Day 6 part 2

2019/src/bin/day6.rs

@@ -14,6 +14,7 @@ fn main() -> io::Result<()> {
     let orbits = parse_input(&input);
 
     println!("Part 1: {}", number_of_orbits(&orbits));
+    println!("Part 2: {}", orbital_transfers(&orbits, "YOU", "SAN"));
 
     Ok(())
 }
@@ -36,6 +37,32 @@ fn number_of_orbits(input: &[Orbit<'_>]) -> usize {
         .sum()
 }
 
+fn orbital_transfers(input: &[Orbit<'_>], from: &str, to: &str) -> usize {
+    let nodes = build_tree(input);
+
+    // Find the to and from nodes
+    let from = nodes
+        .iter()
+        .find(|node| node.name == from)
+        .expect("unable to find from node");
+    let to = nodes
+        .iter()
+        .find(|node| node.name == to)
+        .expect("unable to find to node");
+
+    // Find the longest shared path they have on the way to COM and then the two remaining parts
+    // give the path between them.
+    let from_nodes_to_com = nodes_to_com(&nodes, from, Vec::new());
+    let to_nodes_to_com = nodes_to_com(&nodes, to, Vec::new());
+    let mut i = 0;
+    while from_nodes_to_com[i].name == to_nodes_to_com[i].name {
+        i += 1;
+    }
+
+    // - 2 since we don't count the path from 'from' to the object it's orbiting. Same for 'to'.
+    from_nodes_to_com[i..].len() + to_nodes_to_com[i..].len() - 2
+}
+
 fn build_tree<'a>(input: &'a [Orbit<'a>]) -> Vec<Node<'a>> {
     // Build the tree from the input
     let mut nodes: Vec<Node<'_>> = Vec::with_capacity(input.len());
@@ -85,11 +112,25 @@ fn count_orbits(nodes: &[Node], node_index: usize, count: usize) -> usize {
     }
 }
 
+fn nodes_to_com<'a>(
+    nodes: &'a [Node],
+    current_node: &'a Node,
+    mut path: Vec<&'a Node<'a>>,
+) -> Vec<&'a Node<'a>> {
+    path.push(current_node);
+    if let Some(parent_index) = current_node.parent {
+        nodes_to_com(nodes, &nodes[parent_index], path)
+    } else {
+        path.reverse();
+        path
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
 
-    const TEST_INPUT: &str = "B)C
+    const TEST_INPUT1: &str = "B)C
 COM)B
 C)D
 D)E
@@ -100,11 +141,32 @@ D)I
 E)J
 J)K
 K)L
+";
+
+    const TEST_INPUT2: &str = "COM)B
+B)C
+C)D
+D)E
+E)F
+B)G
+G)H
+D)I
+E)J
+J)K
+K)L
+K)YOU
+I)SAN
 ";
 
     #[test]
     fn test_number_of_orbits() {
-        let orbits = parse_input(TEST_INPUT);
+        let orbits = parse_input(TEST_INPUT1);
         assert_eq!(number_of_orbits(&orbits), 42);
     }
+
+    #[test]
+    fn test_orbital_transfers() {
+        let orbits = parse_input(TEST_INPUT2);
+        assert_eq!(orbital_transfers(&orbits, "YOU", "SAN"), 4);
+    }
 }