From 835a1e0c2deae83d35e45d193e00b57dee1c3e67 Mon Sep 17 00:00:00 2001
From: Adam Wick <awick@uhsure.com>
Date: Mon, 6 Dec 2021 08:42:21 -0800
Subject: [PATCH] Fishie fishie fishie.

---
 src/bin/day6.rs | 127 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 127 insertions(+)
 create mode 100644 src/bin/day6.rs

diff --git a/src/bin/day6.rs b/src/bin/day6.rs
new file mode 100644
index 0000000..d7a857c
--- /dev/null
+++ b/src/bin/day6.rs
@@ -0,0 +1,127 @@
+#[cfg(test)]
+const TEST_DATA: &[u8] = &[3, 4, 3, 1, 2];
+const REAL_DATA: &[u8] = &[
+    3, 4, 1, 2, 1, 2, 5, 1, 2, 1, 5, 4, 3, 2, 5, 1, 5, 1, 2, 2, 2, 3, 4, 5, 2, 5, 1, 3, 3, 1, 3, 4,
+    1, 5, 3, 2, 2, 1, 3, 2, 5, 1, 1, 4, 1, 4, 5, 1, 3, 1, 1, 5, 3, 1, 1, 4, 2, 2, 5, 1, 5, 5, 1, 5,
+    4, 1, 5, 3, 5, 1, 1, 4, 1, 2, 2, 1, 1, 1, 4, 2, 1, 3, 1, 1, 4, 5, 1, 1, 1, 1, 1, 5, 1, 1, 4, 1,
+    1, 1, 1, 2, 1, 4, 2, 1, 2, 4, 1, 3, 1, 2, 3, 2, 4, 1, 1, 5, 1, 1, 1, 2, 5, 5, 1, 1, 4, 1, 2, 2,
+    3, 5, 1, 4, 5, 4, 1, 3, 1, 4, 1, 4, 3, 2, 4, 3, 2, 4, 5, 1, 4, 5, 2, 1, 1, 1, 1, 1, 3, 1, 5, 1,
+    3, 1, 1, 2, 1, 4, 1, 3, 1, 5, 2, 4, 2, 1, 1, 1, 2, 1, 1, 4, 1, 1, 1, 1, 1, 5, 4, 1, 3, 3, 5, 3,
+    2, 5, 5, 2, 1, 5, 2, 4, 4, 1, 5, 2, 3, 1, 5, 3, 4, 1, 5, 1, 5, 3, 1, 1, 1, 4, 4, 5, 1, 1, 1, 3,
+    1, 4, 5, 1, 2, 3, 1, 3, 2, 3, 1, 3, 5, 4, 3, 1, 3, 4, 3, 1, 2, 1, 1, 3, 1, 1, 3, 1, 1, 4, 1, 2,
+    1, 2, 5, 1, 1, 3, 5, 3, 3, 3, 1, 1, 1, 1, 1, 5, 3, 3, 1, 1, 3, 4, 1, 1, 4, 1, 1, 2, 4, 4, 1, 1,
+    3, 1, 3, 2, 2, 1, 2, 5, 3, 3, 1, 1,
+];
+
+#[derive(Debug, PartialEq)]
+struct LanternFishies {
+    data: [usize; 9],
+}
+
+impl<'a> From<&'a [u8]> for LanternFishies {
+    fn from(state: &[u8]) -> LanternFishies {
+        let mut data = [0; 9];
+
+        for x in state.iter() {
+            data[*x as usize] += 1;
+        }
+
+        LanternFishies { data }
+    }
+}
+
+impl LanternFishies {
+    fn count(&self) -> usize {
+        self.data.iter().sum()
+    }
+
+    fn step(&mut self) {
+        let base_data = self.data;
+
+        self.data.fill(0);
+
+        self.data[6] += base_data[0];
+        self.data[8] += base_data[0];
+        for (i, curval) in base_data.iter().enumerate().skip(1) {
+            self.data[i - 1] += curval;
+        }
+    }
+
+    fn steps(&mut self, count: usize) {
+        for _ in 0..count {
+            self.step();
+        }
+    }
+}
+
+#[test]
+fn example() {
+    let mut values = LanternFishies::from(TEST_DATA);
+    assert_eq!(
+        LanternFishies {
+            data: [0, 1, 1, 2, 1, 0, 0, 0, 0]
+        },
+        values
+    );
+    assert_eq!(5, values.count());
+    values.step();
+    assert_eq!(
+        LanternFishies {
+            data: [1, 1, 2, 1, 0, 0, 0, 0, 0]
+        },
+        values
+    );
+    assert_eq!(5, values.count());
+    values.step();
+    assert_eq!(
+        LanternFishies {
+            data: [1, 2, 1, 0, 0, 0, 1, 0, 1]
+        },
+        values
+    );
+    assert_eq!(6, values.count());
+    values.step();
+    assert_eq!(
+        LanternFishies {
+            data: [2, 1, 0, 0, 0, 1, 1, 1, 1]
+        },
+        values
+    );
+    assert_eq!(7, values.count());
+    values.step();
+    assert_eq!(
+        LanternFishies {
+            data: [1, 0, 0, 0, 1, 1, 3, 1, 2]
+        },
+        values
+    );
+    assert_eq!(9, values.count());
+}
+
+#[test]
+fn regression() {
+    let mut test_values = LanternFishies::from(TEST_DATA);
+    test_values.steps(18);
+    assert_eq!(26, test_values.count());
+    test_values.steps(80 - 18);
+    assert_eq!(5934, test_values.count());
+    test_values.steps(256 - 80);
+    assert_eq!(26984457539, test_values.count());
+
+    let mut real_values = LanternFishies::from(REAL_DATA);
+    real_values.steps(80);
+    assert_eq!(365131, real_values.count());
+    real_values.steps(256 - 80);
+    assert_eq!(1650309278600, real_values.count());
+}
+
+fn main() {
+    let mut fishies = LanternFishies::from(REAL_DATA);
+    fishies.steps(80);
+    println!("After 80 days, there will be {} fishies.", fishies.count());
+    fishies.steps(256 - 80);
+    println!(
+        "After 256 days, there will be {} fishies. (That's a lot of fishies!)",
+        fishies.count()
+    );
+}