 12th, a new door creaks open, revealing not a chocolate, but a wonderfully clever puzzle — a dispatch from the North Pole needing urgent help. These challenges are wrapped in a charming Christmas story, inviting you to use any programming language you wish to help the elves save the holiday. One year we might be gathering star fruit in a jungle, another we’re navigating the ocean depths to find Santa’s keys, or even calibrating a trebuchet to launch ourselves to a snow island.

Every puzzle is a beautifully wrapped, two-part gift. Solving the first part is like tying the perfect bow; it’s a satisfying challenge that earns you a single, shimmering gold star. But the moment you succeed, the gift tag flips over to reveal the second part. This is where the true enchantment lies, often twisting the initial problem in a delightful way or scaling it to epic proportions. Conquering this deeper challenge is like opening the box to see the incredible toy inside, earning you a second, even brighter, gold star. And the beautiful simplicity of it all? Despite the complex logic you might build, the answer you submit for each part is always a single, simple string — usually a number, but sometimes a secret password or a sequence of characters.

This magical event is a big deal in the global developer community. For hundreds of thousands of us, the true joy was never in the stressful global race, but in the cozy, communal workshop. It’s about the warm glow of a problem solved, the shared cheer in online communities as everyone shows off their clever creations, and the pure joy of adding a new spell to your personal book of magic.

This year, in a wonderful change that feels like a true Christmas gift, the event is returning to its coziest roots! After ten incredible years, its creator has retired the global leaderboard, ending the stressful “grand, festive race” and putting the focus squarely on what we always loved most. The puzzles are now a more focused 12-day sprint, and the only leaderboards are the private ones we share with friends and colleagues. This is where the real magic has always been — letting your name twinkle like a constellation for all your friends to see. It’s a chance to code not for a deadline, but for the sparkling delight of the craft itself.

The First Test Case: `assertTrue(this.isFun())` #

Before writing a single line of Java, before setting up a project, and long before the first puzzle is released, the most important preparation for the Advent of Code takes place in your mind. In test automation, we often say that you can’t test quality into a product at the end; it has to be built in from the beginning. The same is true for a 12-day coding sprint. You can’t force success through sheer will when you’re tired and frustrated. The foundation for high performance has to be laid first, and my foundation is simple: I treat it as a celebration.

For me, Advent of Code is a cherished holiday ritual. Each problem is a new present to unwrap, a clever puzzle crafted with care. This perspective isn’t just about feeling festive; it’s a powerful strategic advantage. The single greatest bug in any competitor’s run is burnout — the Grinch that steals your motivation. Even with a shorter 12-day run, the daily commitment is intense, and mindset rooted in joy is the ultimate defense. When a problem is fiendishly difficult, I don’t feel the pressure of the leaderboard. I feel the challenge of untying a particularly tricky ribbon on a gift. This simple reframing provides the resilience to show up every morning, excited and ready for a new challenge. And now, this philosophy is the official spirit of Advent of Code! With the global race retired, your only measure of success is your own joy and learning.

This philosophy is something I carry in my professional life as well. You get quality out by putting quality in. By focusing on a high-quality enjoyable experience for myself, I find that a high-quality result — a fast, correct, and elegant solution — follows naturally. Before you build your framework, first frame your mind. Your first test case must always be to assert that the process is fun.

The Magic of a Communal Workshop #

While the image of a solitary coder racing against the clock is a powerful one, the true, lasting magic of Advent of Code isn’t found in isolation — it’s discovered in community. The North Pole isn’t just one elf’s workshop; it’s a bustling, cheerful village where everyone shares their craft. For me, sharing the journey is what elevates it from a personal challenge to a cherished Christmas tradition.

This spirit comes alive right here at Allegro. While the global leaderboard was exciting, the true, lasting fun — as many of us always felt — is in our private leaderboards, where we have a friendly sleigh race among colleagues. In my team, we have a dedicated channel that becomes our virtual fireplace throughout December. It’s where we gather after solving a day’s puzzle to share our different “recipes”, marvel at an elegant solution someone discovered, or help a fellow elf who’s gotten their tinsel in a tangle.

For me, this tradition became a true family adventure. My husband also competes (and is a fantastic elf himself, finishing 94th on the 2024 global leaderboard!), which transforms our home into a second North Pole workshop. Our evenings were filled with long, happy conversations, excitedly sketching out different approaches on notepads and cheering each other on. This shared experience is where the most profound growth happens. Seeing how a teammate, or even your husband, solves the same problem with a completely different spell or a tool you’d never considered is the fastest way to learn. The true gift of the AoC community was never the global competition; it’s the collective wisdom that makes every participant a better, more creative craftsperson.

