Complete day 15

day-15/src/main.rs

@@ -0,0 +1,121 @@
+use std::collections::HashMap;
+
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    // Part 1
+    let numbers = parse_numbers("1,20,11,6,12,0")?;
+    let result = play(&numbers, 2020);
+    println!("{}", result);
+
+    // Part 2
+    let numbers = parse_numbers("1,20,11,6,12,0")?;
+    let result = play(&numbers, 30000000);
+    println!("{}", result);
+
+    Ok(())
+}
+
+fn parse_numbers(input: &str) -> Result<Vec<u32>, std::num::ParseIntError> {
+    input.split(',').map(|number| number.parse()).collect()
+}
+
+fn play(numbers: &Vec<u32>, turns: usize) -> u32 {
+    let mut said = HashMap::new();
+    for (turn, number) in numbers.iter().enumerate() {
+        said.insert(*number, (turn + 1, 0));
+    }
+    let mut last = *numbers.last().expect("Must have at least one number");
+
+    for turn in numbers.len()..turns {
+        match said.get(&last) {
+            Some((_, 0)) | None => {
+                last = 0;
+
+                let entry = said.entry(last).or_insert((0, 0));
+                *entry = (turn + 1, entry.0);
+            }
+            Some((last_turn, last_last_turn)) => {
+                last = (last_turn - last_last_turn) as u32;
+
+                let entry = said.entry(last).or_insert((0, 0));
+                *entry = (turn + 1, entry.0);
+            }
+        };
+    }
+
+    last
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_simple() -> Result<(), Box<dyn std::error::Error>> {
+        let input = "0,3,6";
+        let numbers = parse_numbers(input)?;
+        let result = play(&numbers, 2020);
+
+        assert_eq!(result, 436);
+        Ok(())
+    }
+
+    #[test]
+    fn test_simple0() -> Result<(), Box<dyn std::error::Error>> {
+        let input = "1,3,2";
+        let numbers = parse_numbers(input)?;
+        let result = play(&numbers, 2020);
+
+        assert_eq!(result, 1);
+        Ok(())
+    }
+
+    #[test]
+    fn test_simple1() -> Result<(), Box<dyn std::error::Error>> {
+        let input = "2,1,3";
+        let numbers = parse_numbers(input)?;
+        let result = play(&numbers, 2020);
+
+        assert_eq!(result, 10);
+        Ok(())
+    }
+
+    #[test]
+    fn test_simple2() -> Result<(), Box<dyn std::error::Error>> {
+        let input = "1,2,3";
+        let numbers = parse_numbers(input)?;
+        let result = play(&numbers, 2020);
+
+        assert_eq!(result, 27);
+        Ok(())
+    }
+
+    #[test]
+    fn test_simple3() -> Result<(), Box<dyn std::error::Error>> {
+        let input = "2,3,1";
+        let numbers = parse_numbers(input)?;
+        let result = play(&numbers, 2020);
+
+        assert_eq!(result, 78);
+        Ok(())
+    }
+
+    #[test]
+    fn test_simple4() -> Result<(), Box<dyn std::error::Error>> {
+        let input = "3,2,1";
+        let numbers = parse_numbers(input)?;
+        let result = play(&numbers, 2020);
+
+        assert_eq!(result, 438);
+        Ok(())
+    }
+
+    #[test]
+    fn test_simple5() -> Result<(), Box<dyn std::error::Error>> {
+        let input = "3,1,2";
+        let numbers = parse_numbers(input)?;
+        let result = play(&numbers, 2020);
+
+        assert_eq!(result, 1836);
+        Ok(())
+    }
+}