aoc/year2019/
day04.rs

1//! # Secure Container
2//!
3//! We speed things up by only checking numbers that have digits in non-decreasing order for pairs.
4//! These numbers become rapidly less dense as the password value increases and there
5//! are only 3003 total of these numbers with 6 digits.
6use crate::util::parse::*;
7use crate::util::slice::*;
8
9pub fn parse(input: &str) -> Vec<u32> {
10    input.iter_unsigned().collect()
11}
12
13/// Password must contain at least one pair.
14pub fn part1(input: &[u32]) -> u32 {
15    let predicate = |first: bool, second: bool, third: bool, fourth: bool, fifth: bool| {
16        first || second || third || fourth || fifth
17    };
18    passwords(input, predicate)
19}
20
21/// Password must contain at least one pair that's not part of a larger group.
22pub fn part2(input: &[u32]) -> u32 {
23    let predicate = |first: bool, second: bool, third: bool, fourth: bool, fifth: bool| {
24        (first && !second)
25            || (!first && second && !third)
26            || (!second && third && !fourth)
27            || (!third && fourth && !fifth)
28            || (!fourth && fifth)
29    };
30    passwords(input, predicate)
31}
32
33fn passwords(input: &[u32], predicate: impl Fn(bool, bool, bool, bool, bool) -> bool) -> u32 {
34    let start = input[0];
35    let end = input[1];
36
37    // Split into six digits.
38    let mut digits = [
39        start / 100000,
40        (start / 10000) % 10,
41        (start / 1000) % 10,
42        (start / 100) % 10,
43        (start / 10) % 10,
44        start % 10,
45    ];
46
47    // Increase the starting number to the next number that has all digits in non-decreasing order
48    // to ensure that the incrementing logic in the search loop works correctly.
49    // For example 223450 => 223455, 120568 => 122222 and 439999 => 444444.
50    for i in 1..6 {
51        if digits[i] < digits[i - 1] {
52            for j in i..6 {
53                digits[j] = digits[i - 1];
54            }
55            break;
56        }
57    }
58
59    let mut n = 0;
60    let mut count = 0;
61
62    while n <= end {
63        // Check current number
64        let first = digits[0] == digits[1];
65        let second = digits[1] == digits[2];
66        let third = digits[2] == digits[3];
67        let fourth = digits[3] == digits[4];
68        let fifth = digits[4] == digits[5];
69
70        if predicate(first, second, third, fourth, fifth) {
71            count += 1;
72        }
73
74        // Find the next number with all digits in non-decreasing order.
75        let mut i = 5;
76        while digits[i] == 9 {
77            i -= 1;
78        }
79
80        let next = digits[i] + 1;
81        while i <= 5 {
82            digits[i] = next;
83            i += 1;
84        }
85
86        // Convert number to `u32`.
87        n = digits.fold_decimal();
88    }
89
90    count
91}
aoc/year2019/day04.rs

aoc/year2019/
day04.rs
