day1.md

Day 1: The Tyranny of the Rocket Equation, Part 1

Part one of excercise is actually trivial. What needs to be done, is to read input line by line, calculate input / 3 - 2 (input is some rocket mass, and the result is fuel needed for travel for this rocket, and sum everything up. All of this can be done easly using iterators:

pub async fn solution(input: impl Stream<Item = i64>) -> i64 {
    input.map(|mass| mass / 3 - 2).sum().await
}

Day 1: The Tyranny of the Rocket Equation, Part 2

After applying the result, second part of excercise got revealed. This time elves found out, that rocket fuel also has own mass, so after fuel calculation, there need to be another round - this time calculation of additional fuel needed to carry the previously calculated fuel batch, and the process needs to be repeated, unless additional fuel needed for carrying the rocket is 0.

First what I decided to do is to create iterator, where every item is fuel needed to carry on mass of previous item, like: [rocket-mass, fuel-for-0, fuel-for-1, ..].

Such an iterator would be infinite, but it is easly visible, that those values are strictly decreasing and will eventually achieve 0.

Creating such an iterator is easy with function std::iter::successors:

successors(Some(mass), |mass| Some(*mass / 3 - 2)).skip(1)

I'm skipping the first element, because this would be the initial mass of the rocket, which is not interesting for me.

Next think to do is to limit the infinite iterator, to stop when first 0 is found, and sum needed fuel:

successors(Some(mass), |mass| Some(*mass / 3 - 2))
    .skip(1)
    .take_while(|fuel| *fuel > 0)
    .sum()

What left is an easy part - calculation need to be done for every input line, and then just sum every partial result:

pub async fn extended(input: impl Stream<Item = i64>) -> i64 {
    let partials = |mass| {
        successors(Some(mass), |mass| Some(*mass / 3 - 2))
            .skip(1)
            .take_while(|fuel| *fuel > 0)
            .sum()
    };

    input.map(partials).sum().await
}