Solution for 2022/day13-part2

2022/day13-part2/Cargo.toml

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+[package]
+name = "day13-part2"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

2022/day13-part2/src/main.rs

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+use std::{cmp::Ordering, iter::zip, str::Chars};
+
+#[derive(Debug)]
+enum Packet {
+    Number(i32),
+    List(Vec<Packet>),
+}
+
+impl Packet {
+    // Parse a single packet line into the Packet data structure.
+    fn from_string(s: &mut Chars) -> Vec<Packet> {
+        // println!("Entering with {}", s.clone().collect::<String>());
+        // We're going to iterate through this character by character.
+        // It is assumed that the opening '[' is already stripped.
+        // Create an empty vector packet to fill in a loop.
+        let mut packet_list: Vec<Packet> = Vec::new();
+        // Now iterate through all the remaining characters.
+        loop {
+            // Collect a single element, i.e. all characters until the first occurence of '[', ']' or ','.
+            // The delimiter itself will be collected into `c`.
+            let mut elem = String::new();
+            let mut c: char;
+            loop {
+                c = s.next().unwrap();
+                if c == '[' || c == ']' || c == ',' {
+                    break;
+                } else {
+                    elem.push(c);
+                }
+            }
+            // Parse the element if it isn't empty and add it to the list.
+            if !elem.is_empty() {
+                let num = Packet::Number(elem.parse().unwrap());
+                packet_list.push(num);
+            }
+
+            // Encountering a new list?
+            // Call recursively and collect everything there.
+            if c == '[' {
+                let sublist = Packet::from_string(s);
+                packet_list.push(Packet::List(sublist));
+            }
+
+            // Encountered the end of the list?
+            // Then we're done here.
+            if c == ']' {
+                break;
+            }
+        }
+        packet_list
+    }
+}
+
+fn main() {
+    // Use command line arguments to specify the input filename.
+    let args: Vec<String> = std::env::args().collect();
+    if args.len() < 2 {
+        panic!("Usage: ./main <input-file>\nNo input file provided. Exiting.");
+    }
+
+    // Next, read the contents of the input file into a string for easier processing.
+    let input = std::fs::read_to_string(&args[1]).expect("Error opening file");
+
+    // --- TASK BEGIN ---
+
+    // Collect all actual packets into a big packet vector.
+    let mut packets: Vec<_> = input
+        .lines()
+        .filter(|l| !l.is_empty())
+        .map(|l| Packet::from_string(&mut l[1..].chars()))
+        .collect();
+    
+    // Create copies of the divider packages and add them to the big vector.
+    packets.push(vec![Packet::List(vec![Packet::Number(2)])]);
+    packets.push(vec![Packet::List(vec![Packet::Number(6)])]);
+
+    // The magic line. Sort the vector using `packet_compare`.
+    packets.sort_unstable_by(packet_compare);
+
+    // Find the indices of the 2 and 6 divider.
+    let idx2 = packets.iter().enumerate().find(|(_, x)| {
+        packet_compare(
+            x,
+            &vec![Packet::List(vec![Packet::Number(2)])],
+        ).is_eq()
+    }).unwrap().0;
+
+    let idx6 = packets.iter().enumerate().find(|(_, x)| {
+        packet_compare(
+            x,
+            &vec![Packet::List(vec![Packet::Number(6)])],
+        ).is_eq()
+    }).unwrap().0;
+
+    print_packet_list(&packets);
+    println!("Decoder key: {}", (idx2 + 1) * (idx6 + 1));
+}
+
+fn print_packet_list(packet_list: &Vec<Vec<Packet>>) {
+    for packet in packet_list {
+        print_packet(packet);
+        // Add newlines after every "top-level" packet.
+        println!();
+    }
+}
+
+fn print_packet(packet: &Vec<Packet>) {
+    print!("[");
+    for e in packet {
+        match e {
+            Packet::Number(x) => {
+                print!("{x},");
+            }
+            Packet::List(l) => {
+                print_packet(l);
+            }
+        }
+    }
+    print!("]");
+}
+
+fn packet_compare(left: &Vec<Packet>, right: &Vec<Packet>) -> Ordering {
+    let mut result: Ordering;
+    let mut zipper = zip(left, right);
+    loop {
+        match zipper.next() {
+            Some(d) => {
+                // Determine the Ordering result for the next two "elements", whatever they may be.
+                result = match d {
+                    (Packet::Number(lv), Packet::Number(rv)) => {
+                        // Compare the integers directly.
+                        lv.cmp(rv)
+                    }
+                    (Packet::Number(lv), Packet::List(rl)) => {
+                        // Left is a number, right is a list.
+                        // Convert the number to a list and then compare those.
+                        let ll: Vec<Packet> = vec![Packet::Number(*lv)];
+                        packet_compare(&ll, rl)
+                    }
+                    (Packet::List(ll), Packet::Number(rv)) => {
+                        // Left is a list, right is a number.
+                        // Convert the number to a list and then compare those.
+                        let rl: Vec<Packet> = vec![Packet::Number(*rv)];
+                        packet_compare(ll, &rl)
+                    }
+                    (Packet::List(ll), Packet::List(rl)) => {
+                        // Recursively step into the lists.
+                        packet_compare(ll, rl)
+                    }
+                };
+            }
+            None => {
+                // Zipper empty?
+                // We need to remember which of the lists ran out first.
+                result = left.len().cmp(&right.len());
+                // And definitely break, ofc, since the zipper is done.
+                break;
+            }
+        }
+        // Have we reached a conclusion already? If so, return.
+        if result != Ordering::Equal {
+            break;
+        }
+    }
+    result
+}