aoc/year2023/
day07.rs

1//! # Camel Cards
2//!
3//! The types of each hand are computed from the frequency of the cards ordered in descending order.
4//! For example a full house would have 1 card with a frequency of 3 and a second with a
5//! frequency of 2, giving `[3, 2]`. Similarly two pair would be `[2, 2, 1]`.
6//! Array comparisons will sort the hand types in order.
7//!
8//! To make comparisons faster the frequencies and the card ranks are packed into a `usize`:
9//!
10//! * `55222` => `0x3200055222`
11//! * `32T3K` => `0x2111032a3d`
12//!
13//! For part two we add the numbers of jokers to the highest frequency (which could already be
14//! jokers!).
15//!
16//! * `QQQJA` => `0x41000ccc1a`
17use crate::util::parse::*;
18
19pub struct Hand {
20    cards: [u8; 5],
21    bid: usize,
22}
23
24pub fn parse(input: &str) -> Vec<Hand> {
25    input
26        .lines()
27        .map(|line| {
28            let (prefix, suffix) = line.split_at(5);
29            let cards = prefix.as_bytes().try_into().unwrap();
30            let bid = suffix.unsigned();
31            Hand { cards, bid }
32        })
33        .collect()
34}
35
36pub fn part1(input: &[Hand]) -> usize {
37    sort(input, 11)
38}
39
40pub fn part2(input: &[Hand]) -> usize {
41    sort(input, 1)
42}
43
44fn sort(input: &[Hand], j: usize) -> usize {
45    let mut hands: Vec<_> = input
46        .iter()
47        .map(|&Hand { cards, bid }| {
48            let rank = cards.map(|b| match b {
49                b'A' => 14,
50                b'K' => 13,
51                b'Q' => 12,
52                b'J' => j,
53                b'T' => 10,
54                _ => b.to_decimal() as usize,
55            });
56
57            let mut freq = [0; 15];
58            rank.iter().for_each(|&r| freq[r] += 1);
59
60            let jokers = freq[1];
61            freq[1] = 0;
62            freq.sort_unstable();
63            freq.reverse();
64            freq[0] += jokers;
65
66            // To speed up comparisons, pack the frequencies and card ranks
67            // into the nibbles of a `usize`.
68            let mut key = 0;
69
70            for f in freq.iter().take(5) {
71                key = (key << 4) | f;
72            }
73            for r in rank {
74                key = (key << 4) | r;
75            }
76
77            (key, bid)
78        })
79        .collect();
80
81    hands.sort_unstable();
82    hands.iter().enumerate().map(|(i, (_, bid))| (i + 1) * bid).sum()
83}
aoc/year2023/day07.rs

aoc/year2023/
day07.rs
