Day 22

2024/day22/Cargo.lock

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+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "day22"
+version = "0.1.0"

2024/day22/Cargo.toml

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+[package]
+name = "day22"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]

2024/day22/README.md

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+# Solution Day 22
+
+One of the few later ones I actually did on the day. The pseudo-random number generator is easy enough to
+implement. If you read correctly, that is… I read "round to the nearest integer" first and was slightly off 
+with my numbers, until I saw that little "down" in there, which makes the whole thing _much_ easier. You don't
+actually need to round, you can just do integer division, which will cut off the fractional part, leaving you
+with the already rounded-down number. 
+
+## Task 1
+
+After implementing the pseudo-random generator, it's easy enough to run it 2000x for each start number and
+sum up the resulting numbers. Nothing special to see here.
+
+## Task 2
+
+This one was more complex. My initial naïve approach was to calculate all 4-digit sub-sequences, search for them
+in all sequences and sum up the assiciated price. While this would probably yield the result _at some point_, the
+complexity is _O(n² * m²)_ (with _n_ being the number of starting numbers and _m_ being the sequence length for each
+starting number, assuming I got my maths right). Let's just say: not fun, definitely too slow.
+
+My less naïve approach involves pre-calculating the first price of each sub-sequence for each starting number and
+then afterwards going through them, summing them up grouped by sub-sequence and finding the maximum number of those
+sums. This involves the heavy use of `HashMap`s (to keep track of the found sub-sequences and their associated number),
+but brings complexity down to _ O(n * m)_ (for all three steps, actually. Pre-calculating the sequences and prices is _O(n * m)_,
+calculating the sums and then finding the maximum is as well). This is then actually fast enough to finish before I
+get bored. 

2024/day22/src/main.rs

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+#![feature(map_try_insert)]
+
+use std::collections::{HashMap, VecDeque};
+
+fn mix_and_prune(intermediate: i64, number: i64) -> i64 {
+    (intermediate ^ number) % 16777216
+}
+
+fn next_secret_number(number: i64) -> i64 {
+    let mut result = mix_and_prune(number * 64, number);
+    result = mix_and_prune(result / 32, result);
+    result = mix_and_prune(result * 2048, result);
+    result
+}
+
+fn nth_secret_number(start: i64, n: u32) -> i64 {
+    let mut result = start;
+    for _ in 0..n {
+        result = next_secret_number(result);
+    }
+    result
+}
+
+fn task1() {
+    let numbers = include_str!("../input.txt")
+        .lines()
+        .map(|line| line.parse::<i64>().unwrap());
+
+    let sum: i64 = numbers.map(|number| nth_secret_number(number, 2000)).sum();
+
+    println!("Task 1: sum {sum}");
+}
+
+fn calculate_sequence(number: i64) -> HashMap<Vec<i64>, i64> {
+    let mut current_number = number;
+    let mut result = HashMap::new();
+    let mut current_pattern = VecDeque::new();
+    for _ in 0..2000 {
+        let next_number = next_secret_number(current_number);
+        let next_last_digit = next_number % 10;
+        let current_last_digit = current_number % 10;
+        current_pattern.push_back(next_last_digit - current_last_digit);
+        if current_pattern.len() == 4 {
+            // we're only ever interested in the first occurence of a pattern in the sequence, because that's what counts
+            let _ = result.try_insert(
+                Vec::from_iter(current_pattern.iter().cloned()),
+                next_last_digit,
+            );
+            current_pattern.pop_front(); // throw away the first digit. The pattern is a sliding window over the sequence
+        }
+        current_number = next_number;
+    }
+    result
+}
+
+fn task2() {
+    let numbers = include_str!("../input.txt")
+        .lines()
+        .map(|line| line.parse::<i64>().unwrap());
+
+    let sequences: Vec<_> = numbers.map(calculate_sequence).collect();
+
+    let mut banana_counters = HashMap::new();
+    for sequence in sequences.iter() {
+        for (pattern, price) in sequence.iter() {
+            banana_counters.insert(
+                pattern,
+                banana_counters.get(pattern).unwrap_or(&0_i64) + price,
+            );
+        }
+    }
+
+    let sum = banana_counters.values().max();
+
+    println!("Task 2: sum: {sum:?}");
+}
+
+fn main() {
+    task1();
+    task2();
+}