More functional take on day 1

1 files changed, 10 insertions, 15 deletions

diff --git a/src/bin/day_1.rs b/src/bin/day_1.rs
index 9f54c70..56ae0f7 100644
--- a/src/bin/day_1.rs
+++ b/src/bin/day_1.rs
@@ -1,10 +1,12 @@
 extern crate advent_of_code_2018;
 use advent_of_code_2018::*;
 
+extern crate im_rc;
+
 use std::error::Error;
 use std::path::PathBuf;
 
-use std::collections::HashSet;
+use im_rc::HashSet;
 
 // cargo watch -cs "cargo run --bin day_1"
 
@@ -17,21 +19,14 @@ fn main() -> Result<(), Box<Error>> {
     let sum: i32 = input_ints.iter().sum();
     println!("Sum: {}", sum);
 
-    let mut seen = HashSet::new();
-    let mut acc = 0;
-    let mut repeat_found = false;
-    while !repeat_found {
-        for i in &input_ints {
-            if seen.contains(&acc) {
-                repeat_found = true;
-                break;
-            } else {
-                seen.insert(acc);
-            }
-            acc += i;
+    let first_repeat = input_ints.iter().cycle().try_fold((0, HashSet::new()), |(acc, seen), &i| {
+        if seen.contains(&acc) {
+            Err(acc)
+        } else {
+            Ok((acc + i, seen.update(acc)))
         }
-    }
-    println!("First repeat: {}", acc);
+    }).err().unwrap();
+    println!("First repeat: {}", first_repeat);
 
     Ok(())
 }