day 15.

part two is slow.

Author: erning

README.md

@@ -50,4 +50,16 @@ Part Two: 2002
 --- Day 12: Rain Risk ---
 Part One: 1007
 Part Two: 41212
+
+--- Day 13: Shuttle Search ---
+Part One: 1835
+Part Two: 247086664214628
+
+--- Day 14: Docking Data ---
+Part One: 7477696999511
+Part Two: 3687727854171
+
+--- Day 15: Rambunctious Recitation ---
+Part One: 1111
+Part Two: 48568
 ```

inputs/15-example.txt

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

inputs/15-input.txt

@@ -0,0 +1 @@
+20,9,11,0,1,2

src/day15.rs

@@ -0,0 +1,82 @@
+use std::collections::HashMap;
+
+fn parse_input(input: &str) -> Vec<usize> {
+    input
+        .trim()
+        .split(',')
+        .map(|s| s.parse().unwrap())
+        .collect()
+}
+
+fn target_number(numbers: Vec<usize>, target: usize) -> usize {
+    let n = numbers.len();
+    let mut visited = numbers
+        .iter()
+        .take(n - 1)
+        .enumerate()
+        .map(|(i, &num)| (num, vec![i + 1]))
+        .collect::<HashMap<_, _>>();
+    let mut last = numbers[n - 1];
+    for i in numbers.len()..target {
+        if let Some(prev) = visited.get_mut(&last) {
+            let j = *prev.last().unwrap();
+            *prev = vec![j, i];
+            last = i - j;
+        } else {
+            visited.insert(last, vec![i]);
+            last = 0;
+        }
+    }
+    last
+}
+
+pub fn part_one(input: &str) -> usize {
+    let numbers = parse_input(input);
+    target_number(numbers, 2020)
+}
+
+pub fn part_two(input: &str) -> usize {
+    let numbers = parse_input(input);
+    target_number(numbers, 30000000)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::read_example;
+
+    #[test]
+    fn example() {
+        let input = read_example(15);
+        assert_eq!(part_one(&input), 436);
+        assert_eq!(part_two(&input), 175594);
+    }
+
+    #[test]
+    fn more_examples() {
+        for (input, expected) in vec![
+            ("1,3,2", 1),
+            ("2,1,3", 10),
+            ("1,2,3", 27),
+            ("2,3,1", 78),
+            ("3,2,1", 438),
+            ("3,1,2", 1836),
+        ] {
+            assert_eq!(part_one(input), expected);
+        }
+    }
+
+    #[test]
+    fn more_examples_part_two() {
+        for (input, expected) in vec![
+            ("1,3,2", 2578),
+            ("2,1,3", 3544142),
+            ("1,2,3", 261214),
+            ("2,3,1", 6895259),
+            ("3,2,1", 18),
+            ("3,1,2", 362),
+        ] {
+            assert_eq!(part_two(input), expected);
+        }
+    }
+}

src/lib.rs

@@ -14,6 +14,7 @@ pub mod day11;
 pub mod day12;
 pub mod day13;
 pub mod day14;
+pub mod day15;
 
 pub fn read_as_string(day: u8, filename: &str) -> String {
     let filename = format!("inputs/{:02}-{}.txt", day, filename);

src/main.rs

@@ -30,6 +30,7 @@ fn main() {
         puzzle!(day12, "Rain Risk"),
         puzzle!(day13, "Shuttle Search"),
         puzzle!(day14, "Docking Data"),
+        puzzle!(day15, "Rambunctious Recitation"),
     ];
 
     let filename = match env::args().find(|a| a == "--example") {