I'm fairly new to go and I've recently migrated a in-memory cache from node to go for concurrency improvements, but the memory usage difference between the two are huge. I've tried to read up on the map memory model in go but haven't been able to find a reason for the difference. I can't see that I'm doing anything special, so I'm looking for guidance here.

The documents that are stored are around 8 KB in size as a JSON file. In node the memory usage for 50000 documents stored as objects is 1,5 GB, and for go maps it is 10 GB.

To me, this doesn't seem reasonable but I can't find the source of the difference. Could anyone here give their take on this?

Sorry if this is a stupid question, but I'm struggling with what I think is ownership in rust.

I'm completely new to rust, but have done low level languages before so I know the concept of pointers pretty well. However, this rust ownership I can't quite wrap my head around.

I'm trying to refactor my solution to AoC Day 4 Part 2 using a struct reference instead of a stand-alone vector.

The error I'm getting, and can't figure out is in the process function at line

cards.iter_mut().for_each(|card | {

The error is

cannot borrow cards as mutable more than once at a time second mutable borrow occurs here

There is a lot of parsing in my code, so I stuck that in a spoiler below.

The relevant code is:

#[derive(Debug)]
struct Card {
    id: u32,
    score: u32,
    copies: u32,
}

fn process(input: &str) -> u32 {
    let mut cards: Vec = parse_cards(input);

    cards.iter_mut().for_each(|card| {
        let copy_from = card.id as usize + 1;
        let copy_to: usize = copy_from + card.score as usize - 1;

        if card.score == 0 || copy_from > cards.len() {
            return;
        }

        for card_copy in cards[copy_from - 1..copy_to].iter() {
            let id = card_copy.id as usize - 1;
            let add = cards[card.id as usize - 1].copies;
            cards[id].copies += add;
        }
    });

    return cards.iter().map(|c| c.copies).sum();
}

Other code:

fn main() {
    let input = include_str!("./input1.txt");
    let output = process(input);
    dbg!(output);
}

fn parse_cards(input: &str) -> Vec {
    return input.lines().map(|line| parse_line(line)).collect();
}

fn parse_line(line: &str) -> Card {
    let mut card_split = line.split(':');
    let id = card_split
        .next()
        .unwrap()
        .replace("Card", "")
        .trim()
        .parse::()
        .unwrap();

    let mut number_split = card_split.next().unwrap().trim().split('|');

    let winning: Vec = number_split
        .next()
        .unwrap()
        .trim()
        .split_whitespace()
        .map(|nbr| nbr.trim().parse::().unwrap())
        .collect();
    let drawn: Vec = number_split
        .next()
        .unwrap()
        .trim()
        .split_whitespace()
        .map(|nbr| nbr.trim().parse::().unwrap())
        .collect();

    let mut score = 0;

    for nbr in &drawn {
        if winning.contains(&nbr) {
            score = score + 1;
        }
    }

    return Card {
        id,
        score,
        copies: 1,
    };
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn full_test() {
        let result = process(
            "Card 1: 41 48 83 86 17 | 83 86  6 31 17  9 48 53
        Card 2: 13 32 20 16 61 | 61 30 68 82 17 32 24 19
        Card 3:  1 21 53 59 44 | 69 82 63 72 16 21 14  1
        Card 4: 41 92 73 84 69 | 59 84 76 51 58  5 54 83
        Card 5: 87 83 26 28 32 | 88 30 70 12 93 22 82 36
        Card 6: 31 18 13 56 72 | 74 77 10 23 35 67 36 11",
        );
        assert!(result != 0);
        assert_eq!(result, 30);
    }
}