Add day 6 part 1

src/day6.rs

@@ -0,0 +1,57 @@
+use std::io::BufRead;
+use std::collections::{HashMap, HashSet};
+
+type Tree = HashMap<String, HashSet<String>>;
+
+fn parse_input<F: BufRead> (input: &mut F) -> Tree {
+    let it = input.lines()
+        .filter_map(|line| line.ok())
+        .map(|line| {
+            let v: Vec<_> = line.split(')').collect();
+            (v[0].to_string(), v[1].to_string())
+        });
+    let mut tree = HashMap::new();
+    for (parent, child) in it {
+        let children = tree.entry(parent).or_insert(HashSet::new());
+        children.insert(child);
+    }
+    tree
+}
+
+fn compute_checksum(tree: &Tree, root: &str, level: usize) -> usize {
+    if let Some(children) = tree.get(root) {
+        level + children.iter()
+            .map(|child| compute_checksum(&tree, child, level + 1))
+            .sum::<usize>()
+    } else {
+        level
+    }
+}
+
+pub fn part1<F: BufRead> (mut input: F) {
+    let tree = parse_input(&mut input);
+    println!("{:?}", compute_checksum(&tree, "COM", 0));
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+    use std::io;
+
+    #[test]
+    fn test_compute_checksum() {
+        let mut input = io::Cursor::new(r#"COM)B
+B)C
+C)D
+D)E
+E)F
+B)G
+G)H
+D)I
+E)J
+J)K
+K)L"#);
+        let tree = parse_input(&mut input);
+        assert_eq!(42, compute_checksum(&tree, "COM", 0));
+    }
+}

src/main.rs

@@ -12,6 +12,8 @@ mod day2;
 mod day3;
 mod day4;
 mod day5;
+mod day6;
+
 mod intcode;
 
 fn main() {
@@ -47,6 +49,7 @@ fn main() {
         day3::part1, day3::part2,
         day4::part1, day4::part2,
         day5::part1, day5::part1,
+        day6::part1,
     ];
 
     if let Some(f) = challenges.get(challenge) {