Advent of Code 2025 - Day 1Secret Entrance

| Problem statement | Source code | Tags: Brute forceMathematics

Part 1

I only learned that it's possible to simulate every single step of the dial—I guess that's more expected of day 1. Anyway, I implemented it the cleverer way, which is to update the position once per turn. Completing one turn resets the dial from 100 to 0, which is % 100. However, in Rust, % is remainder, which can return negative numbers if the LHS is negative. The modulo operation is provided by rem_euclid.

let mut pos = 50;
let mut count = 0;
for line in data {
    let dir = &line[0..1];
    let dist: i32 = line[1..].parse().unwrap();
    pos = match dir {
        "L" => (pos - dist).rem_euclid(100),
        "R" => (pos + dist).rem_euclid(100),
        c => panic!("Unknown direction: {}", c),
    };
    if pos == 0 {
        count += 1;
    }
}

Part 2

Now we need to detect if a movement from $pos$ to $pos + dist$ passes through 0. The trick is to imagine that the circular path is "unrolled" into a straight line, in which case every multiple of 100 corresponds to a position of 0. So we can calculate the number of multiples of 100 in the range $(pos, pos + dist]$ (or $[pos - dist, pos)$ when moving left). The number of multiples of 100 that are less than or equal to $x$ is $\lfloor x / 100\rfloor$. So the number of multiples of 100 in the range $(pos, pos + dist]$ is $\lfloor (pos + dist) / 100 \rfloor - \lfloor pos / 100 \rfloor$ (those less than or equal to $pos + dist$, but not less than or equal to $pos$). For $[pos - dist, pos)$, it's $\lfloor (pos - 1) / 100 \rfloor - \lfloor (pos - dist - 1) / 100 \rfloor$. Floor division (not rounding to zero) is provided by div_euclid.

let mut pos = 50;
let mut count = 0;
for line in data {
    let dir = &line[0..1];
    let dist: i32 = line[1..].parse().unwrap();
    pos = match dir {
        "L" => {
            count += (pos - 1i32).div_euclid(100) - (pos - dist - 1).div_euclid(100);
            (pos - dist).rem_euclid(100)
        }
        "R" => {
            count += (pos + dist).div_euclid(100) - pos.div_euclid(100);
            (pos + dist).rem_euclid(100)
        }
        c => panic!("Unknown direction: {}", c),
    };
}