Advent of Code 2023

This is a collection of solutions to the Advent of Code 2023 programming problems implemented in the C programming language. I have done my best to minimize time complexity and running time.

Some of the more complex problems were implemented in a higher-level programming language before being converted to C. The equivalent code is included here as a reference.

Usage

These programs are cross-platform, having been tested on Windows and Linux operating systems.

Important: Do not use Windows-style line endings (CR LF, i.e. \r\n). Only use Unix-style line endings (LF, i.e. \n).

Important: I have tested each program on the example test cases and two puzzle input files - not a large sample size! I have avoided assumptions about the input, sometimes even at the cost of performance. However, to avoid memory allocation, all buffers have fixed sizes. Ensure that all buffers (see #define statements) have sufficient capacity before running. Not doing so could result in a stack smashing, segmentation fault, or worse.

Summary

Note: The times below are expressed as orders of magnitude, not precise figures. For example, a time of 0.00010 seconds indicates an average running time between 0.00005 seconds and 0.00050 seconds.

Day	Problem	Domain	Result	Time	Implementation
1	Trebuchet?!	Strings, Tries	Sum	0.0001	Primitive trie
2	Cube Conundrum	Strings	Sum	0.0001
3	Gear Ratios	Dynamic programming	Sum	0.0001	Sliding window technique
4	Scratchcards	Sets	Sum	0.0001
5	If You Give A Seed A Fertilizer	Functions, Sorting algorithms	Minimum	0.0001	Function composition, System of linear equations
6	Wait For It	Physics, Algebra	Product	0.0001	Quadratic formula
7	Camel Cards	Statistics, Dictionaries, Sorting algorithms	Sum	0.0001	Frequency map with mode-tracking
8	Haunted Wasteland	Graph theory, Number theory, Dictionaries	Least Common Multiple	0.0001	Euclidean algorithm, map-based graph, base-36 encoding
9	Mirage Maintenance	Numerical analysis	Sum	0.0001	Lagrange polynomial
10	Pipe Maze	Geometry, Graph theory, Pathfinding algorithms	Area	0.0001	Flood-fill, matrix-based graph
11	Cosmic Expansion	Geometry	Sum	0.0001	Taxicab geometry
12	Hot Springs	Automata theory, Regular expressions, Dictionaries	Sum	0.01	Non-deterministic finite automaton, iterable dictionary
13	Point of Incidence	Binary arithmetic	Sum	0.0001	Bit array, bit matrix
14	Parabolic Reflector Dish	Strings, Dictionaries	Sum	0.0001	Character matrix, cycle detection
15	Lens Library	Cryptography, Hash functions, Dictionaries, Strings	Sum	0.0001	Iterable ordered dictionary, Length-prefixed string
16	The Floor Will Be Lava	Stacks, Sets, Dictionaries	Maximum	0.01	Array stack, array set, array map, character matrix
17	Clumsy Crucible	Graph theory, Pathfinding algorithms, Priority queues	Minimum	0.01	Dijkstra's algorithm, circular buffer, state matrix
18	Lavaduct Lagoon	Geometry	Area	0.0001	Shoelace formula, Pick's theorem
19	Aplenty	Parsing algorithms, Recursion, Stacks, Functions, Dictionaries	Count	0.0001	Recursive descent parser, dictionary
20	Pulse Propagation	Graph theory, Number theory, Dictionaries, Sets, Circuit design	Product	0.01	Euclidean algorithm, map-based graph, hash set, length-prefixed string
21	Step Counter	Graph theory, Sets, Priority queues	Count	0.001	Breadth-first search, hash set, circular buffer, matrix-based graph
22	Sand Slabs	Geometry, Graph theory, Sorting algorithms, Stacks, Sets	Count	0.01	Object-based graph, hash set
24	Never Tell Me The Odds	Physics, Linear algebra	Sum	0.01	Line–line intersection, Gaussian elimination
25	Snowverload	Graph theory	Product	0.001	Max-flow min-cut theorem, Ford–Fulkerson algorithm, adjacency list, iterable graph

Constraints

I am working within the following constraints to ensure high code quality.

• Adhere to the project style guide.
• Final solutions must be implemented in the C programming language following the C99 standard.
  • Assume signed integer overflow is defined based on two's complement.
• All solutions must be standalone files with no user-defined headers and no external dependencies beyond the C standard library (libc) and the C mathematics library (libm). The first and second problems for a given day must be implemented separately.
• Bounds checking is not required for data structures whose capacity is defined by a macro.
• The return values of all C standard library functions must be checked, except for those returned from the following:
  • fprintf
  • memcpy
  • printf

Omissions

• The solution must produce the correct result for the examples, not just the personalized puzzle input. This means that the efficient solutions to Day 10(b) and Day 21(b) are unfeasible. I have omitted both.
• By design, there is no efficient solution to Day 23(a) and (b). I have omitted both.