Added Solution for 2021 day 06

2021/day06_lanternfish/Cargo.toml

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+[package]
+name = "day06_lanternfish"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

2021/day06_lanternfish/challenge.txt

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+The sea floor is getting steeper. Maybe the sleigh keys got carried this way?
+
+A massive school of glowing [lanternfish](https://en.wikipedia.org/wiki/Lanternfish) swims past. They must spawn quickly to reach such large numbers - maybe *exponentially* quickly? You should model their growth rate to be sure.
+
+Although you know nothing about this specific species of lanternfish, you make some guesses about their attributes. Surely, each lanternfish creates a new lanternfish once every *7* days.
+
+However, this process isn't necessarily synchronized between every lanternfish - one lanternfish might have 2 days left until it creates another lanternfish, while another might have 4. So, you can model each fish as a single number that represents *the number of days until it creates a new lanternfish*.
+
+Furthermore, you reason, a *new* lanternfish would surely need slightly longer before it's capable of producing more lanternfish: two more days for its first cycle.
+
+So, suppose you have a lanternfish with an internal timer value of `3`:
+
+* After one day, its internal timer would become `2`.
+* After another day, its internal timer would become `1`.
+* After another day, its internal timer would become `0`.
+* After another day, its internal timer would reset to `6`, and it would create a *new* lanternfish with an internal timer of `8`.
+* After another day, the first lanternfish would have an internal timer of `5`, and the second lanternfish would have an internal timer of `7`.
+
+A lanternfish that creates a new fish resets its timer to `6`, *not `7`* (because `0` is included as a valid timer value). The new lanternfish starts with an internal timer of `8` and does not start counting down until the next day.
+
+Realizing what you're trying to do, the submarine automatically produces a list of the ages of several hundred nearby lanternfish (your puzzle input). For example, suppose you were given the following list:
+
+```
+3,4,3,1,2
+```
+
+This list means that the first fish has an internal timer of `3`, the second fish has an internal timer of `4`, and so on until the fifth fish, which has an internal timer of `2`. Simulating these fish over several days would proceed as follows:
+
+```
+Initial state: 3,4,3,1,2
+After  1 day:  2,3,2,0,1
+After  2 days: 1,2,1,6,0,8
+After  3 days: 0,1,0,5,6,7,8
+After  4 days: 6,0,6,4,5,6,7,8,8
+After  5 days: 5,6,5,3,4,5,6,7,7,8
+After  6 days: 4,5,4,2,3,4,5,6,6,7
+After  7 days: 3,4,3,1,2,3,4,5,5,6
+After  8 days: 2,3,2,0,1,2,3,4,4,5
+After  9 days: 1,2,1,6,0,1,2,3,3,4,8
+After 10 days: 0,1,0,5,6,0,1,2,2,3,7,8
+After 11 days: 6,0,6,4,5,6,0,1,1,2,6,7,8,8,8
+After 12 days: 5,6,5,3,4,5,6,0,0,1,5,6,7,7,7,8,8
+After 13 days: 4,5,4,2,3,4,5,6,6,0,4,5,6,6,6,7,7,8,8
+After 14 days: 3,4,3,1,2,3,4,5,5,6,3,4,5,5,5,6,6,7,7,8
+After 15 days: 2,3,2,0,1,2,3,4,4,5,2,3,4,4,4,5,5,6,6,7
+After 16 days: 1,2,1,6,0,1,2,3,3,4,1,2,3,3,3,4,4,5,5,6,8
+After 17 days: 0,1,0,5,6,0,1,2,2,3,0,1,2,2,2,3,3,4,4,5,7,8
+After 18 days: 6,0,6,4,5,6,0,1,1,2,6,0,1,1,1,2,2,3,3,4,6,7,8,8,8,8
+
+```
+
+Each day, a `0` becomes a `6` and adds a new `8` to the end of the list, while each other number decreases by 1 if it was present at the start of the day.
+
+In this example, after 18 days, there are a total of `26` fish. After 80 days, there would be a total of `*5934*`.
+
+Find a way to simulate lanternfish. *How many lanternfish would there be after 80 days?*
+
+Your puzzle answer was `366057`.
+
+\--- Part Two ---
+----------
+
+Suppose the lanternfish live forever and have unlimited food and space. Would they take over the entire ocean?
+
+After 256 days in the example above, there would be a total of `*26984457539*` lanternfish!
+
+*How many lanternfish would there be after 256 days?*
+
+Your puzzle answer was `1653559299811`.
+
+Both parts of this puzzle are complete! They provide two gold stars: \*\*
+
+At this point, you should [return to your Advent calendar](/2021) and try another puzzle.
+
+If you still want to see it, you can [get your puzzle input](6/input).

