Solve the day 19 puzzle

Author: rvprasad

day19/Cargo.toml

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

day19/README.md

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+Execute this code via `cargo run <filename>`.
+
+### Observations
+

day19/src/main.rs

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+use std::collections::{HashMap, HashSet};
+use std::env;
+use std::fs::File;
+use std::io::{BufRead, BufReader};
+
+fn main() {
+    let args = env::args().collect::<Vec<String>>();
+    if let Ok((patterns, designs)) = get_patterns_and_designs(&args[1]) {
+        println!("{:?}", solve(&patterns, &designs))
+    }
+}
+
+fn solve(patterns: &HashSet<String>, designs: &Vec<String>) -> (usize, u64) {
+    fn calculate_num_of_realizations<'a>(
+        design: &'a str,
+        patterns: &'a HashSet<String>,
+        feasible_designs: &mut HashMap<&'a str, u64>,
+        infeasible_designs: &mut HashSet<&'a str>,
+    ) -> u64 {
+        if feasible_designs.contains_key(design) {
+            return feasible_designs[design];
+        }
+
+        let num_of_realizations = patterns
+            .iter()
+            .filter_map(|p| design.strip_prefix(p).map(|s| (p, s)))
+            .map(|(_, suffix)| {
+                calculate_num_of_realizations(
+                    suffix,
+                    patterns,
+                    feasible_designs,
+                    infeasible_designs,
+                ) as u64
+            })
+            .sum::<u64>();
+
+        feasible_designs.insert(design, num_of_realizations);
+        num_of_realizations
+    }
+
+    let mut feasible_designs_to_num_of_realizations = HashMap::<&str, u64>::new();
+    feasible_designs_to_num_of_realizations.insert("", 1);
+    for d in designs {
+        calculate_num_of_realizations(
+            d,
+            &patterns,
+            &mut feasible_designs_to_num_of_realizations,
+            &mut HashSet::<&str>::new(),
+        );
+    }
+
+    designs.iter().fold((0, 0), |acc, d| {
+        let tmp = feasible_designs_to_num_of_realizations[d.as_str()];
+        (acc.0 + if tmp > 0u64 { 1 } else { 0 }, acc.1 + tmp)
+    })
+}
+
+fn get_patterns_and_designs(filename: &String) -> std::io::Result<(HashSet<String>, Vec<String>)> {
+    let (patterns, designs) = BufReader::new(File::open(filename)?)
+        .lines()
+        .flatten()
+        .fold(
+            (HashSet::<String>::new(), vec![]),
+            |(mut patterns, mut designs), line| {
+                if line.contains(",") {
+                    patterns.extend(line.split(", ").map(String::from));
+                } else if !line.is_empty() {
+                    designs.push(line);
+                }
+                (patterns, designs)
+            },
+        );
+
+    Ok((patterns, designs))
+}