complete day 3

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+/// --- Day 3: Rucksack Reorganization ---
+///
+/// One Elf has the important job of loading all of the rucksacks with supplies for the jungle
+/// journey.  Unfortunately, that Elf didn't quite follow the packing instructions, and so a few
+/// items now need to be rearranged.
+///
+/// Each rucksack has two large compartments.  All items of a given type are meant to go into
+/// exactly one of the two compartments.  The Elf that did the packing failed to follow this rule
+/// for exactly one item type per rucksack.
+///
+/// The Elves have made a list of all of the items currently in each rucksack (your puzzle input),
+/// but they need your help finding the errors.  Every item type is identified by a single lowercase
+/// or uppercase letter (that is, a and A refer to different types of items).
+///
+/// The list of items for each rucksack is given as characters all on a single line.  A given
+/// rucksack always has the same number of items in each of its two compartments, so the first half
+/// of the characters represent items in the first compartment, while the second half of the
+/// characters represent items in the second compartment.
+///
+/// For example, suppose you have the following list of contents from six rucksacks:
+///
+/// ```
+/// vJrwpWtwJgWrhcsFMMfFFhFp
+/// jqHRNqRjqzjGDLGLrsFMfFZSrLrFZsSL
+/// PmmdzqPrVvPwwTWBwg
+/// wMqvLMZHhHMvwLHjbvcjnnSBnvTQFn
+/// ttgJtRGJQctTZtZT
+/// CrZsJsPPZsGzwwsLwLmpwMDw
+/// ```
+///
+///     The first rucksack contains the items vJrwpWtwJgWrhcsFMMfFFhFp, which means its first
+///     compartment contains the items vJrwpWtwJgWr, while the second compartment contains the
+///     items hcsFMMfFFhFp.  The only item type that appears in both compartments is lowercase p.
+///     The second rucksack's compartments contain jqHRNqRjqzjGDLGL and rsFMfFZSrLrFZsSL.  The only
+///     item type that appears in both compartments is uppercase L.
+///     The third rucksack's compartments contain PmmdzqPrV and vPwwTWBwg; the only common item
+///     type is uppercase P.
+///     The fourth rucksack's compartments only share item type v.
+///     The fifth rucksack's compartments only share item type t.
+///     The sixth rucksack's compartments only share item type s.
+///
+/// To help prioritize item rearrangement, every item type can be converted to a priority:
+///
+///     Lowercase item types a through z have priorities 1 through 26.
+///     Uppercase item types A through Z have priorities 27 through 52.
+///
+/// In the above example, the priority of the item type that appears in both compartments of each
+/// rucksack is 16 (p), 38 (L), 42 (P), 22 (v), 20 (t), and 19 (s); the sum of these is 157.
+///
+/// Find the item type that appears in both compartments of each rucksack. What is the sum of the
+/// priorities of those item types?
+use clap::Parser;
+
+use std::fs::File;
+use std::io::prelude::*;
+use std::io::BufReader;
+use std::path::PathBuf;
+
+const FILEPATH: &'static str = "examples/input.txt";
+
+#[derive(Parser, Debug)]
+#[clap(author, version, about, long_about = None)]
+struct Cli {
+    #[clap(short, long, default_value = FILEPATH)]
+    file: PathBuf,
+}
+
+fn main() {
+    let args = Cli::parse();
+
+    let file = File::open(&args.file).unwrap();
+    let reader = BufReader::new(file);
+    let res = reader
+        .lines()
+        .map(|l| l.unwrap())
+        .inspect(|l| {
+            if l.len() % 2 != 0 {
+                panic!("odd number of items in rucksack")
+            }
+        })
+        .map(|l| {
+            let (l, r) = l.split_at(l.len() / 2);
+            (l.to_owned(), r.to_owned())
+        })
+        .map(|(lhs, rhs)| {
+            lhs.chars()
+                .filter(|l| rhs.contains(*l))
+                .map(|l| {
+                    let b = l as u8;
+                    if b > b'a' {
+                        (b - b'a') as u16 + 1
+                    } else {
+                        (b - b'A') as u16 + 27
+                    }
+                })
+                .next()
+                .unwrap()
+        })
+        .sum::<u16>();
+
+    println!(
+        "The sum of the priorities of the item that appear in both compartments of each rucksack is {}",
+        res
+    );
+}