Advent Of Code 2025

My solutions for the Advent of Code 2025, using .NET 10 and C# 14.

A few rules and goals I set for myself:

• All solutions are self-contained, meaning:
  • Each solution has exactly one code file and one project file
  • No external dependencies / NuGet packages
  • No shared libraries or extensions
• Use top-level statements only and reduce file length
  • No namespaces, no explicit main method
  • All code files are less than 100 lines
  • Moving usings to the project files is allowed
  • All while still maintaining a decent level of readability
• Find reasonably efficient algorithms
  • Each solution should complete in less than one second
  • The required memory should be less than 1 GB

A few things I learned / tried out with this project:

• The Enumerable.SequenceEqual method for quickly comparing arrays
• Record classes with primary constructors for concise immutable types
• The PriorityQueue collection type for an efficient sorted collection
• The BitOperations.PopCount method for counting the bits set in an unsigned number
• The new SLNX solution format with a lot less boilerplate compared to SLN