Added Solution for 2020 day 15

2020/day15_rambunctious_recitation/Cargo.toml

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+[package]
+name = "day15_rambunctious_recitation"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

2020/day15_rambunctious_recitation/challenge.txt

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+You catch the airport shuttle and try to book a new flight to your vacation island. Due to the storm, all direct flights have been cancelled, but a route is available to get around the storm. You take it.
+
+While you wait for your flight, you decide to check in with the Elves back at the North Pole. They're playing a *memory game* and are ever so excited to explain the rules!
+
+In this game, the players take turns saying *numbers*. They begin by taking turns reading from a list of *starting numbers* (your puzzle input). Then, each turn consists of considering the *most recently spoken number*:
+
+* If that was the *first* time the number has been spoken, the current player says *`0`*.
+* Otherwise, the number had been spoken before; the current player announces *how many turns apart* the number is from when it was previously spoken.
+
+So, after the starting numbers, each turn results in that player speaking aloud either *`0`* (if the last number is new) or an *age* (if the last number is a repeat).
+
+For example, suppose the starting numbers are `0,3,6`:
+
+* *Turn 1*: The `1`st number spoken is a starting number, *`0`*.
+* *Turn 2*: The `2`nd number spoken is a starting number, *`3`*.
+* *Turn 3*: The `3`rd number spoken is a starting number, *`6`*.
+* *Turn 4*: Now, consider the last number spoken, `6`. Since that was the first time the number had been spoken, the `4`th number spoken is *`0`*.
+* *Turn 5*: Next, again consider the last number spoken, `0`. Since it *had* been spoken before, the next number to speak is the difference between the turn number when it was last spoken (the previous turn, `4`) and the turn number of the time it was most recently spoken before then (turn `1`). Thus, the `5`th number spoken is `4 - 1`, *`3`*.
+* *Turn 6*: The last number spoken, `3` had also been spoken before, most recently on turns `5` and `2`. So, the `6`th number spoken is `5 - 2`, *`3`*.
+* *Turn 7*: Since `3` was just spoken twice in a row, and the last two turns are `1` turn apart, the `7`th number spoken is *`1`*.
+* *Turn 8*: Since `1` is new, the `8`th number spoken is *`0`*.
+* *Turn 9*: `0` was last spoken on turns `8` and `4`, so the `9`th number spoken is the difference between them, *`4`*.
+* *Turn 10*: `4` is new, so the `10`th number spoken is *`0`*.
+
+(The game ends when the Elves get sick of playing or dinner is ready, whichever comes first.)
+
+Their question for you is: what will be the *`2020`th* number spoken? In the example above, the `2020`th number spoken will be `436`.
+
+Here are a few more examples:
+
+* Given the starting numbers `1,3,2`, the `2020`th number spoken is `1`.
+* Given the starting numbers `2,1,3`, the `2020`th number spoken is `10`.
+* Given the starting numbers `1,2,3`, the `2020`th number spoken is `27`.
+* Given the starting numbers `2,3,1`, the `2020`th number spoken is `78`.
+* Given the starting numbers `3,2,1`, the `2020`th number spoken is `438`.
+* Given the starting numbers `3,1,2`, the `2020`th number spoken is `1836`.
+
+Given your starting numbers, *what will be the `2020`th number spoken?*
+
+Your puzzle answer was `959`.
+
+\--- Part Two ---
+----------
+
+Impressed, the Elves issue you a challenge: determine the `30000000`th number spoken. For example, given the same starting numbers as above:
+
+* Given `0,3,6`, the `30000000`th number spoken is `175594`.
+* Given `1,3,2`, the `30000000`th number spoken is `2578`.
+* Given `2,1,3`, the `30000000`th number spoken is `3544142`.
+* Given `1,2,3`, the `30000000`th number spoken is `261214`.
+* Given `2,3,1`, the `30000000`th number spoken is `6895259`.
+* Given `3,2,1`, the `30000000`th number spoken is `18`.
+* Given `3,1,2`, the `30000000`th number spoken is `362`.
+
+Given your starting numbers, *what will be the `30000000`th number spoken?*
+
+Your puzzle answer was `116590`.
+
+Both parts of this puzzle are complete! They provide two gold stars: \*\*
+
+At this point, you should [return to your Advent calendar](/2020) and try another puzzle.
+
+Your puzzle input was `18,11,9,0,5,1`.

2020/day15_rambunctious_recitation/src/lib.rs

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+use std::num::ParseIntError;
+
+pub fn run(input: &str) -> Result<(usize, usize), ParseIntError> {
+    let initial: Vec<_> = input.split(',').map(|n| n.parse::<usize>()).collect::<Result<Vec<_>, _>>()?;
+    let mut said_last = vec![0; 30_000_000];
+    initial.iter().enumerate().for_each(|(idx, i)| {said_last[*i] = idx+1});
+    let mut last_number = initial[initial.len()-1];
+    let mut next_number = initial.iter().rev().skip(1).position(|n| *n == last_number).map(|n| n+1).unwrap_or(0);
+    (initial.len()+1..=2020).for_each(|turn| {
+        last_number = next_number;
+        if said_last[last_number] == 0 {
+            next_number = 0;
+        } else {
+            next_number = turn - said_last[last_number];
+        }
+        said_last[last_number] = turn;
+    });
+    let first = last_number;
+    (2021..=30_000_000).for_each(|turn| {
+        last_number = next_number;
+        if said_last[last_number] == 0 {
+            next_number = 0;
+        } else {
+            next_number = turn - said_last[last_number];
+        }
+        said_last[last_number] = turn;
+    });
+    let second = last_number;
+    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((436, 175594)));
+    }
+
+    #[test]
+    fn test_challenge() {
+        let challenge_input = read_file("tests/challenge_input");
+        assert_eq!(run(&challenge_input), Ok((959, 116590)));
+    }
+}

2020/day15_rambunctious_recitation/tests/challenge_input

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+18,11,9,0,5,1

2020/day15_rambunctious_recitation/tests/sample_input

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+0,3,6