day06 task 2

src/tasks/day06.rs

@@ -1,3 +1,4 @@
+use std::collections::HashMap;
 use std::collections::VecDeque;
 
 pub fn run() {
@@ -10,6 +11,7 @@ pub fn run() {
         })
         .collect();
     task1(&orbits);
+    task2(&orbits);
 }
 
 fn task1(orbit_list: &Vec<(&str, &str)>) {
@@ -28,5 +30,57 @@ fn task1(orbit_list: &Vec<(&str, &str)>) {
         })
         .sum();
     println!("Task 1: there are {} direct and indirect orbits", sum);
-    //405096 too high
+}
+
+fn task2(orbit_list: &Vec<(&str, &str)>) {
+    let mut queue = VecDeque::new();
+    queue.push_back("COM"); // com is where everything floats around
+    let parents: HashMap<_, _> = (0..)
+        .scan((orbit_list, queue), |(all, queue), _| {
+            if let Some(next) = queue.pop_front() {
+                let children = all
+                    .iter()
+                    .filter(|(a, _)| *a == next)
+                    .map(|(_, b)| *b)
+                    .collect::<Vec<_>>();
+                children.iter().for_each(|b| queue.push_back(b));
+                Some(
+                    children
+                        .into_iter()
+                        .map(|child| (child, next))
+                        .collect::<Vec<_>>(),
+                )
+            } else {
+                None
+            }
+        })
+        .flatten()
+        .collect();
+    // graph maps from the name of an object A to all objects that either orbit
+    // A or that A directly orbits
+    let ancestors = |start: &str| {
+        let mut result = std::iter::successors(Some(start.to_string()), |last| {
+            parents.get(&last.as_ref()).map(|l| l.to_string())
+        })
+        .collect::<Vec<_>>();
+        result.reverse();
+        result
+    };
+    let a_san = ancestors("SAN");
+    let a_you = ancestors("YOU");
+
+    let last_common_ancestor = a_san
+        .iter()
+        .zip(a_you.iter())
+        .take_while(|(a, b)| a == b)
+        .map(|(a, _)| a)
+        .last()
+        .unwrap();
+    let dist_to_ancestor = |ancestors: &Vec<String>, target: &String| {
+        ancestors.iter().rev().position(|x| x == target).unwrap()
+    };
+    let result = dist_to_ancestor(&a_san, &last_common_ancestor) - 1
+        + dist_to_ancestor(&a_you, &last_common_ancestor)
+        - 1; // two times -1 because we search for the path length for yours and santas orbit-center
+    println!("Task 2: distance is {}", result);
 }

src/tasks/mod.rs

@@ -1,5 +1,4 @@
 pub mod day01;
 pub mod day03;
 pub mod day04;
-pub mod day05;
 pub mod day06;