feat: solve day 6, part 1 & 2

input/6.txt

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

src/day6.rs

@@ -0,0 +1,81 @@
+use std::num::ParseIntError;
+
+pub fn parse_input(input: &str) -> Result<Vec<i32>, ParseIntError> {
+    input.trim_end().split_terminator(',')
+        .map(|s| s.parse())
+        .collect()
+}
+
+fn simulate_step(fishes: &mut Vec<i32>) {
+    for i in 0..fishes.len() {
+        if fishes[i] == 0 {
+            fishes[i] = 7;
+            fishes.push(9);
+        }
+    }
+
+    for i in 0..fishes.len() {
+        if fishes[i] > 0 {
+            fishes[i] -= 1;
+        }
+    }
+}
+
+pub fn part1(input: &[i32]) -> u32 {
+    let mut fishes = input.to_vec();
+
+    for _ in 0..80 {
+        simulate_step(&mut fishes)
+    }
+
+    fishes.len() as u32
+}
+
+// Instead of storing each fishes cycle as individual values group them up
+// by there cycles. Because it dosen't matter where the fish is in the list.
+// So just make an array of size 9 for the 9 possible fish cycle timers.
+// And one extra group, for accounting for the delay that when the timer is 0, 
+// they produce a new fish only on the next turn.
+pub fn part2(input: &[i32]) -> u64 {
+    let mut groups: [u64; 10] = [0; 10];
+
+    for fish in input.iter() {
+        groups[*fish as usize + 1] += 1;
+    }
+
+    for _ in 0..256 {
+        for i in 1..10 {
+            groups[i-1] += groups[i];
+            groups[i] = 0;
+        }
+        groups[7] += groups[0];
+        groups[9] += groups[0];
+        groups[0] = 0;
+    }
+
+    let mut count: u64 = 0;
+    for amount in groups.iter() {
+        count += amount;
+    }
+    count
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn part1_example() {
+        let input = [3,4,3,1,2];
+        let result = part1(&input);
+        assert_eq!(result, 5934);
+    }
+
+    #[test]
+    fn part2_example() {
+        let input = [3,4,3,1,2];
+        let result = part2(&input);
+        assert_eq!(result, 26984457539 as u64);
+    }
+}
+

src/main.rs

@@ -3,6 +3,7 @@ mod day2;
 mod day3;
 mod day4;
 mod day5;
+mod day6;
 
 use std::{env, process};
 use std::fs::File;