Solution Day 19

2024/day19/Cargo.lock

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+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "day19"
+version = "0.1.0"

2024/day19/Cargo.toml

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+[package]
+name = "day19"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]

2024/day19/README.txt

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+# Solution Day 19
+
+The full solutions are getting rarer and rarer. Today's topic was building strings from a fixed set of substrings.
+
+## Task 1
+
+The initial solution to task 1 stopped after finding one solution (see the commented out code for `is_pattern_possible(...)`).
+My main idea was checking, whether a substring (towel) started the pattern and then recursively check whether the remaining
+pattern could also be formed. If so, the pattern would be valid.
+
+Initially I ran into an endless loop with one of my inputs. Reason being: the recursive check lead to a situation, where the
+implementation would check the same invalid suffix again and again, every time finding a way to start it, but not finish it.
+I could solve this by tracking the known bad suffixes in a separate `HashMap` and short-circuiting before entering the
+recursive loop again.
+
+## Task 2
+
+Task 2 looks like a slight extension of Task 1 (just count all combinations instead of stopping at the first), but it proved
+to be much harder initially. With my approach of tracking the known bad suffixes, I simply could not prevent the endless
+loop even at the first input. Even now I'm not 100% sure, why. I'm not even sure, whether this is really an endless loop or just
+very, very inefficient checking the same suffix again and again.
+
+After some more thought I decided to switch to a memoization technique: for each string store the number of known solutions and
+short-circuit when you already know the solution. This did the trick and I got my solution. This way I'm even able to express
+Task 1 in terms of the solver for Task 2: a valid pattern is any pattern, where the number of possible combinations is greater 0.

2024/day19/src/main.rs

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+use std::collections::{HashMap, HashSet};
+
+fn load_input() -> (Vec<String>, Vec<String>) {
+    let mut data = include_str!("../input.txt").lines();
+
+    let mut available_towels: Vec<String> = data
+        .next()
+        .unwrap()
+        .split(", ")
+        .map(|s| s.trim().to_string())
+        .filter(|s| s.len() > 0)
+        .collect();
+    available_towels.sort_by_key(|s| 100 - s.len());
+
+    data.next();
+
+    let desired_patterns: Vec<String> = data.map(|s| s.to_string()).collect();
+
+    (available_towels, desired_patterns)
+}
+
+// fn is_pattern_possible(
+//     desired_pattern: &str,
+//     available_towels: &Vec<String>,
+//     known_bad: &mut HashSet<String>, // known bad patterns that cannot be built
+// ) -> bool {
+//     if known_bad.contains(desired_pattern) {
+//         return false;
+//     }
+//     for towel in available_towels {
+//         if desired_pattern.starts_with(towel) {
+//             if desired_pattern.len() == towel.len() {
+//                 return true; // complete match, we found something
+//             }
+
+//             if is_pattern_possible(&desired_pattern[towel.len()..], available_towels, known_bad) {
+//                 return true;
+//             }
+//         }
+//     }
+//     known_bad.insert(desired_pattern.to_string());
+//     return false;
+// }
+
+fn find_possible_solutions(
+    desired_pattern: &str,
+    available_towels: &Vec<String>,
+    known_solutions: &mut HashMap<String, usize>,
+) -> usize {
+    if let Some(known) = known_solutions.get(desired_pattern) {
+        return *known;
+    }
+
+    let mut possible_solutions = 0;
+    for towel in available_towels {
+        if desired_pattern.starts_with(towel) {
+            if desired_pattern.len() == towel.len() {
+                // exact match, count and continue
+                possible_solutions += 1;
+            } else {
+                possible_solutions += find_possible_solutions(
+                    &desired_pattern[towel.len()..],
+                    &available_towels,
+                    known_solutions,
+                );
+            }
+        }
+    }
+
+    known_solutions.insert(desired_pattern.to_string(), possible_solutions);
+    possible_solutions
+}
+
+fn task1() {
+    let (available_towels, desired_patterns) = load_input();
+
+    let total_possible_patterns = desired_patterns
+        .iter()
+        .filter(|&pattern| {
+            find_possible_solutions(&pattern, &available_towels, &mut HashMap::new()) > 0
+        })
+        .count();
+
+    println!("Task 1: possible patterns: {total_possible_patterns}");
+}
+
+fn task2() {
+    let (available_towels, desired_patterns) = load_input();
+
+    let total_possible_combinations = desired_patterns.iter().fold(0, |existing, pattern| {
+        existing + find_possible_solutions(&pattern, &available_towels, &mut HashMap::new())
+    });
+
+    println!("Task 2: possible combinations: {total_possible_combinations}");
+}
+
+fn main() {
+    task1();
+    task2();
+}