Posted: 2023-12-11
Last modified: 2024-12-01 @ 4568f16

Advent of Code 2023: Day 09

This post is part of the following series: Advent of Code 2023, Advent of Code.

Code for solutions available on GitHub.

Another post under the influence of covid. Like day 8, I’ll be brief.

Mirage Maintenance

This is a pretty simple one. The input is a list of time sequences:

0 3 6 9 12 15
1 3 6 10 15 21
10 13 16 21 30 45

And our job is to predict the next number for each sequence and sum up the values of all predictions. A prediction is made by repeatedly taking a discrete derivative of the sequence until reaching a constant state, predicting the next value of that, then bubbling up. See the actual problem description for a really good explanation of the problem. I feel like I can’t reword in in a better way than the problem was already put.

Basically, If we have a sequence of all zeroes, the next value will be zero. If this is not the case, we create a new sequence from the pairwise difference of numbers in the sequence, predict the next value of that, and the next value of the current sequence is the last value plus the predicted value of the sequence of differences. Here’s the function that does just that (with help from the itertools crate):

fn predict_value(values: Vec<i32>) -> i32 {
    if values.iter().all(|x| *x == 0) {
        return 0;
    }

    let lower_sequence = values
        .iter()
        .tuple_windows()
        .map(|(a, b)| b - a)
        .collect_vec();
    let last = values.last().unwrap();
    predict_value(lower_sequence) + last
}

That’s it. That’s the puzzle.

Part 2

Part 2 asks to predict a value backwards in time, i.e. what was the value before the first? Just reverse each sequence and call the same function.

Other posts in Advent of Code 2023

  1. Advent of Code 2023: Day 01
  2. Advent of Code 2023: Day 02
  3. Advent of Code 2023: Day 03
  4. Advent of Code 2023: Day 04
  5. Advent of Code 2023: Day 05
  6. Advent of Code 2023: Day 06
  7. Advent of Code 2023: Day 07
  8. Advent of Code 2023: Day 08
  9. Advent of Code 2023: Day 09
  10. Advent of Code 2023: Day 10
  11. Advent of Code 2023: Day 11

Other posts in Advent of Code

  1. Advent of Code 2023: Day 01
  2. Advent of Code 2023: Day 02
  3. Advent of Code 2023: Day 03
  4. Advent of Code 2023: Day 04
  5. Advent of Code 2023: Day 05
  6. Advent of Code 2023: Day 06
  7. Advent of Code 2023: Day 07
  8. Advent of Code 2023: Day 08
  9. Advent of Code 2023: Day 09
  10. Advent of Code 2023: Day 10
  11. Advent of Code 2023: Day 11
  12. Advent of Code 2024: Day 01
  13. Advent of Code 2024: Day 02
  14. Advent of Code 2024: Day 03
  15. Advent of Code 2024: Day 04
  16. Advent of Code 2024: Day 05
  17. Advent of Code 2024: Day 06
  18. Advent of Code 2024: Day 07
  19. Advent of Code 2024: Day 08