Day 6

Cargo.lock

@@ -19,3 +19,7 @@ version = "0.1.0"
 [[package]]
 name = "day_5"
 version = "0.1.0"
+
+[[package]]
+name = "day_6"
+version = "0.1.0"

Cargo.toml

@@ -5,4 +5,5 @@ members = [
     "day_3",
     "day_4",
     "day_5",
+    "day_6",
 ]

day_6/Cargo.toml

@@ -0,0 +1,9 @@
+[package]
+name = "day_6"
+version = "0.1.0"
+authors = ["Smaug123 <patrick+github@patrickstevens.co.uk>"]
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

day_6/input.txt

@@ -0,0 +1 @@
+11	11	13	7	0	15	5	5	4	4	1	1	7	1	15	11

day_6/src/main.rs

@@ -0,0 +1,108 @@
+use std::collections::HashMap;
+use std::collections::HashSet;
+
+fn input() -> Vec<u32> {
+    let input = include_str!("../input.txt");
+    input
+        .trim()
+        .split_whitespace()
+        .map(|i| {
+            i.parse()
+                .unwrap_or_else(|_| panic!("{} wasn't a valid u32", i))
+        })
+        .collect::<Vec<u32>>()
+}
+
+pub fn part_1(v: &mut [u32]) -> u32 {
+    let len = v.len() as u32;
+    let mut seen: HashSet<Vec<u32>> = HashSet::new();
+    let mut count = 0;
+    let (mut max_pos, mut max) = v
+        .iter()
+        .cloned()
+        .enumerate()
+        .max_by_key(|(_, x)| *x)
+        .unwrap();
+    while seen.insert(v.to_vec()) {
+        let extras = max % len;
+        let all = max / len;
+        v[max_pos] = 0;
+        for i in 0..(extras as usize) {
+            v[(max_pos + i + 1) % v.len()] += 1;
+        }
+        max = 0;
+        for (i, item) in v.iter_mut().enumerate() {
+            *item += all;
+            if *item > max {
+                max = *item;
+                max_pos = i;
+            }
+        }
+        count += 1;
+    }
+    count
+}
+
+pub fn part_2(v: &mut [u32]) -> u32 {
+    let len = v.len() as u32;
+    let mut seen: HashMap<Vec<u32>, u32> = HashMap::new();
+    let mut count = 0;
+    let (mut max_pos, mut max) = v
+        .iter()
+        .cloned()
+        .enumerate()
+        .max_by_key(|(_, x)| *x)
+        .unwrap();
+    loop {
+        if let Some(existing) = seen.insert(v.to_vec(), count) {
+            return count - existing;
+        }
+        let extras = max % len;
+        let all = max / len;
+        v[max_pos] = 0;
+        for i in 0..(extras as usize) {
+            v[(max_pos + i + 1) % v.len()] += 1;
+        }
+        max = 0;
+        for (i, item) in v.iter_mut().enumerate() {
+            *item += all;
+            if *item > max {
+                max = *item;
+                max_pos = i;
+            }
+        }
+        count += 1;
+    }
+}
+
+fn main() {
+    let input = input();
+    println!("part 1 => {}", part_1(&mut input.clone()));
+    let mut input = input;
+    println!("part 2 => {}", part_2(&mut input));
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn part1_known() {
+        assert_eq!(part_1(&mut vec![0, 2, 7, 0]), 5);
+    }
+
+    #[test]
+    fn part2_known() {
+        assert_eq!(part_2(&mut vec![0, 2, 7, 0]), 4);
+    }
+
+    #[test]
+    fn test_day_6() {
+        let input = input();
+        let answer = part_1(&mut input.clone());
+        assert_eq!(answer, 4074);
+        let mut input = input;
+        let answer = part_2(&mut input);
+        assert_eq!(answer, 2793);
+    }
+}