Solutions for 2022, as well as 2015-2018 and 2019 up to day 11

2015/day25-let_it_snow/Cargo.toml

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+[package]
+name = "day25-let_it_snow"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

2015/day25-let_it_snow/challenge.txt

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+\--- Day 25: Let It Snow ---
+----------
+
+Merry Christmas! Santa is booting up his weather machine; looks like you might get a [white Christmas](1) after all.
+
+The weather machine beeps! On the console of the machine is a copy protection message asking you to [enter a code from the instruction manual](https://en.wikipedia.org/wiki/Copy_protection#Early_video_games). Apparently, it refuses to run unless you give it that code. No problem; you'll just look up the code in the--
+
+"Ho ho ho", Santa ponders aloud. "I can't seem to find the manual."
+
+You look up the support number for the manufacturer and give them a call. Good thing, too - that 49th star wasn't going to earn itself.
+
+"Oh, that machine is quite old!", they tell you. "That model went out of support six minutes ago, and we just finished shredding all of the manuals. I bet we can find you the code generation algorithm, though."
+
+After putting you on hold for twenty minutes (your call is *very* important to them, it reminded you repeatedly), they finally find an engineer that remembers how the code system works.
+
+The codes are printed on an infinite sheet of paper, starting in the top-left corner. The codes are filled in by diagonals: starting with the first row with an empty first box, the codes are filled in diagonally up and to the right. This process repeats until the [infinite paper is covered](https://en.wikipedia.org/wiki/Cantor's_diagonal_argument). So, the first few codes are filled in in this order:
+
+```
+   | 1   2   3   4   5   6  
+---+---+---+---+---+---+---+
+ 1 |  1   3   6  10  15  21
+ 2 |  2   5   9  14  20
+ 3 |  4   8  13  19
+ 4 |  7  12  18
+ 5 | 11  17
+ 6 | 16
+
+```
+
+For example, the 12th code would be written to row `4`, column `2`; the 15th code would be written to row `1`, column `5`.
+
+The voice on the other end of the phone continues with how the codes are actually generated. The first code is `20151125`. After that, each code is generated by taking the previous one, multiplying it by `252533`, and then keeping the remainder from dividing that value by `33554393`.
+
+So, to find the second code (which ends up in row `2`, column `1`), start with the previous value, `20151125`. Multiply it by `252533` to get `5088824049625`. Then, divide that by `33554393`, which leaves a remainder of `31916031`. That remainder is the second code.
+
+"Oh!", says the voice. "It looks like we missed a scrap from one of the manuals. Let me read it to you." You write down his numbers:
+
+```
+   |    1         2         3         4         5         6
+---+---------+---------+---------+---------+---------+---------+
+ 1 | 20151125  18749137  17289845  30943339  10071777  33511524
+ 2 | 31916031  21629792  16929656   7726640  15514188   4041754
+ 3 | 16080970   8057251   1601130   7981243  11661866  16474243
+ 4 | 24592653  32451966  21345942   9380097  10600672  31527494
+ 5 |    77061  17552253  28094349   6899651   9250759  31663883
+ 6 | 33071741   6796745  25397450  24659492   1534922  27995004
+
+```
+
+"Now remember", the voice continues, "that's not even all of the first few numbers; for example, you're missing the one at 7,1 that would come before 6,2. But, it should be enough to let your-- oh, it's time for lunch! Bye!" The call disconnects.
+
+Santa looks nervous. Your puzzle input contains the message on the machine's console. *What code do you give the machine?*
+
+Your puzzle answer was `9132360`.
+
+\--- Part Two ---
+----------
+
+The machine springs to life, then falls silent again. It beeps. "Insufficient fuel", the console reads. "*Fifty stars* are required before proceeding. *One star* is available."
+
+..."one star is available"? You check the fuel tank; sure enough, a lone star sits at the bottom, awaiting its friends. Looks like you need to provide 49 yourself.
+
+If you like, you can .
+
+Both parts of this puzzle are complete! They provide two gold stars: \*\*
+
+At this point, all that is left is for you to [admire your Advent calendar](/2015).
+
+If you still want to see it, you can [get your puzzle input](25/input).

2015/day25-let_it_snow/src/lib.rs

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+pub fn run(input: &str) -> usize {
+    let components: Vec<_> = input.split(' ').collect();
+    assert_eq!(components.len(), 19);
+    let row = components[16][..components[16].len()-1].parse::<usize>().unwrap();
+    let col = components[18][..components[18].len()-2].parse::<usize>().unwrap();
+    code(get_sequence_number(row, col))
+}
+
+pub fn get_sequence_number(row: usize, col: usize) -> usize {
+     if row == 1 {
+        (col)*(col+1)/2    
+    } else {
+        // there is probably some way to calculate this directly, but I'm to tired to find it..
+        get_sequence_number(row-1, col+1) - 1
+    }
+}
+
+fn code(sequence_number: usize) -> usize {
+    let mut res = 20151125;
+    for _ in 1..sequence_number {
+        res = (res * 252533) % 33554393;
+    }
+    res
+}
+
+#[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)[..])
+    }
+
+    #[test]
+    fn find_id() {
+        let expected = vec![
+                vec![1, 3, 6, 10, 15, 21],
+                vec![2, 5, 9, 14, 20],
+                vec![4, 8, 13, 19],
+                vec![7, 12, 18],
+                vec![11, 17],
+                vec![16]
+            ];
+        for row in 0..expected.len() {
+            for col in 0..expected[row].len() {
+                assert_eq!(get_sequence_number(row+1, col+1), expected[row][col]);
+            }
+        }
+    }
+
+    #[test]
+    fn test_challenge() {
+        let challenge_input = read_file("tests/challenge_input");
+        assert_eq!(run(&challenge_input), 9132360);
+    }
+}

2015/day25-let_it_snow/tests/challenge_input

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+To continue, please consult the code grid in the manual.  Enter the code at row 2981, column 3075.