Rust

use crate::utils::read_lines;

pub fn solution1() {
    let lines = read_lines("src/day3/input.txt");
    let sum = lines
        .map(|line| {
            let mut sum = 0;
            let mut command_bytes = Vec::new();
            for byte in line.bytes() {
                match (byte, command_bytes.as_slice()) {
                    (b')', [.., b'0'..=b'9']) => {
                        handle_mul(&mut command_bytes, &mut sum);
                    }
                    _ if matches_mul(byte, &command_bytes) => {
                        command_bytes.push(byte);
                    }
                    _ => {
                        command_bytes.clear();
                    }
                }
            }

            sum
        })
        .sum::<usize>();

    println!("Sum of multiplication results = {sum}");
}

pub fn solution2() {
    let lines = read_lines("src/day3/input.txt");

    let mut can_mul = true;
    let sum = lines
        .map(|line| {
            let mut sum = 0;
            let mut command_bytes = Vec::new();
            for byte in line.bytes() {
                match (byte, command_bytes.as_slice()) {
                    (b')', [.., b'0'..=b'9']) if can_mul => {
                        handle_mul(&mut command_bytes, &mut sum);
                    }
                    (b')', [b'd', b'o', b'(']) => {
                        can_mul = true;
                        command_bytes.clear();
                    }
                    (b')', [.., b't', b'(']) => {
                        can_mul = false;
                        command_bytes.clear();
                    }
                    _ if matches_do_or_dont(byte, &command_bytes)
                        || matches_mul(byte, &command_bytes) =>
                    {
                        command_bytes.push(byte);
                    }
                    _ => {
                        command_bytes.clear();
                    }
                }
            }

            sum
        })
        .sum::<usize>();

    println!("Sum of enabled multiplication results = {sum}");
}

fn matches_mul(byte: u8, command_bytes: &[u8]) -> bool {
    matches!(
        (byte, command_bytes),
        (b'm', [])
            | (b'u', [.., b'm'])
            | (b'l', [.., b'u'])
            | (b'(', [.., b'l'])
            | (b'0'..=b'9', [.., b'(' | b'0'..=b'9' | b','])
            | (b',', [.., b'0'..=b'9'])
    )
}

fn matches_do_or_dont(byte: u8, command_bytes: &[u8]) -> bool {
    matches!(
        (byte, command_bytes),
        (b'd', [])
            | (b'o', [.., b'd'])
            | (b'n', [.., b'o'])
            | (b'\'', [.., b'n'])
            | (b'(', [.., b'o' | b't'])
            | (b't', [.., b'\''])
    )
}

fn handle_mul(command_bytes: &mut Vec<u8>, sum: &mut usize) {
    let first_num_index = command_bytes
        .iter()
        .position(u8::is_ascii_digit)
        .expect("Guarunteed to be there");
    let comma_index = command_bytes
        .iter()
        .position(|&c| c == b',')
        .expect("Guarunteed to be there.");

    let num1 = bytes_to_num(&command_bytes[first_num_index..comma_index]);
    let num2 = bytes_to_num(&command_bytes[comma_index + 1..]);

    *sum += num1 * num2;
    command_bytes.clear();
}

fn bytes_to_num(bytes: &[u8]) -> usize {
    bytes
        .iter()
        .rev()
        .enumerate()
        .map(|(i, digit)| (*digit - b'0') as usize * 10usize.pow(i as u32))
        .sum::<usize>()
}

Definitely not my prettiest code ever. It would probably look nicer if I used regex or some parsing library, but I took on the self-imposed challenge of not using third party libraries. Also, this is already further than I made it last year!

Python

def process(input, part2=False):
    if part2:
        input = re.sub(r'don\'t\(\).+?do\(\)', '', input) # remove everything between don't() and do()
    total = [ int(i[0]) * int(i[1]) for i in re.findall(r'mul\((\d+),(\d+)\)', input) ]
    return sum(total)

Given the structure of the input file, we just have to ignore everything between don’t() and do(), so remove those from the instructions before processing.

