Day 1 complete

day_01/src/main.rs

@@ -1,3 +1,4 @@
+use std::collections::HashMap;
 use std::env;
 use std::error::Error;
 use std::fs::File;
@@ -18,8 +19,19 @@ fn parse_line(line: &str) -> Result<(i64, i64), Box<dyn Error>> {
     return Ok((first_element, second_element));
 }
 
-/// Part one of day 1
-/// Calculate the difference after sorting each list from smallest to biggest
+/// Part 1
+///
+/// Maybe the lists are only off by a small amount! To find out, pair
+/// up the numbers and measure how far apart they are. Pair up the
+/// smallest number in the left list with the smallest number in the
+/// right list, then the second-smallest left number with the
+/// second-smallest right number, and so on.
+///
+/// Within each pair, figure out how far apart the two numbers are;
+/// you'll need to add up all of those distances. For example, if you
+/// pair up a 3 from the left list with a 7 from the right list, the
+/// distance apart is 4; if you pair up a 9 with a 3, the distance
+/// apart is 6.
 fn calculate_diff(mut left_list: Vec<i64>, mut right_list: Vec<i64>) -> i64 {
     // sort lists
     left_list.sort();
@@ -35,6 +47,29 @@ fn calculate_diff(mut left_list: Vec<i64>, mut right_list: Vec<i64>) -> i64 {
     accumulator
 }
 
+/// Part 2
+///
+/// This time, you'll need to figure out exactly how often each
+/// number from the left list appears in the right list. Calculate
+/// a total similarity score by adding up each number in the left
+/// list after multiplying it by the number of times that number
+/// appears in the right list.
+fn calculate_similarity(left_list: Vec<i64>, right_list: Vec<i64>) -> i64 {
+    // calculate number of occurrences of each id in right list
+    let mut right_occurrences: HashMap<i64, i64> = HashMap::new();
+    for v in right_list {
+        *right_occurrences.entry(v).or_insert(0) += 1;
+    }
+
+    // Accumulate result
+    let mut accumulator = 0;
+    for v in left_list {
+        accumulator += v * right_occurrences.get(&v).unwrap_or(&0);
+    }
+
+    accumulator
+}
+
 fn main() -> Result<(), Box<dyn Error>> {
     // Handle command input
     let args: Vec<String> = env::args().collect();
@@ -60,7 +95,14 @@ fn main() -> Result<(), Box<dyn Error>> {
         right_list.push(parsed_line.1);
     }
 
-    println!("Result: {}", calculate_diff(left_list, right_list));
+    println!(
+        "Difference: {}",
+        calculate_diff(left_list.clone(), right_list.clone())
+    );
+    println!(
+        "Similarity: {}",
+        calculate_similarity(left_list, right_list)
+    );
 
     return Ok(());
 }