Day 01 - No Time for a Taxicab

Language: Rust

Problem https://adventofcode.com/2016/day/1

Part 1: Instructions like R2 or L5: turn, then walk that many blocks. Starting at the origin facing North, find the Manhattan distance to the final position.

I track direction as an index into a directions array and update x and y by the full step count at once:

let directions = [(0, 1), (1, 0), (0, -1), (-1, 0)]; // N, E, S, W

dir = match turn {
    "R" => (dir + 1) % 4,
    "L" => (dir + 3) % 4,
    _ => panic!("Invalid turn"),
};

let (dx, dy) = directions[dir];
x += dx * steps;
y += dy * steps;

Answer is x.abs() + y.abs().

Part 2: Find the first location visited twice. Since each instruction can cover multiple blocks, I walk one step at a time and check every intermediate position against a HashSet:

for _ in 0..steps {
    x += dx;
    y += dy;
    if !visited.insert((x, y)) {
        return (x.abs() + y.abs()).to_string();
    }
}

HashSet::insert returns false if the value was already present, so the check and insert happen in one call.

Solution: https://github.com/Elyrial/AdventOfCode/blob/main/src/solutions/year2016/day01.rs

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