aoc/year2015/day11.rs
1//! # Corporate Policy
2//!
3//! Like the [previous day] we rely on the special structure of the input to solve
4//! more efficiently than the general case.
5//!
6//! The key observation is that if there are no straights or pairs in the first 4 digits then the
7//! next lowest valid sequence will end in a string of the form `aabcc`,
8//! compressing 2 pairs and a straight into 5 digits.
9//!
10//! * If the current string ends with `xxyzz` then increment the third digit and wrap around
11//! to `aabcc`.
12//! * The 5 digit sequence cannot start with any letter from `g` to `o` inclusive or it would
13//! contain an invalid character somewhere in the sequence.
14//!
15//! [previous day]: crate::year2015::day10
16use std::str::from_utf8;
17
18type Password = [u8; 8];
19type Input = [Password; 2];
20
21pub fn parse(input: &str) -> Input {
22 let password = clean(input.trim().as_bytes().try_into().unwrap());
23
24 // No pairs in the first 4 characters
25 let pair = |i, j| password[i] == password[j];
26 assert!(!(pair(0, 1) | pair(1, 2) | pair(2, 3)));
27
28 // No straights in the first 4 characters
29 let sequence = |i, j| password[j] > password[i] && password[j] - password[i] == 1;
30 assert!(!(sequence(1, 2) && (sequence(0, 1) || sequence(2, 3))));
31
32 // No potential carry in the third character
33 assert_ne!(password[2], b'z');
34
35 let first = next_password(password);
36 let second = next_password(first);
37 [first, second]
38}
39
40pub fn part1(input: &Input) -> &str {
41 from_utf8(&input[0]).unwrap()
42}
43
44pub fn part2(input: &Input) -> &str {
45 from_utf8(&input[1]).unwrap()
46}
47
48/// Sanitize the input to make sure it has no invalid characters. We increment the first invalid
49/// character found, for example `abcixyz` becomes `abcjaaa`.
50fn clean(mut password: Password) -> Password {
51 if let Some(index) = password.iter().position(|&d| matches!(d, b'i' | b'o' | b'l')) {
52 password[index] += 1;
53 password[index + 1..].fill(b'a');
54 }
55 password
56}
57
58/// Find the next valid 5 digit sequence of form `aabcc`.
59fn next_password(mut password: Password) -> Password {
60 // If the sequence would contain any illegal character, then skip to the next possible
61 // valid sequence starting with `p`.
62 if (b'g'..b'o').contains(&password[3]) {
63 return fill(password, b'p');
64 }
65
66 // If there's room then check if a sequence starting with the current character is
67 // higher than the current password.
68 if password[3] <= b'x' {
69 let candidate = fill(password, password[3]);
70 if candidate > password {
71 return candidate;
72 }
73 }
74
75 // Otherwise we need to increment the first digit of the sequence.
76 if password[3] == b'x' {
77 // If it starts with `x` then increment the third digit and wrap around.
78 password[2] += if matches!(password[2], b'h' | b'n' | b'k') { 2 } else { 1 };
79 fill(password, b'a')
80 } else if password[3] == b'f' {
81 // If it would enter the invalid range from `g` to `o` then take the next valid start `p`.
82 fill(password, b'p')
83 } else {
84 // Otherwise increment the first digit.
85 fill(password, password[3] + 1)
86 }
87}
88
89/// Creates a sequence of form `aabcc` from an arbitrary starting character.
90fn fill(mut password: Password, start: u8) -> Password {
91 password[3] = start;
92 password[4] = start;
93 password[5] = start + 1;
94 password[6] = start + 2;
95 password[7] = start + 2;
96 password
97}