The Pre-December Sprint — Stocking the Magical Workshop #

Long before the first snowflake of December falls, and well before the first puzzle appears, the real Christmas magic is already underway in the workshop. A brilliant December is built on a well-prepared November. In my day job, we call this “sprint zero”, but here, it’s far more enchanting. It’s the time of year when the workshop is filled with the happy sounds of an elf polishing their tools and stocking the shelves with every magical ingredient they’ll need for the busy season ahead.

The foundation of a joyful and successful December is my personal, enchanted toolkit. It’s built on a wonderful secret that all seasoned elves know: Santa — being a fan of the classics — often reimagines his favorite toys year after year. The puzzles may look brand new, but they are often built with the same timeless, magical patterns. My job in November is to master these patterns and craft the perfect tools to handle them.

Studying from Santa’s Great Book of Toys #

My preparation begins by poring over “Santa’s Great Book” — the treasure trove of puzzles from years past. Within its pages, you can see the recurring designs that are beloved in the North Pole. Sometimes the magic is straightforward, but often the true enchantment is revealed in Part 2, where a simple task is scaled up to billions, forcing you to find a cleverer, faster solution. Recognizing these patterns is the key. The most cherished ones include:

The Enchanted Maze (Grid Traversal): The most common pattern of all. Weaving a path through a gingerbread village or a forest of candy canes, using spells like Breadth-First Search (BFS) for the quickest route or Depth-First Search (DFS) to explore every nook and cranny.
The Reindeer’s Quickest Route (Shortest Path): A classic holiday challenge where some snowy paths have a higher “cost” than others. Finding the most efficient path requires the glowing light of Dijkstra’s lantern.
The Elves’ Custom Computer (Parsing & Interpreters): Santa sometimes provides a list of unique instructions — like jmp, acc, and nop — for a strange new toy-making machine. The challenge here is to build a small, magical interpreter in your code to execute the list and see what happens.
The Toy That Takes a Billion Years (The Scaling Problem): This is a favorite trick from the North Pole. Part 1 will ask you to simulate a process for a few steps, which works perfectly with a simple loop. But Part 2 will ask you to run it for ten billion steps, a task that would take your computer centuries. This is where the real magic is needed. The solution is never to run the simulation; it’s to find a mathematical shortcut, a hidden cycle, or to use powerful optimization spells like memoization to avoid re-doing the same work over and over.
The Grand Tapestry (Complex State-Space Search): The ultimate challenge, where you must find the optimal path through a dizzying number of choices. This isn’t just about finding a path on a map, but about weaving together the perfect sequence of decisions — like plotting Santa’s entire route on Christmas Eve. This often requires enchanting recursive techniques to explore the possibilities.

Forging the Magical Toolkit #

Recognizing these classic designs is the first step, but a true master elf takes it further. They don’t just build a toy once; they perfect the design, forging magical tools to build it again, perfectly, every single time. A master’s workshop contains two kinds of magic, each forged with care for a different purpose:

The Enchanted Artifacts (The Reusable Library): First are the grand artifacts, forged for the big, recurring challenges. After solving a classic puzzle, I lovingly refactor the core logic from a one-off creation into a gleaming, reusable tool for my enchanted toolbox. My Pathfinder.java isn’t just a class; it’s a magical compass that never fails to find the shortest path. My Grid.java utility is an enchanted map that draws itself and always knows its boundaries. These powerful tools instantly solve the most complex parts of a problem.
The Everyday Incantations (IDE Snippets): Complementing the grand artifacts are the nimble incantations, crafted for the small, repetitive tasks. These are custom snippets (also known as live templates) I’ve enchanted into my IDE to handle the boilerplate within a solution. If a puzzle involves a map of the workshop, a grid-loop spell instantly unfolds a perfect for y… for x… structure. If I’m using a queue for a search, a while-queue snippet produces the while (!queue.isEmpty()) {…} block.

This combination is the key to fluid, joyful creation. The grand artifacts provide the power, while the swift incantations provide the precision, allowing me to stay focused on the creative heart of the puzzle with both speed and delight.

While it can be tempting to look for another elf’s finished book of spells, I highly encourage you not to use someone else’s toolkit. The most powerful magic you will ever wield is the magic you craft yourself. The true power of an enchanted compass isn’t in having it; it’s in the memory of the long, quiet hours spent forging it. That is where you gain the deep understanding to adapt when Santa brings you a challenge you’ve never seen before. Your own, handcrafted toolkit, infused with your personal style, will always be the best and most powerful one for you.

The Micro-Automations: The Elf’s Workshop Routine #

While the grand framework is my sleigh, the workshop is filled with tiny, magical tools that save precious seconds and keep my focus on the fun part — the puzzle itself. As any good elf knows, efficiency is key to Christmas joy. These small automations are my secret to a smooth and happy workshop, even when the clock is ticking.

