day01 redone in a more functional way

in task2, RefCell is used since the passed state in 'scan' seems to be
immutable, although it shouldn't.

src/tasks/day01.rs

@@ -2,13 +2,10 @@ use std::collections::HashSet;
 use utils;
 
 pub fn task1() {
-    let contents = utils::read_file("input/day01.txt");
-
-    let mut frequency = 0;
-    for line in contents.split_whitespace() {
-        let number: i32 = line.parse().unwrap();
-        frequency += number;
-    }
+    let frequency: i32 = utils::read_file("input/day01.txt")
+        .lines()
+        .map(|line| line.parse::<i32>().unwrap())
+        .sum();
 
     println!("Result: {}", frequency);
 }
@@ -16,21 +13,23 @@ pub fn task1() {
 pub fn task2() {
     let contents = utils::read_file("input/day01.txt");
 
-    let mut seen = HashSet::new();
+    let mut seen: HashSet<i32> = HashSet::new();
+    seen.insert(0);
+    let start_state = (std::cell::RefCell::new(seen), std::cell::RefCell::new(0));
+    let final_state = contents
+        .lines()
+        .map(|line| line.parse::<i32>().unwrap())
+        .cycle()
+        .scan(&start_state, |(hash_set, current_freq), freq_change| {
+            *current_freq.borrow_mut() += freq_change;
+            if hash_set.borrow_mut().contains(&current_freq.borrow()) {
+                Some((current_freq.borrow(), true))
+            } else {
+                let value: i32 = *current_freq.borrow_mut();
+                hash_set.borrow_mut().insert(value);
+                Some((current_freq.borrow(), false))
+            }
+        }).find(|state| state.1);
 
-    let mut frequency = 0;
-    seen.insert(frequency);
-    let changes: Vec<i32> = contents.lines().map(|line| line.parse().unwrap()).collect();
-
-    loop {
-        for number in changes.iter() {
-            frequency += number;
-            if seen.contains(&frequency) {
-                println!("{} was there already!", frequency);
-                return;
-            }
-
-            seen.insert(frequency);
-        }
-    }
+    println!("{:?} was there already!", final_state.unwrap().0);
 }