Day 4

src/bin/day04.rs

@@ -0,0 +1,67 @@
+use std::env;
+use std::fs;
+
+use std::collections::HashSet;
+
+use anyhow::{Context, Result};
+use regex::Regex;
+
+fn main() -> Result<()> {
+    let filename = env::args()
+        .nth(1)
+        .context("./day04 <path to puzzle input>")?;
+    let input = fs::read_to_string(filename)?;
+
+    let re = Regex::new(r"\d+")?;
+
+    let lines = input.lines().collect::<Vec<_>>();
+
+    let mut count: Vec<usize> = vec![1; lines.len()];
+
+    let mut part1 = 0;
+    for (line_num, line) in lines.into_iter().enumerate() {
+        let mut hs = HashSet::new();
+
+        let (_, card_text) = line.split_once(':').unwrap();
+        let (winning, mine) = card_text.split_once('|').unwrap();
+
+        let winning_nums = re
+            .find_iter(winning)
+            .map(|m| m.as_str().parse::<u32>().unwrap());
+
+        let my_nums = re
+            .find_iter(mine)
+            .map(|m| m.as_str().parse::<u32>().unwrap());
+
+        for i in winning_nums {
+            hs.insert(i);
+        }
+
+        let mut num_matches: usize = 0;
+
+        for i in my_nums {
+            if hs.contains(&i) {
+                num_matches += 1;
+            }
+        }
+
+        for i in line_num + 1..line_num + num_matches + 1 {
+            if i >= count.len() {
+                break;
+            }
+
+            count[i] += count[line_num];
+        }
+
+		if num_matches > 0 {
+			part1 += 1 << (num_matches - 1);
+		}
+    }
+
+    let part2 = count.into_iter().sum::<usize>();
+
+    println!("1) {}", part1);
+    println!("2) {}", part2);
+
+    Ok(())
+}