Discussion on: Advent of Code 2020 Solution Megathread - Day 1: Report Repair

[Benjamin Trent]

Here is my solution in rust (minus all the code required to grab the input and parse it)

fn sums_to<'a>(
    sum: &'_ usize,
    curr: &'a usize,
    rest: &'a [usize],
) -> Option<(&'a usize, &'a usize)> {
    if rest.len() == 0 {
        return Option::None;
    }
    for re in rest {
        if re + curr == *sum {
            return Option::Some((curr, re));
        }
    }
    sums_to(sum, &rest[0], &rest[1..])
}

fn tri_sums_to<'a>(
    sum: &'_ usize,
    x: &'a usize,
    rest: &'a [usize],
) -> Option<(&'a usize, &'a usize, &'a usize)> {
    if rest.len() == 0 {
        return Option::None;
    }
    if x < sum {
        let sub_sum = *sum - x;
        for i in 0..(rest.len() - 1) {
            if let Some((a, b)) = sums_to(&sub_sum, &rest[i], &rest[i + 1..]) {
                return Option::Some((x, a, b));
            }
        }
    }
    tri_sums_to(sum, &rest[0], &rest[1..])
}

async fn day1_1() -> Result<usize> {
    let input = input_to_vec(1).await?;
    let (v1, v2) = sums_to(&2020, &input[0], &input[1..]).unwrap();
    Ok(v1 * v2)
}

async fn day1_2() -> Result<usize> {
    let input = input_to_vec(1).await?;
    let (v1, v2, v3) = tri_sums_to(&2020, &input[0], &input[1..]).unwrap();
    Ok(v1 * v2 * v3)
}

[Christopher Kruse]

Hey - I'm working on the problems in Rust this year, too! I'm putting all mine in Github, but will share here, too.

(I found a cool cargo plugin that provides helpful macros and makes doing AoC stuff easier - cargo-aoc)

use aoc_runner_derive::{aoc, aoc_generator};

#[aoc_generator(day1)]
fn parse_input_day1(input: &str) -> Vec<usize> {
    input
        .lines()
        .map(|v| v.parse().expect("Failed to parse input!"))
        .collect()
}

#[aoc(day1, part1)]
fn find_2020_entries(input: &Vec<usize>) -> usize {
    let pair = input.iter().enumerate().find_map(|(idx, &item)| {
        match input
            .iter()
            .enumerate()
            .find(|(idx2, &second)| idx2 != &idx && (item + second == 2020))
        {
            Some((_, second)) => Some(item * second),
            None => None,
        }
    });
    pair.unwrap_or(1)
}

#[aoc(day1, part2)]
fn find_2020_entries_with_three(input: &Vec<usize>) -> usize {
    let result = input.iter().enumerate().find_map(|(idx, &item)| {
        input.iter().enumerate().find_map(|(idx2, &second)| {
            match input.iter().enumerate().find(|(idx3, &third)| {
                idx2 != idx && &idx != idx3 && &idx2 != idx3 && (item + second + third == 2020)
            }) {
                Some((_, third)) => Some(item * second * third),
                None => None,
            }
        })
    });
    result.unwrap_or(1)
}