2023-08: Solve part two

src/year_2023/day_08_haunted_wasteland/part_two.rs

@@ -4,9 +4,9 @@ use crate::utils::input_reader;
 use crate::year_2023::day_08_haunted_wasteland::common::{parse_line, Node};
 
 pub fn part_two() -> String {
-    if let Ok(lines) = input_reader::read_lines("./input/2023/day_08/input_test_part_2.txt") {
-        let mut node_map: HashMap<String, &Node> = HashMap::new();
-        let mut start_nodes: Vec<&Node> = Vec::new();
+    if let Ok(lines) = input_reader::read_lines("./input/2023/day_08/input.txt") {
+        let mut node_map: HashMap<String, Node> = HashMap::new();
+        let mut start_nodes: Vec<String> = Vec::new();
         let mut instructions: Vec<char> = Vec::new();
         let mut s: String;
 
@@ -25,48 +25,56 @@ pub fn part_two() -> String {
             let node = parse_line(&s);
             let value = node.value.clone();
 
-            node_map.insert(value.clone(), &node.clone());
+            node_map.insert(value.clone(), node);
 
             if value.ends_with('A') {
-                let cloned_node = node.clone();
-                start_nodes.push(&cloned_node);
+                start_nodes.push(value.clone())
             }
         }
 
-        let mut curr_nodes: Vec<&Node> = start_nodes;
+        let mut counts: Vec<usize> = Vec::new();
+
+        for curr in start_nodes {
+
+            let mut curr_node = &node_map[&curr];
             let mut count = 0;
-        let mut cond = true;
 
-        while cond {
+            while !curr_node.value.ends_with('Z') {
                 let i = count % instructions.len();
-            let c = instructions[i];
 
-            let mut new_curr: Vec<&Node> = Vec::new();
-            cond = false;
-
-            for n in curr_nodes {
-                let nn: &Node;
                 if instructions[i] == 'L' {
-                    nn = &node_map[&n.left];
+                    curr_node = &node_map[&curr_node.left];
                 } else if instructions[i] == 'R' {
-                    nn = &node_map[&n.right];
-                } else {
-                    panic!("Unknown instruction!")
+                    curr_node = &node_map[&curr_node.right];
                 }
 
-                if !nn.value.clone().ends_with('Z') {
-                    cond = true;
-                }
-
-                new_curr.push(nn);
-            }
-
-            curr_nodes = new_curr;
                 count += 1;
             }
 
-        return count.to_string();
+            counts.push(count);
+        }
+
+        let result = counts.iter().fold(1, |acc, &x| lcm(acc, x));
+
+        return result.to_string();
     } else {
         panic!("Failed to read lines from input file");
     }
 }
+
+fn lcm(a : usize, b : usize) -> usize {
+    a * b / gcd(a, b)
+}
+
+fn gcd(a : usize, b : usize) -> usize {
+
+    if b > a {
+        return gcd(b, a);
+    }
+    
+    if b == 0 {
+        a
+    } else {
+        gcd(b, a % b)
+    }
+}