Nime
Metadata
Table of contents

Advent of Code 2015 Day 1

Day 1: Not Quite Lisp

https://adventofcode.com/2015/day/1

Santa’s weather machine’s snow function is powered by stars. During Advent of Code, each puzzle solution rewards you with a star. Hooray, a convenient way to help.

Santa’s in an apartment building and has directions that tell him to go up or down 1 floor.

  • ( go up one floor
  • ) go down one floor

An example input looks like this:

input.txt
(()(()(

The building is very tall (and deep!), Santa will never reach the top (or bottom) floor.

Santa starts at the ground floor (number 0).

Part 1

The question asks what floor Santa ends at after following the instructions.

This asks to turn a sequence of something into a singular answer.

Ideal for your favourite language’s method that does that. Rust has fold, JavaScript has reduce.

Main code for part 1

day_01.rs
pub fn part_1(input: &str) -> usize {
    input.chars().fold(0, |floor, c| match c {
        '(' => floor + 1,
        ')' => floor - 1,
        _ => panic!("invalid input"),
    })
}

Part 2

The question asks for the position of the first character that causes Santa to enter the basement.

The basement starts at -1.

This was fun, I made 3 variants that do the same thing.

Old reliable

A workhorse. A for loop.

let mut floor = 0;
for (idx, c) in input.chars().enumerate() {
    match c {
        '(' => floor += 1,
        ')' => floor -= 1,
        _ => panic!("invalid input"),
    }
    if floor < 0 {
        return idx + 1;
    }
}
panic!("Santa never enters the basement");

A short-circuiting fold.

In other words, one that stops as soon as a condition is met.

input
    .chars()
    .enumerate()
    .try_fold(0, |floor, (idx, c)| match c {
        '(' => Ok(floor + 1),
        ')' if floor > 0 => Ok(floor - 1),
        ')' => Err(idx + 1),
        _ => panic!("invalid input"),
    })
    .unwrap_err()

My favourite

And finally, my favourite, a solution that creates a sequence of the floor Santa is currently on.

input
    .chars()
    // create iterator where each item is the current floor Santa is on, starting at floor 0
    .scan(0, |floor, c| {
        match c {
            '(' => *floor += 1,
            ')' => *floor -= 1,
            _ => panic!("invalid input"),
        }
        Some(*floor)
    })
    // find the first index where santa enters the basement
    .position(|floor| floor < 0)
    .unwrap()
    + 1

Main code for part 2

day_01.rs
pub fn part_2(input: &str) -> usize {
    input
        .chars()
        // create iterator where each item is the current floor Santa is on, starting at floor 0
        .scan(0, |floor, c| {
            match c {
                '(' => *floor += 1,
                ')' => *floor -= 1,
                _ => panic!("invalid input"),
            }
            Some(*floor)
        })
        // find the first index where santa enters the basement
        .position(|floor| floor < 0)
        .unwrap()
        + 1
}

Final code

day_01.rs
pub fn part_1(input: &str) -> usize {
    input.chars().fold(0, |floor, c| match c {
        '(' => floor + 1,
        ')' => floor - 1,
        _ => panic!("invalid input"),
    })
}


pub fn part_2(input: &str) -> usize {
    input
        .chars()
        // create iterator where each item is the current floor Santa is on, starting at floor 0
        .scan(0, |floor, c| {
            match c {
                '(' => *floor += 1,
                ')' => *floor -= 1,
                _ => panic!("invalid input"),
            }
            Some(*floor)
        })
        // find the first index where santa enters the basement
        .position(|floor| floor < 0)
        .unwrap()
        + 1
}