2020: day01: part 2

aoc2020/src/day01.rs

@@ -8,6 +8,7 @@ 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)
 }
@@ -18,13 +19,24 @@ fn part1(input: &str) -> Result<u64> {
         .map(|line| line.parse::<u64>().map_err(|e| err!("{}", e)))
         .collect::<Result<Vec<u64>>>()?;
 
-    let (a, b) = find_2020_sum(&entries)?;
+    let (a, b) = find_2020_2_sum(&entries)?;
 
     Ok(a * b)
 }
 
-fn find_2020_sum(entries: &[u64]) -> Result<(u64, u64)> {
-    for i in 0..(entries.len()) {
+fn part2(input: &str) -> Result<u64> {
+    let entries = input
+        .lines()
+        .map(|line| line.parse::<u64>().map_err(|e| err!("{}", e)))
+        .collect::<Result<Vec<u64>>>()?;
+
+    let (a, b, c) = find_2020_3_sum(&entries)?;
+
+    Ok(a * b * c)
+}
+
+fn find_2020_2_sum(entries: &[u64]) -> Result<(u64, u64)> {
+    for i in 0..entries.len() {
         for j in 0..entries.len() {
             if i == j {
                 continue;
@@ -38,19 +50,53 @@ fn find_2020_sum(entries: &[u64]) -> Result<(u64, u64)> {
     Err(err!("couldn't find 2 elements that sum to 2020"))
 }
 
+fn find_2020_3_sum(entries: &[u64]) -> Result<(u64, u64, u64)> {
+    for i in 0..entries.len() {
+        for j in 0..entries.len() {
+            for k in 0..entries.len() {
+                if i == j || j == k || k == i {
+                    continue;
+                }
+
+                let a = entries[i];
+                let b = entries[j];
+                let c = entries[k];
+
+                if a + b + c == 2020 {
+                    return Ok((a, b, c));
+                }
+            }
+        }
+    }
+
+    Err(err!("couldn't find 2 elements that sum to 2020"))
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
 
+    static PROVIDED: &[u64] = &[1721, 979, 366, 299, 675, 1456];
+
     #[test]
     fn part1_provided() {
-        let (a, b) = find_2020_sum(&vec![1721, 979, 366, 299, 675, 1456]).unwrap();
-        assert_eq!(a, 1721);
-        assert_eq!(b, 299);
+        let (a, b) = find_2020_2_sum(PROVIDED).unwrap();
+        assert_eq!(a + b, 2020);
     }
 
     #[test]
     fn part1_real() {
         assert_eq!(part1(INPUT).unwrap(), 1014171);
     }
+
+    #[test]
+    fn part2_provided() {
+        let (a, b, c) = find_2020_3_sum(PROVIDED).unwrap();
+        assert_eq!(a + b + c, 2020);
+    }
+
+    #[test]
+    fn part2_real() {
+        assert_eq!(part2(INPUT).unwrap(), 46584630);
+    }
 }