A live coding walkthrough of Advent of Code 2025 Day 11 in Rust. Part one involves counting all paths from a source node to a destination in a directed graph, solved using the pathfinding crate's count_paths function with a BTreeMap-based parser using nom. Part two adds a constraint requiring paths to visit two specific intermediate nodes (FFT and DAC), solved by redefining the graph node to include boolean state tracking whether each required node has been visited, effectively expanding the graph while leveraging built-in caching to handle the large path counts efficiently. The solution runs in ~100 microseconds for part one and ~277 microseconds for part two.
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