adventofcode2024/day_01/src/main.rs

use std::collections::HashMap;
use std::env;
use std::error::Error;
use std::fs::File;
use std::io::{BufReader, prelude::*};

fn parse_line(line: &str) -> Result<(i64, i64), Box<dyn Error>> {
    // Split line on whitespace
    // Expecting "88789   64926"
    // Should end up with two string, if not something is wrong.
    let split_line: Vec<_> = line
        .splitn(2, char::is_whitespace)
        .map(|elm| elm.trim())
        .collect();

    // Parse and return our two elements
    let first_element = split_line[0].parse::<i64>()?;
    let second_element = split_line[1].parse::<i64>()?;
    return Ok((first_element, second_element));
}

/// Part 1
///
/// Maybe the lists are only off by a small amount! To find out, pair
/// up the numbers and measure how far apart they are. Pair up the
/// smallest number in the left list with the smallest number in the
/// right list, then the second-smallest left number with the
/// second-smallest right number, and so on.
///
/// Within each pair, figure out how far apart the two numbers are;
/// you'll need to add up all of those distances. For example, if you
/// pair up a 3 from the left list with a 7 from the right list, the
/// distance apart is 4; if you pair up a 9 with a 3, the distance
/// apart is 6.
fn calculate_diff(mut left_list: Vec<i64>, mut right_list: Vec<i64>) -> i64 {
    // sort lists
    left_list.sort();
    right_list.sort();

    let mut accumulator = 0;
    for (index, left_value) in left_list.iter().enumerate() {
        let right_value = right_list[index];
        let absolute_diff = (left_value - right_value).abs();
        accumulator += absolute_diff;
    }

    accumulator
}

/// Part 2
///
/// This time, you'll need to figure out exactly how often each
/// number from the left list appears in the right list. Calculate
/// a total similarity score by adding up each number in the left
/// list after multiplying it by the number of times that number
/// appears in the right list.
fn calculate_similarity(left_list: Vec<i64>, right_list: Vec<i64>) -> i64 {
    // calculate number of occurrences of each id in right list
    let mut right_occurrences: HashMap<i64, i64> = HashMap::new();
    for v in right_list {
        *right_occurrences.entry(v).or_insert(0) += 1;
    }

    // Accumulate result
    let mut accumulator = 0;
    for v in left_list {
        accumulator += v * right_occurrences.get(&v).unwrap_or(&0);
    }

    accumulator
}

fn main() -> Result<(), Box<dyn Error>> {
    // Handle command input
    let args: Vec<String> = env::args().collect();
    if args.len() != 2 {
        panic!(
            "{} must be run with a single argument of files name!",
            &args[0]
        )
    }
    let input_file_string = &args[1];

    let input_file = File::open(input_file_string)?;
    let input_reader = BufReader::new(input_file);

    let mut left_list: Vec<i64> = Vec::new();
    let mut right_list: Vec<i64> = Vec::new();

    for line in input_reader.lines() {
        let line = line?;
        let parsed_line = parse_line(&line)?;
        //println!("({},{})", parsed_line.0, parsed_line.1);
        left_list.push(parsed_line.0);
        right_list.push(parsed_line.1);
    }

    println!(
        "Difference: {}",
        calculate_diff(left_list.clone(), right_list.clone())
    );
    println!(
        "Similarity: {}",
        calculate_similarity(left_list, right_list)
    );

    return Ok(());
}