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Nicky Meuleman

Posted on • Originally published at nickymeuleman.netlify.app

# Advent of Code 2022 Day 25

## Day 25: Full of Hot Air

TL;DR: my solution in Rust

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 `=`.

SNAFU decimal
0 0
1 1
2 2
- -1
= -2

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!