Day 01 - Inverse Captcha

Language: Rust

Problem https://adventofcode.com/2017/day/1

Part 1: A string of digits treated as a circular list. Sum all digits that match the next one, with the last digit wrapping back to the first.

I zip the string with itself offset by one to compare consecutive pairs, then handle the wrap manually at the end:

for (c1, c2) in input.chars().zip(input.chars().skip(1)) {
    if c1 == c2 {
        sum += c1.to_digit(10).unwrap();
    }
}
if let (Some(first), Some(last)) = (input.chars().next(), input.chars().last()) {
    if first == last {
        sum += first.to_digit(10).unwrap();
    }
}

Part 2: Now compare each digit to the one halfway around the list. Since both halves are symmetric, every match contributes twice. I zip the first half with the second half and multiply each match by 2:

let ans: u32 = input.chars()
    .take(input.len() / 2)
    .zip(input.chars().skip(input.len() / 2))
    .filter(|(a, b)| a == b)
    .map(|(c, _)| 2 * c.to_digit(10).unwrap())
    .sum();

Solution: https://github.com/Elyrial/AdventOfCode/blob/main/src/solutions/year2017/day01.rs

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