Advent of Code 2022 Day 25

Day 25: Full of Hot Air

https://adventofcode.com/2022/day/25

The expedition reached the last part, a trip to the north pole via hot air balloon!

It is so cold on the mountain that the fuel needs to be pre-heated. A machine that does this needs to know the total amount to process before it will work.

Today’s input is the fuel requirements for each balloon, written in a weird number format.

An example input looks like this:

1=-0-2
12111
2=0=
21
2=01
111
20012
112
1=-1=
1-12
12
1=
122

So the amount the machine (nicknamed “Bob”) has to process is the sum of all those numbers.

The numbers are in a format called SNAFU.

It’s similar to decimal. Where decimal is based on powers of 10, SNAFU is based on powers of 5.

• In decimal, starting from the right, you have a ones place, a tens place, a hundreds place, …
• In SNAFU, starting from the right, you have a ones place, a fives place, a twenty-fives place, …

The digits are similar too, with two weird ones, -, and =.

Some examples converting decimal to SNAFU:

• 10 in decimal is two fives and no ones, in SNAFU it is written 20.
• 8 in decimal iss two fives minus two ones, in SNAFU it is written 2=.

And one in the other direction:

For the SNAFU number 2=-01

• 2 in the 625s place
• = in the 125s place
• - in the 25s place
• 0 in the 5s place
• 1 in the 1s place

(2 * 625) + (-2 * 125) + (-1 * 25) + (0 * 5) + (1 * 1). That’s 1250 + -250 + -25 + 0 + 1. 976

Part 1

The question asks what the SNAFU number you have to supply to “Bob“‘s console is.

Helpers

A function that converts SNAFU string to a decimal number.

With a running total that starts at 0: Look at a single SNAFU digit at a time, converting it to decimal. To get the new running total: multiply the current running total by 5 and add the converted digit.

fn to_decimal(snafu: &str) -> i64 {
    snafu.chars().fold(0, |decimal, snafu_digit| {
        let decimal_digit = ['=', '-', '0', '1', '2']
            .into_iter()
            .position(|c| c == snafu_digit)
            .unwrap() as i64
            - 2;
        decimal * 5 + decimal_digit
    })
}

A function that converts a decimal number to a SNAFU string.

Recursion!

It calculates the last SNAFU digit and adds it the the end of all the other SNAFU digits. It does this by using that base 5/power of 5 property. Keeping into account SNAFU digits start at -2.

fn to_snafu(decimal: i64) -> String {
    if decimal == 0 {
        return String::new();
    }

    let decimal_remainder = decimal % 5;
    let snafu_digit = ['0', '1', '2', '=', '-'][decimal_remainder as usize];

    let new_decimal = (decimal + 2) / 5;
    let mut snafu = to_snafu(new_decimal);
    snafu.push(snafu_digit);

    snafu
}

Using those helpers, the final code:

• Converts all input SNAFU numbers to decimal.
• Sums all those decimal numbers.
• Converts that decimal sum back to SNAFU.

Final code

pub fn part_1(input: &str) -> String {
    let sum = input.lines().map(to_decimal).sum();
    to_snafu(sum)
}

fn to_decimal(snafu: &str) -> i64 {
    snafu.chars().fold(0, |decimal, snafu_digit| {
        let decimal_digit = ['=', '-', '0', '1', '2']
            .into_iter()
            .position(|c| c == snafu_digit)
            .unwrap() as i64
            - 2;
        decimal * 5 + decimal_digit
    })
}

fn to_snafu(decimal: i64) -> String {
    if decimal == 0 {
        return String::new();
    }

    let new_decimal = (decimal + 2) / 5;
    let mut snafu = to_snafu(new_decimal);

    let decimal_remainder = decimal % 5;
    let snafu_digit = ['0', '1', '2', '=', '-'][decimal_remainder as usize];
    snafu.push(snafu_digit);

    snafu
}

Part 2

An Elf hands you a single star. Combined with the 49 you gathered on the expedition you make a smoothie and deliver it to the reindeer!