2021/day06_lanternfish/src/lib.rs

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+use std::num::ParseIntError;
+
+pub fn run(input: &str) -> Result<(usize, usize), ParseIntError> {
+    let fish: Vec<_> = input.split(',').map(|i| i.parse::<u8>()).collect::<Result<Vec<_>, _>>()?;
+    let mut ages: Vec<usize> = (0..=8).map(|age| fish.iter().filter(|fish_age| **fish_age == age).count()).collect();
+    for _ in 0..80 {
+        let new = ages[0];
+        (0..8).for_each(|age| ages[age] = ages[age+1]);
+        ages[6] += new;
+        ages[8] = new;
+    }
+    let first = ages.iter().sum();
+    for _ in 80..256 {
+        let new = ages[0];
+        (0..8).for_each(|age| ages[age] = ages[age+1]);
+        ages[6] += new;
+        ages[8] = new;
+    }
+    let second = ages.iter().sum();
+    Ok((first, second))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::fs::read_to_string;
+
+    fn read_file(name: &str) -> String {
+        read_to_string(name).expect(&format!("Unable to read file: {name}")[..]).trim().to_string()
+    }
+
+    #[test]
+    fn test_sample() {
+        let sample_input = read_file("tests/sample_input");
+        assert_eq!(run(&sample_input), Ok((5934, 26984457539)));
+    }
+
+    #[test]
+    fn test_challenge() {
+        let challenge_input = read_file("tests/challenge_input");
+        assert_eq!(run(&challenge_input), Ok((366057, 1653559299811)));
+    }
+}

2021/day06_lanternfish/tests/challenge_input

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+1,2,1,3,2,1,1,5,1,4,1,2,1,4,3,3,5,1,1,3,5,3,4,5,5,4,3,1,1,4,3,1,5,2,5,2,4,1,1,1,1,1,1,1,4,1,4,4,4,1,4,4,1,4,2,1,1,1,1,3,5,4,3,3,5,4,1,3,1,1,2,1,1,1,4,1,2,5,2,3,1,1,1,2,1,5,1,1,1,4,4,4,1,5,1,2,3,2,2,2,1,1,4,3,1,4,4,2,1,1,5,1,1,1,3,1,2,1,1,1,1,4,5,5,2,3,4,2,1,1,1,2,1,1,5,5,3,5,4,3,1,3,1,1,5,1,1,4,2,1,3,1,1,4,3,1,5,1,1,3,4,2,2,1,1,2,1,1,2,1,3,2,3,1,4,5,1,1,4,3,3,1,1,2,2,1,5,2,1,3,4,5,4,5,5,4,3,1,5,1,1,1,4,4,3,2,5,2,1,4,3,5,1,3,5,1,3,3,1,1,1,2,5,3,1,1,3,1,1,1,2,1,5,1,5,1,3,1,1,5,4,3,3,2,2,1,1,3,4,1,1,1,1,4,1,3,1,5,1,1,3,1,1,1,1,2,2,4,4,4,1,2,5,5,2,2,4,1,1,4,2,1,1,5,1,5,3,5,4,5,3,1,1,1,2,3,1,2,1,1

2021/day06_lanternfish/tests/sample_input

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+3,4,3,1,2