From 3dea0ef91405c9de1057c8e430858b47f0984aa5 Mon Sep 17 00:00:00 2001
From: Shadowfacts <me@shadowfacts.net>
Date: Mon, 6 Dec 2021 10:16:51 -0500
Subject: [PATCH] Day 6

---
 input/day6.txt |  1 +
 src/day6.rs    | 41 +++++++++++++++++++++++++++++++++++++++++
 src/main.rs    |  3 ++-
 3 files changed, 44 insertions(+), 1 deletion(-)
 create mode 100644 input/day6.txt
 create mode 100644 src/day6.rs

diff --git a/input/day6.txt b/input/day6.txt
new file mode 100644
index 0000000..c696217
--- /dev/null
+++ b/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
diff --git a/src/day6.rs b/src/day6.rs
new file mode 100644
index 0000000..253dada
--- /dev/null
+++ b/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>()
+    );
+}
diff --git a/src/main.rs b/src/main.rs
index 41d3cbb..f057f1c 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -5,7 +5,8 @@ mod day2;
 mod day3;
 mod day4;
 mod day5;
+mod day6;
 
 fn main() {
-    day5::day5();
+    day6::day6();
 }