2020: day 15 part 2

2 files changed, 41 insertions, 0 deletions

diff --git a/y2020/src/d15.rs b/y2020/src/d15.rs
index 7a02dba..1c0048e 100644
--- a/y2020/src/d15.rs
+++ b/y2020/src/d15.rs
@@ -1,6 +1,7 @@
 use std::collections::HashMap;
 
 const COUNT: usize = 2020;
+const PART2_COUNT: usize = 30000000;
 
 pub fn part1<T: Iterator<Item = String>>(e: T) -> usize {
     let mut i = e.collect::<String>().split(",")
@@ -24,6 +25,29 @@ pub fn part1<T: Iterator<Item = String>>(e: T) -> usize {
     ls
 }
 
+
+pub fn part2<T: Iterator<Item = String>>(e: T) -> usize {
+    let mut i = e.collect::<String>().split(",")
+        .map(|n| n.parse().unwrap())
+        .collect::<Vec<usize>>();
+
+    // add the starting values to HashMap
+    let mut m: HashMap<usize, usize> = HashMap::new();
+    i.iter()
+        .enumerate()
+        .for_each(|(i, x)| { m.insert(*x, i + 1); });
+
+    // loop through each turn updating HashMap
+    let mut ls = i.remove(i.len() - 1);
+    for t in i.len() + 1..PART2_COUNT {
+        let s = m.get(&ls).unwrap_or(&t).to_owned();
+        m.insert(ls, t);
+        ls = t - s;
+    }
+
+    ls
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -35,10 +59,24 @@ mod tests {
         (1836, ["3,1,2"])
     ];
 
+    static EX2: [(usize, [&str; 1]); 7] = [
+        (175594, ["0,3,6"]), (2578, ["1,3,2"]),
+        (3544142, ["2,1,3"]), (261214, ["1,2,3"]),
+        (6895259, ["2,3,1"]), (18, ["3,2,1"]),
+        (362, ["3,1,2"])
+    ];
+
     #[test]
     fn test() {
         for (a, i) in EX.iter() {
             assert_eq!(*a, part1(i.iter().map(|x| x.to_string())));
         }
     }
+
+    #[test]
+    fn test2() {
+        for (a, i) in EX2.iter() {
+            assert_eq!(*a, part2(i.iter().map(|x| x.to_string())));
+        }
+    }
 }
diff --git a/y2020/src/main.rs b/y2020/src/main.rs
index 5c9beb6..088eecc 100644
--- a/y2020/src/main.rs
+++ b/y2020/src/main.rs
@@ -146,5 +146,8 @@ fn main() {
     if args.is_empty() || args.contains("15") {
         let a = d15::part1(get_input_for_day(15));
         println!("2020: day 15 part 1: {}", a);
+
+        let a = d15::part2(get_input_for_day(15));
+        println!("2020: day 15 part 2: {}", a);
     }
 }