Add another part 2 solution that needs massive optimization

2023/day8/README.md

@@ -47,3 +47,49 @@ ZZZ = (ZZZ, ZZZ)
 ```
 
 Starting at AAA, follow the left/right instructions. **How many steps are required to reach ZZZ**?
+
+## Part 2
+
+The sandstorm is upon you and you aren't any closer to escaping the wasteland. You had the camel follow the 
+instructions, but you've barely left your starting position. It's going to take **significantly more steps** to escape!
+
+What if the map isn't for people - what if the map is for **ghosts?** Are ghosts even bound by the laws of spacetime? 
+Only one way to find out.
+
+After examining the maps a bit longer, your attention is drawn to a curious fact: the number of nodes with names 
+ending in A is equal to the number ending in Z! If you were a ghost, you'd probably just **start at every node that 
+ends with A** and follow all of the paths at the same time until they all simultaneously end up at nodes that end with 
+`Z`.
+
+For example:
+
+```
+LR
+
+11A = (11B, XXX)
+11B = (XXX, 11Z)
+11Z = (11B, XXX)
+22A = (22B, XXX)
+22B = (22C, 22C)
+22C = (22Z, 22Z)
+22Z = (22B, 22B)
+XXX = (XXX, XXX)
+```
+
+Here, there are two starting nodes, 11A and 22A (because they both end with A). As you follow each left/right 
+instruction, use that instruction to **simultaneously** navigate away from both nodes you're currently on. Repeat this 
+process until **all** of the nodes you're currently on end with Z. (If only some of the nodes you're on end with Z, they 
+act like any other node and you continue as normal.) In this example, you would proceed as follows:
+
+- Step 0: You are at `11A` and `22A`.`
+- Step 1: You choose all of the **left** paths, leading you to `11B` and `22B`.
+- Step 2: You choose all of the **right** paths, leading you to **11Z** and `22C`.
+- Step 3: You choose all of the left paths, leading you to `11B` and **22Z**.
+- Step 4: You choose all of the right paths, leading you to **11Z** and `22B`.
+- Step 5: You choose all of the left paths, leading you to `11B` and `22C`.
+- Step 6: You choose all of the right paths, leading you to **11Z** and **22Z**.
+
+So, in this example, you end up entirely on nodes that end in Z after **6** steps.
+
+Simultaneously start on every node that ends with A. 
+**How many steps does it take before you're only on nodes that end with `Z`?**

2023/day8/src/example-input2.txt

@@ -0,0 +1,10 @@
+LR
+
+11A = (11B, XXX)
+11B = (XXX, 11Z)
+11Z = (11B, XXX)
+22A = (22B, XXX)
+22B = (22C, 22C)
+22C = (22Z, 22Z)
+22Z = (22B, 22B)
+XXX = (XXX, XXX)

2023/day8/src/main.rs

@@ -86,6 +86,37 @@ impl Network {
             }
         }
     }
+
+    fn count_steps_part_two(&self) -> usize {
+        let mut count = 0usize;
+        let mut next_keys: Vec<_> = self.map.keys().filter(|k| k.ends_with('A')).collect();
+        let mut current: Vec<_> = next_keys
+            .iter()
+            .map(|k| self.map.get(*k).unwrap())
+            .collect();
+
+        loop {
+            for x in self.direction_list.iter() {
+                count += 1;
+                next_keys = current
+                    .iter()
+                    .map(|c| match x {
+                        Direction::Left => &c.0,
+                        Direction::Right => &c.1,
+                    })
+                    .collect();
+
+                if next_keys.iter().all(|k| k.ends_with('Z')) {
+                    return count;
+                }
+
+                current = next_keys
+                    .iter()
+                    .map(|k| self.map.get(*k).unwrap())
+                    .collect();
+            }
+        }
+    }
 }
 
 fn part_one() {
@@ -95,18 +126,36 @@ fn part_one() {
     println!("Part 1: Step count from AAA to ZZZ {}", step_count);
 }
 
+fn part_two() {
+    let network = Network::parse(FILE_STR);
+    let step_count = network.count_steps_part_two();
+
+    println!(
+        "Part 2: Step count for steps to locations ending in Z {}",
+        step_count
+    );
+}
+
 fn main() {
     part_one();
+    part_two();
 }
 
 #[cfg(test)]
 mod tests {
     const EXAMPLE_FILE_STR: &str = include_str!("example-input.txt");
+    const EXAMPLE2_FILE_STR: &str = include_str!("example-input2.txt");
     use super::*;
 
     #[test]
-    fn first_example() {
+    fn part_one_example() {
         let network = Network::parse(EXAMPLE_FILE_STR);
         assert_eq!(6, network.count_steps());
     }
+
+    #[test]
+    fn part_two_example() {
+        let network = Network::parse(EXAMPLE2_FILE_STR);
+        assert_eq!(6, network.count_steps_part_two());
+    }
 }