day_25.rs

// --- Day 25: Full of Hot Air ---
//
// As the expedition finally reaches the extraction point, several large hot air balloons drift
// down to meet you. Crews quickly start unloading the equipment the balloons brought: many hot air
// balloon kits, some fuel tanks, and a fuel heating machine.

// The fuel heating machine is a new addition to the process. When this mountain was a volcano, the
// ambient temperature was more reasonable; now, it's so cold that the fuel won't work at all
// without being warmed up first.

// The Elves, seemingly in an attempt to make the new machine feel welcome, have already attached a
// pair of googly eyes and started calling it "Bob".

// To heat the fuel, Bob needs to know the total amount of fuel that will be processed ahead of
// time so it can correctly calibrate heat output and flow rate. This amount is simply the sum of
// the fuel requirements of all of the hot air balloons, and those fuel requirements are even
// listed clearly on the side of each hot air balloon's burner.

// You assume the Elves will have no trouble adding up some numbers and are about to go back to
// figuring out which balloon is yours when you get a tap on the shoulder. Apparently, the fuel
// requirements use numbers written in a format the Elves don't recognize; predictably, they'd like
// your help deciphering them.

// You make a list of all of the fuel requirements (your puzzle input), but you don't recognize the
// number format either. For example:

// 1=-0-2
// 12111
// 2=0=
// 21
// 2=01
// 111
// 20012
// 112
// 1=-1=
// 1-12
// 12
// 1=
// 122
//
// Fortunately, Bob is labeled with a support phone number. Not to be deterred, you call and ask
// for help.

// "That's right, just supply the fuel amount to the-- oh, for more than one burner? No problem,
// you just need to add together our Special Numeral-Analogue Fuel Units. Patent pending! They're
// way better than normal numbers for--"

// You mention that it's quite cold up here and ask if they can skip ahead.

// "Okay, our Special Numeral-Analogue Fuel Units - SNAFU for short - are sort of like normal
// numbers. You know how starting on the right, normal numbers have a ones place, a tens place, a
// hundreds place, and so on, where the digit in each place tells you how many of that value you
// have?"

// "SNAFU works the same way, except it uses powers of five instead of ten. Starting from the
// right, you have a ones place, a fives place, a twenty-fives place, a
// one-hundred-and-twenty-fives place, and so on. It's that easy!"

// You ask why some of the digits look like - or = instead of "digits".

// "You know, I never did ask the engineers why they did that. Instead of using digits four through
// zero, the digits are 2, 1, 0, minus (written -), and double-minus (written =). Minus is worth
// -1, and double-minus is worth -2."

// "So, because ten (in normal numbers) is two fives and no ones, in SNAFU it is written 20. Since
// eight (in normal numbers) is two fives minus two ones, it is written 2=."

// "You can do it the other direction, too. Say you have the SNAFU number 2=-01. That's 2 in the
// 625s place, = (double-minus) in the 125s place, - (minus) in the 25s place, 0 in the 5s place,
// and 1 in the 1s place. (2 times 625) plus (-2 times 125) plus (-1 times 25) plus (0 times 5)
// plus (1 times 1). That's 1250 plus -250 plus -25 plus 0 plus 1. 976!"

// "I see here that you're connected via our premium uplink service, so I'll transmit our handy
// SNAFU brochure to you now. Did you need anything else?"

// You ask if the fuel will even work in these temperatures.

// "Wait, it's how cold? There's no way the fuel - or any fuel - would work in those conditions!
// There are only a few places in the-- where did you say you are again?"

// Just then, you notice one of the Elves pour a few drops from a snowflake-shaped container into
// one of the fuel tanks, thank the support representative for their time, and disconnect the call.

// The SNAFU brochure contains a few more examples of decimal ("normal") numbers and their SNAFU
// counterparts:

//   Decimal          SNAFU
//         1              1
//         2              2
//         3             1=
//         4             1-
//         5             10
//         6             11
//         7             12
//         8             2=
//         9             2-
//        10             20
//        15            1=0
//        20            1-0
//      2022         1=11-2
//     12345        1-0---0
// 314159265  1121-1110-1=0
//
// Based on this process, the SNAFU numbers in the example above can be converted to decimal
// numbers as follows:

//  SNAFU  Decimal
// 1=-0-2     1747
//  12111      906
//   2=0=      198
//     21       11
//   2=01      201
//    111       31
//  20012     1257
//    112       32
//  1=-1=      353
//   1-12      107
//     12        7
//     1=        3
//    122       37
//
// In decimal, the sum of these numbers is 4890.

// As you go to input this number on Bob's console, you discover that some buttons you expected are
// missing. Instead, you are met with buttons labeled =, -, 0, 1, and 2. Bob needs the input value
// expressed as a SNAFU number, not in decimal.

// Reversing the process, you can determine that for the decimal number 4890, the SNAFU number you
// need to supply to Bob's console is 2=-1=0.

// The Elves are starting to get cold. What SNAFU number do you supply to Bob's console?

fn to_snafu(v: isize) -> String {
    if v == 0 {
        String::new()
    } else {
        let mut s = to_snafu((v + 2) / 5);
        s.push(['0', '1', '2', '=', '-'][v.rem_euclid(5) as usize]);
        s
    }
}

fn from_snafu(s: &str) -> isize {
    let mut v = 0;
    for c in s.chars() {
        v = v * 5
            + match c {
                '2' => 2,
                '1' => 1,
                '0' => 0,
                '-' => -1,
                '=' => -2,
                _ => unreachable!(),
            };
    }
    v
}

pub fn part_1(input: &str) -> String {
    let mut sum = 0;
    for line in input.lines() {
        if line.is_empty() {
            continue;
        }
        sum += from_snafu(line);
    }

    to_snafu(sum)
}

#[cfg(test)]
pub mod tests {
    use crate::day_25::part_1;
    const INPUTS: &str = r#"1=-0-2
12111
2=0=
21
2=01
111
20012
112
1=-1=
1-12
12
1=
122"#;

    #[test]
    pub fn test_day_25_example_part1() {
        assert_eq!(part_1(INPUTS), "2=-1=0");
    }

    #[test]
    pub fn test_day_25_part1() {
        assert_eq!(
            part_1(include_str!("input/day_25.txt")),
            "2=2-1-010==-0-1-=--2"
        );
    }
}