Day 01 Part 02 Solution

src/bin/01.rs

@@ -24,18 +24,56 @@ pub fn part_one(input: &str) -> Option<u32> {
         .collect::<Vec<_>>();
 
     // compute pair-wise distances
-    let distances = (0..pairs.len()).map(|line_index| {
-        (0..sorted.len()).map(|list_index| {
-            sorted[list_index][line_index]
-        }).reduce(|acc, e| acc.abs_diff(e)).unwrap()
-    }).collect::<Vec<_>>();
+    let distances = (0..pairs.len())
+        .map(|line_index| {
+            (0..sorted.len())
+                .map(|list_index| sorted[list_index][line_index])
+                .reduce(|acc, e| acc.abs_diff(e))
+                .unwrap()
+        })
+        .collect::<Vec<_>>();
 
     // sum the distances
     Some(distances.iter().sum())
 }
 
 pub fn part_two(input: &str) -> Option<u32> {
-    None
+    // convert string into vector of vectors of numbers in the same configuration
+    let pairs = input
+        .lines()
+        .map(|line| {
+            line.split("   ")
+                .map(|item| item.parse::<u32>().unwrap())
+                .collect::<Vec<_>>()
+        })
+        .collect::<Vec<_>>();
+
+    // split the lists into individual vectors
+    let lists = (0..pairs[0].len())
+        .map(|list_index| {
+            pairs
+                .iter()
+                .map(|line| line[list_index])
+                .collect::<Vec<_>>()
+        })
+        .collect::<Vec<_>>();
+
+    // build counts of items in second list
+    let mut second_set_counts = std::collections::HashMap::new();
+    lists[1].iter().for_each(|item| {
+        second_set_counts
+            .entry(item)
+            .and_modify(|e| *e += 1)
+            .or_insert(1);
+    });
+
+    // multiply items in first list by counts from second list and sum up
+    let similarity = lists[0]
+        .iter()
+        .map(|item| item * second_set_counts.get(item).unwrap_or(&0))
+        .sum();
+
+    Some(similarity)
 }
 
 #[cfg(test)]
@@ -51,6 +89,6 @@ mod tests {
     #[test]
     fn test_part_two() {
         let result = part_two(&advent_of_code::template::read_file("examples", DAY));
-        assert_eq!(result, None);
+        assert_eq!(result, Some(31));
     }
 }