Day 15 - Part 2, brute force

day15/README.md

@@ -5,3 +5,13 @@ https://adventofcode.com/2020/day/15
 ## Part 1
 
 In order to be memory efficient, we should use a HashMap of which number was last spoken at which turn. We load it up with the input numbers and begin looping turns starting from input length.
+
+## Part 2
+
+Let's see if the brute-force solution from Part 1 works in reasonable time for this much larger N... Well, if you consider 2 seconds when compiled with `--release` flag reasonable, then I guess it worked. :)
+
+Still, could we find a pattern that helps us figure out a much more efficient algorithm?
+
+For example, the input `3,2,1` is interesting:
+  - the 2020th number spoken is 438
+  - the 30000000th number spoken is 18

day15/src/main.rs

@@ -1,6 +1,9 @@
 use std::collections::HashMap;
 
-fn solve(lines: Vec<String>) -> u32 {
+const PART1: u32 = 2020;
+const PART2: u32 = 30000000;
+
+fn solve(lines: &[String], target_turn: u32) -> u32 {
     let starting_numbers: Vec<u32> = lines[0].split(',').map(|s| s.parse().unwrap()).collect();
 
     // HashMap<K, V>, where number K was last spoken at turn V
@@ -27,7 +30,7 @@ fn solve(lines: Vec<String>) -> u32 {
         number_just_spoken = next_number;
         turn += 1;
 
-        if turn == 2020 {
+        if turn == target_turn {
             break;
         }
     }
@@ -37,25 +40,31 @@ fn solve(lines: Vec<String>) -> u32 {
 
 fn main() {
     let lines = utils::read_lines();
-    println!("Part 1: {}", solve(lines));
+    println!("Part 1: {}", solve(&lines, PART1));
+    println!("Part 2: {}", solve(&lines, PART2));
 }
 
 #[cfg(test)]
 mod tests {
     use super::*;
 
     #[test]
-    fn solve_works_with_initial_example() {
-        assert_eq!(solve(vec!["0,3,6".to_string()]), 436);
+    fn part1_works_with_initial_example() {
+        assert_eq!(solve(&vec!["0,3,6".to_string()], PART1), 436);
+    }
+
+    #[test]
+    fn part1_works_with_other_examples() {
+        assert_eq!(solve(&vec!["1,3,2".to_string()], PART1), 1);
+        assert_eq!(solve(&vec!["2,1,3".to_string()], PART1), 10);
+        assert_eq!(solve(&vec!["1,2,3".to_string()], PART1), 27);
+        assert_eq!(solve(&vec!["2,3,1".to_string()], PART1), 78);
+        assert_eq!(solve(&vec!["3,2,1".to_string()], PART1), 438);
+        assert_eq!(solve(&vec!["3,1,2".to_string()], PART1), 1836);
     }
 
     #[test]
-    fn solve_works_with_other_examples() {
-        assert_eq!(solve(vec!["1,3,2".to_string()]), 1);
-        assert_eq!(solve(vec!["2,1,3".to_string()]), 10);
-        assert_eq!(solve(vec!["1,2,3".to_string()]), 27);
-        assert_eq!(solve(vec!["2,3,1".to_string()]), 78);
-        assert_eq!(solve(vec!["3,2,1".to_string()]), 438);
-        assert_eq!(solve(vec!["3,1,2".to_string()]), 1836);
+    fn part2_works_with_initial_example() {
+        assert_eq!(solve(&vec!["0,3,6".to_string()], PART2), 175594);
     }
 }