Day 6

input/day6.txt

@@ -0,0 +1 @@
+1,3,4,1,1,1,1,1,1,1,1,2,2,1,4,2,4,1,1,1,1,1,5,4,1,1,2,1,1,1,1,4,1,1,1,4,4,1,1,1,1,1,1,1,2,4,1,3,1,1,2,1,2,1,1,4,1,1,1,4,3,1,3,1,5,1,1,3,4,1,1,1,3,1,1,1,1,1,1,1,1,1,1,1,1,1,5,2,5,5,3,2,1,5,1,1,1,1,1,1,1,1,1,1,1,1,2,1,1,1,1,5,1,1,1,1,5,1,1,1,1,1,4,1,1,1,1,1,3,1,1,1,1,1,1,1,1,1,1,1,3,1,2,4,1,5,5,1,1,5,3,4,4,4,1,1,1,2,1,1,1,1,1,1,2,1,1,1,1,1,1,5,3,1,4,1,1,2,2,1,2,2,5,1,1,1,2,1,1,1,1,3,4,5,1,2,1,1,1,1,1,5,2,1,1,1,1,1,1,5,1,1,1,1,1,1,1,5,1,4,1,5,1,1,1,1,1,1,1,1,1,1,1,1,1,2,1,1,1,1,5,4,5,1,1,1,1,1,1,1,5,1,1,3,1,1,1,3,1,4,2,1,5,1,3,5,5,2,1,3,1,1,1,1,1,3,1,3,1,1,2,4,3,1,4,2,2,1,1,1,1,1,1,1,5,2,1,1,1,2

src/day6.rs

@@ -0,0 +1,41 @@
+pub fn day6() {
+    // let input = "3,4,3,1,2";
+    let input = include_str!("../input/day6.txt");
+
+    let initial_fishes = input
+        .trim()
+        .split(",")
+        .map(|s| s.parse::<usize>().unwrap())
+        .collect::<Vec<_>>();
+
+    count_fishes_after(&initial_fishes, 18);
+    count_fishes_after(&initial_fishes, 80);
+    count_fishes_after(&initial_fishes, 256);
+}
+
+fn count_fishes_after(initial_fishes: &[usize], days: u32) {
+    let mut fish_batches = [0_u64; 9];
+
+    for f in initial_fishes {
+        fish_batches[*f] += 1;
+    }
+
+    for _ in 1..=days {
+        let old_batches = fish_batches;
+        fish_batches = [0; 9];
+
+        for counter in 0..=7 {
+            fish_batches[counter] = old_batches[counter + 1];
+        }
+
+        // each fish previously at 0 spawns a new fish with counter 8 and resets its counter to 6
+        fish_batches[8] = old_batches[0];
+        fish_batches[6] += old_batches[0];
+    }
+
+    println!(
+        "total fishes after {} days: {}",
+        days,
+        fish_batches.iter().sum::<u64>()
+    );
+}

src/main.rs

@@ -5,7 +5,8 @@ mod day2;
 mod day3;
 mod day4;
 mod day5;
+mod day6;
 
 fn main() {
-    day5::day5();
+    day6::day6();
 }