Day 19

Author: finnhartshorn

data/examples/19.txt

@@ -0,0 +1,10 @@
+r, wr, b, g, bwu, rb, gb, br
+
+brwrr
+bggr
+gbbr
+rrbgbr
+ubwu
+bwurrg
+brgr
+bbrgwb

src/bin/19.rs

@@ -0,0 +1,101 @@
+use std::collections::HashMap;
+
+advent_of_code::solution!(19);
+
+pub fn part_one(input: &str) -> Option<u32> {
+    let mut lines = input.lines();
+    let towels: Vec<&str> = lines.next().unwrap().split(", ").collect();
+    lines.next(); // Skip empty line
+
+    let mut memo: HashMap<&str, bool> = HashMap::new();
+
+    Some(
+        lines
+            .filter_map(|line| {
+                if get_if_possible(line, &towels, &mut memo) {
+                    Some(1)
+                } else {
+                    None
+                }
+            })
+            .sum(),
+    )
+}
+
+fn get_num_possible_combinations<'a>(
+    towel: &'a str,
+    possible_towels: &[&str],
+    memo: &mut HashMap<&'a str, u64>,
+) -> u64 {
+    if towel.is_empty() {
+        return 1;
+    }
+    if memo.contains_key(towel) {
+        return memo[towel];
+    }
+    let p = possible_towels
+        .iter()
+        .map(|pt| {
+            if let Some(new_towel) = towel.strip_prefix(pt) {
+                get_num_possible_combinations(new_towel, possible_towels, memo)
+            } else {
+                0
+            }
+        })
+        .sum();
+    memo.insert(towel, p);
+    p
+}
+
+fn get_if_possible<'a>(
+    towel: &'a str,
+    possible_towels: &[&str],
+    memo: &mut HashMap<&'a str, bool>,
+) -> bool {
+    if towel.is_empty() {
+        return true;
+    }
+    if memo.contains_key(towel) {
+        return memo[towel];
+    }
+    let p = possible_towels.iter().any(|pt| {
+        if let Some(new_towel) = towel.strip_prefix(pt) {
+            get_if_possible(new_towel, possible_towels, memo)
+        } else {
+            false
+        }
+    });
+    memo.insert(towel, p);
+    p
+}
+
+pub fn part_two(input: &str) -> Option<u64> {
+    let mut lines = input.lines();
+    let towels: Vec<&str> = lines.next().unwrap().split(", ").collect();
+    lines.next(); // Skip empty line
+
+    let mut memo: HashMap<&str, u64> = HashMap::new();
+
+    Some(
+        lines
+            .map(|line| get_num_possible_combinations(line, &towels, &mut memo))
+            .sum(),
+    )
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_part_one() {
+        let result = part_one(&advent_of_code::template::read_file("examples", DAY));
+        assert_eq!(result, Some(6));
+    }
+
+    #[test]
+    fn test_part_two() {
+        let result = part_two(&advent_of_code::template::read_file("examples", DAY));
+        assert_eq!(result, Some(16));
+    }
+}