[TASK] Solved Day 15

src/day15.rs

@@ -0,0 +1,41 @@
+use super::util;
+
+pub fn solve() {
+    let lines = util::read_file("input/day15.txt");
+    let starting_numbers = lines[0].split(",").map(|s| s.parse::<usize>().unwrap()).collect::<Vec<_>>();
+
+    let part1 = play_until(2020, &starting_numbers);
+    println!("Day X Part 1: {}", part1);
+
+
+    let part2 = play_until(30_000_000, &starting_numbers);
+    println!("Day X Part 2: {}", part2);
+}
+
+fn play_until(max_turn: usize, starting_numbers: &Vec<usize>) -> usize {
+
+    // Because numbers can never be bigger than the number of turns, this works:
+    let mut last_occurrence = vec!{0usize; max_turn};
+
+    // Note: we ignore the last number here; it will be the "previous_number" later.
+    for i in 0..(starting_numbers.len() - 1) {
+        let n = starting_numbers[i];
+        last_occurrence[n] = i + 1;
+    }
+
+    let mut previous_number = starting_numbers[starting_numbers.len() - 1];
+    for turn in starting_numbers.len()..max_turn {
+
+        let prev_occurrence = last_occurrence[previous_number];
+        last_occurrence[previous_number] = turn;
+        if prev_occurrence == 0 {
+            previous_number = 0;
+        } else {
+            previous_number = turn - prev_occurrence;
+        }
+        println!("Turn {}: {}", turn + 1, previous_number);
+    }
+
+    return previous_number;
+
+}