Day 1 on Day (almost) 3, sorry I'm late

src/advent_of_code/mod.rs

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+mod sun_01;
+
+pub fn today() {
+    let key = sun_01::sun_01("./assets/day_1_input").unwrap();
+}

src/advent_of_code/sun_01.rs

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+use std::{fs::read_to_string, ops::Sub};
+
+type Number = i32;
+type Key = i32;
+
+pub fn sun_01(input: &str) -> Result<Key, ()> {
+    let mut list_1: Vec<Number> = vec![];
+    let mut list_2: Vec<Number> = vec![];
+    let mut key: Number = 0;
+
+    for line in read_to_string(input).unwrap().lines() {
+        if let Some((one, two)) = line.split_once("   ") {
+            list_1.push(one.parse::<Number>().unwrap());
+            list_2.push(two.parse::<Number>().unwrap());
+        }
+    }
+
+    if list_1.len() != list_2.len() {
+        return Err(());
+    }
+
+    // 1. Pair up the numbers by size and compare the distance between them: Sorting
+    list_1.sort();
+    list_2.sort();
+
+    // 2. Calc the distance between the pair elements
+    for index in 0..list_1.len() {
+        key += calc_distance(list_1[index], list_2[index]);
+    }
+
+    Ok(Key)
+}
+
+fn calc_distance<T>(num_1: T, num_2: T) -> T
+where
+    T: PartialOrd + Sub<Output = T>,
+{
+    if num_1 > num_2 {
+        return num_1 - num_2;
+    }
+    return num_2 - num_1;
+}

src/main.rs

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+mod advent_of_code;
+
+fn main() {
+    advent_of_code::today();
+    println!("Happy new year!");
+}