I couldn’t figure it out in haskell, so I went with bash for the first part

Shell

cat example | grep -Eo "mul\([[:digit:]]{1,3},[[:digit:]]{1,3}\)" | cut -d "(" -f 2 | tr -d ")" | tr "," "*" | paste -sd+ | bc

but this wouldn’t rock anymore in the second part, so I had to resort to python for it

Python

import sys

f = "\n".join(sys.stdin.readlines())

f = f.replace("don't()", "\ndon't()\n")
f = f.replace("do()", "\ndo()\n")

import re

enabled = True
muls = []
for line in f.split("\n"):
    if line == "don't()":
        enabled = False
    if line == "do()":
        enabled = True
    if enabled:
        for match in re.finditer(r"mul\((\d{1,3}),(\d{1,3})\)", line):
            muls.append(int(match.group(1)) * int(match.group(2)))
        pass
    pass

print(sum(muls))

Rust feat. pest

No Zalgo here! I wasted a huge amount of time by not noticing that the second part’s example input was different - my code worked fine but my test failed 🤦‍♂️

pest.rs is lovely, although part two made my PEG a bit ugly.

part1    =  { SOI ~ (mul_expr | junk)+ ~ EOI }
part2    =  { (enabled | disabled)+ ~ EOI }
mul_expr =  { "mul(" ~ number ~ "," ~ number ~ ")" }
number   =  { ASCII_DIGIT{1,3} }
junk     = _{ ASCII }
on       = _{ "do()" }
off      = _{ "don't()" }
enabled  = _{ (SOI | on) ~ (!(off) ~ (mul_expr | junk))+ }
disabled = _{ off ~ (!(on) ~ junk)+ }

use std::fs;

use color_eyre::eyre;
use pest::Parser;
use pest_derive::Parser;

#[derive(Parser)]
#[grammar = "memory.pest"]
pub struct MemoryParser;

fn parse(input: &str, rule: Rule) -> eyre::Result<usize> {
    let sum = MemoryParser::parse(rule, input)?
        .next()
        .expect("input must be ASCII")
        .into_inner()
        .filter(|pair| pair.as_rule() == Rule::mul_expr)
        .map(|pair| {
            pair.into_inner()
                .map(|num| num.as_str().parse::<usize>().unwrap())
                .product::<usize>()
        })
        .sum();
    Ok(sum)
}

fn part1(filepath: &str) -> eyre::Result<usize> {
    let input = fs::read_to_string(filepath)?;
    parse(&input, Rule::part1)
}

fn part2(filepath: &str) -> eyre::Result<usize> {
    let input = fs::read_to_string(filepath)?;
    parse(&input, Rule::part2)
}

fn main() -> eyre::Result<()> {
    color_eyre::install()?;

    let part1 = part1("d03/input.txt")?;
    let part2 = part2("d03/input.txt")?;
    println!("Part 1: {part1}\nPart 2: {part2}");
    Ok(())
}

Kotlin

fun part1(input: String): Int {
    val pattern = "mul\\((\\d{1,3}),(\\d{1,3})\\)".toRegex()
    var sum = 0
    pattern.findAll(input).forEach { match ->
        val first = match.groups[1]?.value?.toInt()!!
        val second = match.groups[2]?.value?.toInt()!!
        sum += first * second

    }
    return sum
}

fun part2(input: String): Int {
    val pattern = "mul\\((\\d{1,3}),(\\d{1,3})\\)|don't\\(\\)|do\\(\\)".toRegex()
    var sum = 0
    var enabled = true
    pattern.findAll(input).forEach { match ->
        if (match.value == "do()") enabled = true
        else if (match.value == "don't()") enabled = false
        else if (enabled) {
            val first = match.groups[1]?.value?.toInt()!!
            val second = match.groups[2]?.value?.toInt()!!
            sum += first * second
        }
    }
    return sum
}