Laying Out the Wrapping Paper: The 5:59 AM Setup #

Seconds before the puzzle is released, I’m not staring at a blank screen. I run a single setup script. Its job isn’t to fetch the puzzle — that’s not available yet — but to prepare my workbench perfectly. It creates the day’s directory and generates my DayXX.java file from a rich template. This isn’t a blank file; it’s a complete, runnable class. It already contains all the boilerplate: the main method, robust file-reading logic that populates a list of strings, and empty part1() and part2() methods ready for the day’s logic. The sleigh is polished and waiting at the starting line, ready to be filled with the day’s ingredients.

The Starting Pistol: Fetching the Ingredients #

The moment the clock strikes 6:00 AM CET, the race begins. My first action is to trigger another script with a hotkey. This one acts as the starting pistol, firing off an authenticated HTTP GET request to the Advent of Code servers to download my personal puzzle input directly into the input.txt file. While that downloads, my eyes are already scanning the problem description. There’s no time wasted with manual copy-pasting; the workshop’s pantry is stocked automatically.

Sending the Letter to Santa: Automated Submission #

Once my code produces the right answer, the final step is a joyful one. My program prints the solution to the console, and my final script acts like a magical owl, instantly capturing this result and delivering it to the North Pole. It sends an authenticated HTTP POST request to the answer endpoint, removing any frantic, last-second fumbling with browser tabs. This final piece of automation ensures that when the workshop has produced the perfect gift, it gets onto Santa’s sleigh without a moment’s delay.

The Daily Yuletide Sprint #

With a joyful mind and a workshop brimming with magical tools, the real fun begins. Each morning in December is a new sprint, a race to build a beautiful, clever toy to share with your workshop community. This is where preparation meets practice, where the elf’s craftsmanship is put to the ultimate test. My daily ritual follows a clear, test-driven rhythm that ensures quality and speed go hand-in-hand.

Decoding the Dispatch from the North Pole #

The moment the puzzle drops, it’s like receiving an urgent, special request directly from Santa. The first fifteen seconds are the most critical, and they are not for coding. They are for reading. Rushing to code with a half-understood idea is the quickest way to end up on the naughty list of failed attempts.

My approach is one of strategic comprehension. I scan the festive story to quickly grasp the core task, but my eyes are trained for specific clues. The most important of these are the highlighted and bolded words in the text. These are not just for decoration; they are deliberate hints from the North Pole, a sprinkle of magical dust to guide your focus. When I see a word like shortest or fewest, my mind instantly summons my Pathfinder compass. When all possible is emphasized, I know I’ll need the recursive magic of a Depth-First Search.

The TDD Carol: Red, Green, Refactor #

Once I’ve understood the request, my hands finally move to the keyboard, but my work is guided by the steady rhythm of a classic carol: Red, Green, Refactor.

Red — The Blueprint: The example case provided in the puzzle is the blueprint for the day’s toy. My first goal is to write a test that fails — to prove my code doesn’t work yet. This confirms I’ve understood the blueprint and have a clear target.
Green — The Joyful Assembly: This is the magical moment of creation. I pull the enchanted tools from my framework, connect them with the day’s unique logic, and watch as the toy comes to life. The goal here is simple: make the test pass. Make the blueprint a reality.
Refactor — The Elf’s Final Polish: Before shipping the toy to Santa (submitting my solution), I take a deep breath and perform the final, crucial step. A master elf never sends out a toy with a rough edge. I quickly review my code to ensure the logic is clear. This is the quality gate where a Test Engineer’s discipline prevents a last-minute bug from spoiling the fun.

When the Magic Fizzles: Finding the Lost Sparkle #

Of course, sometimes a toy just doesn’t work as planned. When my code fails on the real input, the festive spirit doesn’t give way to panic. An engineer sees this not as a failure, but as a fascinating new puzzle: a wind-up reindeer with a stuck gear.

My debugging process is a systematic search for the “lost sparkle”. I don’t randomly change code and hope for the best. I perform a Root Cause Analysis. Using System.out.println is like holding a magical magnifying glass over my workshop. I check the state of my data at each critical step of the process, looking for the exact moment the magic fizzles. This turns debugging from a frustrating chore into a methodical and often rewarding treasure hunt.

Conclusion #

In the end, mastering the Advent of Code puzzles requires more than just fast typing. It requires velocity — speed with direction and quality. The discipline of a Test Automation Engineer provided the quality, my experience with the “AoC Canon” provided the direction, and a genuine love for Christmas provided the joyful persistence to maintain that velocity for 12 straight days. The engineering mindset built a robust sleigh, but the festive spirit provided the magic that made it fly.

The true prize of this challenge, as its creator has so wonderfully affirmed this year, has never been the rank you achieve. It’s finishing the season feeling smarter, more capable, and still filled with the holiday spirit. It’s the deep satisfaction of a project well-tested and successfully deployed. So this year, I encourage you to not just solve the puzzles, but to frame them. Build your own tools, test your own assumptions, and most importantly, have fun. Embrace a Test-Driven Christmas — you’ll be amazed at what you can achieve.

tech aoc christmas