2020: day15: part 2

aoc2020/input/day15_provided.txt

@@ -1 +1,7 @@
 0,3,6
+1,3,2
+2,1,3
+1,2,3
+2,3,1
+3,2,1
+3,1,2

aoc2020/src/day15.rs

@@ -1,33 +1,51 @@
-use std::collections::HashMap;
 use std::fmt::Write;
 
 use anyhow::Result;
 
 const INPUT: &str = include_str!("../input/day15.txt");
+const PART1_TURNS: usize = 2020;
+const PART2_TURNS: usize = 30_000_000;
 
 pub fn run() -> Result<String> {
     let mut res = String::with_capacity(128);
 
     writeln!(res, "part 1: {}", part1(INPUT)?)?;
+    writeln!(res, "part 2: {}", part2(INPUT)?)?;
 
     Ok(res)
 }
 
 fn part1(input: &str) -> Result<u64> {
-    let starting_numbers = input.trim_end()
+    let starting_numbers = input
+        .trim_end()
         .split(',')
         .map(|num| num.parse().map_err(anyhow::Error::new))
         .collect::<Result<Vec<u64>>>()?;
 
-    let mut spoken_numbers = HashMap::new();
+    Ok(get_last_spoken_number(&starting_numbers, PART1_TURNS))
+}
+
+fn part2(input: &str) -> Result<u64> {
+    let starting_numbers = input
+        .trim_end()
+        .split(',')
+        .map(|num| num.parse().map_err(anyhow::Error::new))
+        .collect::<Result<Vec<u64>>>()?;
+
+    Ok(get_last_spoken_number(&starting_numbers, PART2_TURNS))
+}
+
+fn get_last_spoken_number(starting_numbers: &[u64], game_length: usize) -> u64 {
+    let mut spoken_numbers = Vec::new();
+    spoken_numbers.resize_with(game_length, Default::default);
     let mut next_number: u64 = 0;
-    let mut turn: usize = 0;
+    let mut turn: usize = 1;
 
     for num in starting_numbers {
-        let last_turn = speak_number(num, turn, &mut spoken_numbers);
+        let last_turn = speak_number(*num, turn, &mut spoken_numbers);
         match last_turn {
-            Some(prev_turn) => next_number = (turn - prev_turn) as u64,
-            None => next_number = 0,
+            0 => next_number = 0,
+            prev_turn => next_number = (turn - prev_turn) as u64,
         }
 
         turn += 1;
@@ -35,32 +53,32 @@ fn part1(input: &str) -> Result<u64> {
 
     let mut last_number: u64 = 0;
 
-    while turn < 2020 {
+    while turn <= game_length {
         // store the number we're about to speak for the solution
         last_number = next_number;
 
         // get the previous time this number was spoken, if any
         let last_turn = speak_number(next_number, turn, &mut spoken_numbers);
         match last_turn {
+            // it's the first time it appeared, we'll say 0 next turn
+            0 => next_number = 0,
             // if it was spoken before, the number we'll say next turn is the difference between the
             // two turns where it was last spoken
-            Some(prev_turn) => next_number = (turn - prev_turn) as u64,
-            // otherwise we'll say 0 next turn
-            None => next_number = 0,
+            prev_turn => next_number = (turn - prev_turn) as u64,
         }
 
         turn += 1
     }
 
-    Ok(last_number)
+    last_number
 }
 
 /// Inserts the turn a number was spoken in the map. Returns the previous turn the number was
 /// spoken, if any
-fn speak_number(n: u64, turn: usize, spoken_numbers: &mut HashMap<u64, usize>) -> Option<usize> {
-    let res = spoken_numbers.get(&n).copied();
+fn speak_number(n: u64, turn: usize, spoken_numbers: &mut [usize]) -> usize {
+    let res = spoken_numbers[n as usize];
 
-    spoken_numbers.insert(n, turn);
+    spoken_numbers[n as usize] = turn;
 
     res
 }
@@ -73,11 +91,31 @@ mod tests {
 
     #[test]
     fn part1_provided() {
-        assert_eq!(part1(PROVIDED).unwrap(), 436);
+        let expected = [436, 1, 10, 27, 78, 438, 1836];
+
+        for (line, expected) in PROVIDED.lines().zip(expected.iter()) {
+            assert_eq!(part1(line).unwrap(), *expected);
+        }
     }
 
     #[test]
     fn part1_real() {
         assert_eq!(part1(INPUT).unwrap(), 447);
     }
+
+    #[test]
+    #[ignore]
+    fn part2_provided() {
+        let expected = [175594, 2578, 3544142, 261214, 6895259, 18, 362];
+
+        for (line, expected) in PROVIDED.lines().zip(expected.iter()) {
+            assert_eq!(part2(line).unwrap(), *expected);
+        }
+    }
+
+    #[test]
+    #[ignore]
+    fn part2_real() {
+        assert_eq!(part2(INPUT).unwrap(), 11721679);
+    }
 }