Advent of Code - day 19 part 1

day19a.rs

use std::fs::File;
use std::io::BufReader;
use std::io::BufRead;
use std::collections::HashSet;

fn main() {
    let input_fname = "day19_input.txt";
    let f = File::open(input_fname).unwrap();

    let mut replacements: Vec<(String, String)> = Vec::new();

    let mut reader = BufReader::new(f);
    for line in (&mut reader).lines() {
        let line = line.unwrap();
        if line.len() == 0 {
            break;
        }
        let v: Vec<&str> = line.splitn(2, " => ").collect();
        replacements.push((v[0].to_string(), v[1].to_string()));
    }
    let molecule = reader.lines().next().unwrap().unwrap();

    let mut output_molecules: HashSet<String> = HashSet::new();

    for &(ref from, ref to) in replacements.iter() {
        for (ix, _) in molecule.match_indices(from) {
            let mut out_molecule = molecule[0 .. ix].to_string();
            out_molecule.push_str(to);
            out_molecule.push_str(&molecule[ix + from.len() ..]);
            output_molecules.insert(out_molecule);
        }
    }

    println!("Unique molecules: {}", output_molecules.len());
}