1 files changed, 48 insertions, 0 deletions
diff --git a/src/bin/day6.rs b/src/bin/day6.rs
new file mode 100644
index 0000000..d8073b4
--- /dev/null
+++ b/src/bin/day6.rs
@@ -0,0 +1,48 @@
+use std::collections::HashMap;
+
+fn main() {
+    let input : Vec<u32> = advent::read_file(6).trim_end()
+                                               .split_terminator(',')
+                                               .map(|n| n.parse::<u32>().unwrap())
+                                               .collect();
+    println!("6a: {}", simulate(&input, 80));
+    println!("6b: {}", simulate(&input, 256));
+}
+
+fn simulate(input: &[u32], days: u32) -> usize {
+    let mut fish = HashMap::new();
+    for days in 0 ..= 8 {
+        fish.insert(days, 0);
+    }
+    for f in input {
+        let count = fish.entry(*f).or_insert(0);
+        *count += 1;
+    }
+
+    for _ in 0 .. days {
+        let mut new_fish = HashMap::new();
+        for remaining in 1 ..= 8 {
+            new_fish.insert(remaining - 1, *fish.get(&remaining).unwrap());
+        }
+        new_fish.insert(8, *fish.get(&0).unwrap());
+
+        let count = new_fish.entry(6).or_insert(0);
+        *count += fish.get(&0).unwrap();
+
+        fish = new_fish;
+    }
+
+    fish.values().sum()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test() {
+        let input = [3,4,3,1,2];
+        assert_eq!(simulate(&input, 18), 26);
+        assert_eq!(simulate(&input, 80), 5934);
+    }
+}