Preparing Your Dev Environment for December Coding Challenges

December brings a certain kind of puzzle: one small problem a day, a fresh input file, two parts, and a leaderboard that rewards people who started twenty minutes ago. The puzzles themselves are rarely the thing that stops you. What stops you is friction — deciding where today's files live, remembering which command runs the tests, pasting the example input for the fourth time, and losing half an hour before you have written a line of real logic.

An evening spent setting up a workspace in late November pays for itself by the third or fourth. Here is a setup that stays out of the way.

Make the dull decisions once

Pick a language and a test runner now, and resist the urge to change halfway through. The best choice is the one where you already know how to split a string, convert between types and reach for a hash map without checking the documentation. If you want to learn something new, that is a fine goal, but accept that you will be slower, and that the harder days are not the moment to discover a language's quirks around integer overflow.

Once chosen, fix these four things in advance:

• Language and runtime version. Note it in the repository so a machine rebuild does not surprise you.
• Test command. One command for the whole day's folder, and a watch mode if your runner has one.
• Editor setup. A snippet for a new solution file, and a quick way to jump between a solution and its input.
• Timing. A wrapper that prints how long each part took. It is useful for spotting when a naive approach will not scale to the real input.

Nothing here is clever. That is the point. Clever belongs in the puzzle solution.

One folder per day, one command to run it

A flat directory of files called solution2.js, solution2final.js and solution2working.js becomes unusable by about 9 December. Numbered folders age much better:

• 01/, 02/, 03/ and so on, zero-padded so they sort correctly.
• input.txt — the real puzzle input, cached locally.
• example.txt — the sample from the puzzle page, including any second sample for part two.
• solution — the file you actually edit.
• test — the fixtures that check your parsing and your answers.
• notes.md — a few lines on the approach and the trap you fell into.

Keep a shared folder for helpers you reuse: grid utilities, a coordinate class, a small priority queue, an interval merger. Every year these accumulate, and every year you will be glad of them. Resist the temptation to build a grand generic framework in advance; add a helper after the second time you need it, not before.

Templates that write the first twenty lines

Most days begin the same way: read the file, trim it, split it into lines, parse, then run part one or part two depending on a flag. That is twenty lines of typing you should never do by hand.

Write a snippet that expands into a skeleton with the file reading, the line splitting, the timed wrapper and a two-branch entry point. Then write a small scaffolding script — a shell script is plenty — that takes a day number, copies a template folder, creates the input and example files, and opens the editor on the new solution file. Running ./newday 07 and having the whole structure appear takes a couple of seconds, and it removes the last excuse for faffing.

Keep the parsing separate

Put parsing in its own function. Every year there is a day where the sample input has a different shape from the real one, or where the last line has no trailing newline, and having a single function to fix beats hunting through the logic. Return a typed structure rather than raw lines wherever you can. It makes the rest of the day read like the puzzle description.

Fixtures first: let the tests keep score

The example in the puzzle statement is a test case that someone else has already written for you. Paste it into example.txt, assert that part one returns the published answer, and you have a fast signal that your parsing and logic agree with the problem setter.

Do not assert on the real input's answer. Parsing the real input and printing the result is usually the right behaviour, because the answer changes per account and you will want to eyeball it before submitting. Guard the example tests behind a line in the test file that skips the slow ones if the sample takes more than a moment.

Common traps worth a test or at least a comment:

• State shared between parts. Part one mutates a grid, part two reads a fresh copy — parse the input again rather than reusing the mutated structure.
• Negative numbers with modulo. Many languages return a negative remainder; add the modulus and take it again.
• Fixed-width integers. Products and sums can quietly overflow in languages that use 32-bit ints by default.
• Off-by-one errors on ranges. Decide early whether your intervals are inclusive and write it down.

Cache inputs, and keep secrets out of the repository

Fetching the input by hand each morning is a small tax. A short script that downloads the day's input into the right folder if it does not already exist removes it. Keep your session cookie in an environment variable, never in the code, and add both the cached inputs and the cookie file to your ignore rules. Be polite about it: one request per day, no retry storms, no parallel hammering of a site that is trying to serve thousands of people at once. Some puzzle sites ask that inputs not be republished, so treat your cached copies as local working files.

Notes, and a plan for the day you get stuck

A notes.md per day sounds precious until day 17 references a structure you built on day 12. Two or three lines is enough: the shape of the data, the algorithm, the complexity, and what went wrong. Future you will read far more of these than you expect.

When a puzzle resists, work through a short checklist before reaching for a bigger idea:

1. Re-read the statement, particularly the sentence you skimmed.
2. Run the example again and print the intermediate structure, not just the final answer.
3. Check the boundary: empty input, a single item, the last line, duplicate values.
4. Timebox it. Forty-five minutes without progress is a good moment to stop, make a note, and come back with fresh eyes.

The puzzle should be the hard part. Everything around it should cost you one command.

A five-minute routine for the whole month

Settle into the same pattern each day. Open the puzzle, read it properly once without touching the keyboard, then scaffold the folder and paste the example in. Write the parser and the example test before you think about the algorithm, and get part one passing on the sample before you run it against your real input. Submit, then refactor only if part two demands it — most days it does not. Finish by writing the two-line note, committing, and closing the laptop.

Do that for twenty-five days and the workspace disappears from your attention entirely. What is left is the interesting part: the puzzle, a decent cup of tea, and the small satisfaction of a green test on a dark morning.

Photo: Pavel Danilyuk / Pexels