Day 8 is about mapping antennas on a 2D grid and marking antinodes where signals resonate in straight lines, first at a single step beyond a pair and then across entire lines. We will walk through a vector-stepping approach in Ruby that keeps the implementation small, readable, and fast. If you want …

In software engineering, it’s typically very good to have a retrospective after a sprint of work. The goal isn’t just to celebrate successes or catalogue frustrations, but to step back and deliberately reflect: what worked well, what didn’t, and what lessons are worth carrying forward? Advent …

Day 12 is the last day of Advent of Code 2025, and the last day is historically quite easy. The puzzle today looks like a polyomino packing puzzle at first glance, but for this input it reduces neatly to a capacity check on occupied cells per region. This was an assumption on my part because of the …

Day 11 is about counting all possible paths through a graph between two specific nodes. This sort of problem is best solved with a depth-first search using memoisation. A tiny bit of state is added in to enforce constraints in Part 2.

The whole thing runs in well under a millisecond per part on my …

Day 10 is by far the hardest puzzle so far in 2025. First a classic bit-flipping puzzle that reduces to XOR over bitmasks, then a minimal-presses counter puzzle modelled as an integer optimisation. If you want to follow the whole series, check out the tag page at Advent of Code, and you can browse …

Day 9 asks for the largest axis-aligned rectangle defined by any two corner coordinates in the input. It then graduates to validating that a candidate rectangle must lie wholly inside the rectilinear loop traced by those corners. Part 1 is a straightforward exhaustive search over corner pairs with …

Day 8 is all about finding edges between 1,000 nodes in a graph, each with a 3D set of integer coordinates. Graphs can be computationally expensive if the wrong types are used, but my solutions run in under a second for each part of the puzzle. Let’s take a look at the puzzle specifics and how …

Day 7 models a downward beam traversing a grid of splitters, and a single dynamic programming pass prepares everything we need for both parts. Part 1 counts where a straight descent is blocked by a splitter and turns into a left–right split, while Part 2 totals the many‑worlds outcomes that land on …

Day 6 presents a neat parsing twist where the same worksheet must be interpreted in two different ways, and a small amount of transposition and reduction logic solves both parts cleanly. We will outline what each part asks for, then walk through a compact Python implementation that builds …

Day 5 serves up two courses: a quick taste test to check whether each ingredient is fresh according to ranges of fresh items. Then a hearty main where we reduce overlapping ranges of the fresh items and identify how many unique ingredients are included. We manipulate the input into integers for …

Day 4 boils down to counting and then repeatedly removing rolls of paper that have fewer than four neighbouring rolls. A small Grid2D helper plus a couple of concise functions are all you need. We will go over the rule for a roll being accessed, show how to parse the grid and inspect eight-way …

Day 3 builds a number from digits by picking a subsequence with the largest possible value, and the same greedy approach powers both parts. We will unpack the problem requirements at a high level, then walk through a compact Python solution that selects the maximum value subsequence per digit …