day05 both parts fast

src/tasks/day05.rs

@@ -2,7 +2,7 @@ use utils;
 
 pub fn task1() {
     let mut input = utils::read_file("input/day05.txt");
-    input = reduce(input);
+    input = reduce(input.as_str());
 
     println!("resulting polymer: {}", input); //11591 too high
     println!("RESULT: {}", input.len());
@@ -16,36 +16,47 @@ pub fn task2() {
             (
                 c,
                 reduce(
-                    input
+                    &input
                         .chars()
                         .filter(|ch| ch.to_lowercase().next().unwrap() != c)
-                        .collect(),
-                ),
+                        .collect::<String>(),
+                ).len(),
             )
-        }).max_by_key(|it| it.1.len())
+        }).min_by_key(|it| it.1)
         .unwrap();
 
-    println!("Best: {} (length {})", best.0, best.1.len()); // 11184 too high
+    println!("Best: {} (length {})", best.0, best.1); // 11184 too high, 4328 too low
 }
 
-fn reduce(mut input: String) -> String {
-    let mut go_on = true;
-    while go_on {
-        go_on = false;
-        let mut index = 0;
-        while index < input.len() - 1 {
-            let a = input.chars().nth(index).unwrap();
-            let b = input.chars().nth(index + 1).unwrap();
-            if a.is_lowercase() && b.is_uppercase() && a == b.to_lowercase().next().unwrap()
-                || b.is_lowercase() && a.is_uppercase() && b == a.to_lowercase().next().unwrap()
-            {
-                // println!("{} and {} match", a, b);
-                input.remove(index);
-                input.remove(index);
-                go_on = true;
-            }
-            index += 1;
-        }
-    }
+fn reduce(input: &str) -> String {
     input
+        .chars()
+        .fold(Vec::<char>::new(), |mut stack, c| {
+            let last: Option<char>;
+            {
+                if let Some(c) = stack.last() {
+                    last = Some(*c)
+                } else {
+                    last = None
+                }
+            }
+
+            if let Some(last) = last {
+                if c.is_lowercase()
+                    && last.is_uppercase()
+                    && c == last.to_lowercase().next().unwrap()
+                    || last.is_lowercase()
+                        && c.is_uppercase()
+                        && last == c.to_lowercase().next().unwrap()
+                {
+                    stack.pop();
+                } else {
+                    stack.push(c);
+                }
+            } else {
+                stack.push(c);
+            }
+            stack
+        }).iter()
+        .collect()
 }