Solutions to puzzles are organized into directories, one directory for each day — 01, 02, 03, …, 24, 25:

01:
  solve-1           # implementation for part 1, expects puzzle input on stdin
  solve-2           # implementation for part 2, expects puzzle input on stdin
  test-input.txt    # example input
  input.txt         # puzzle input (.gitignore-d)

To invoke puzzle implementations from the project root, you can run:

./solve <day> [part [--test|-t]]

For instance, ./solve 5 and ./solve 5 1 will invoke day 5 part 1. Running ./solve 5 2 will invoke part 2.

Adding --test or -t after the part number, will use input from test-input.txt instead of input.txt.