aoc/year2016/
day07.rs

1//! # Internet Protocol Version 7
2//!
3//! It's faster to treat the entire input as one big stream, using line breaks to increment
4//! the count if an address is valid.
5//!
6//! For part two there are at most 26 * 26 = 676 possible ABA or BAB sequences so we can use
7//! a fixed size array to keep track of which ones we've seen for the current address so far.
8pub fn parse(input: &str) -> &[u8] {
9    input.as_bytes()
10}
11
12pub fn part1(input: &[u8]) -> usize {
13    let mut count = 0;
14    let mut inside = false;
15    let mut positive = false;
16    let mut negative = false;
17
18    for w in input.windows(4) {
19        if w[0].is_ascii_lowercase() {
20            if w[0] == w[3] && w[1] == w[2] && w[0] != w[1] {
21                if inside {
22                    negative = true;
23                } else {
24                    positive = true;
25                }
26            }
27        } else if w[0] == b'[' {
28            inside = true;
29        } else if w[0] == b']' {
30            inside = false;
31        } else {
32            // Next line
33            if positive && !negative {
34                count += 1;
35            }
36            positive = false;
37            negative = false;
38        }
39    }
40
41    if positive && !negative { count + 1 } else { count }
42}
43
44pub fn part2(input: &[u8]) -> usize {
45    let mut count = 0;
46    let mut version = 0;
47    let mut inside = false;
48    let mut positive = false;
49    let mut aba = [usize::MAX; 676];
50    let mut bab = [usize::MAX; 676];
51
52    for w in input.windows(3) {
53        if w[1].is_ascii_lowercase() {
54            if w[0] == w[2] && w[0] != w[1] {
55                let first = (w[0] - b'a') as usize;
56                let second = (w[1] - b'a') as usize;
57
58                if inside {
59                    // Reverse the order of letters
60                    let index = 26 * second + first;
61                    bab[index] = version;
62                    positive |= aba[index] == version;
63                } else {
64                    let index = 26 * first + second;
65                    aba[index] = version;
66                    positive |= bab[index] == version;
67                }
68            }
69        } else if w[1] == b'[' {
70            inside = true;
71        } else if w[1] == b']' {
72            inside = false;
73        } else {
74            // Next line
75            if positive {
76                count += 1;
77            }
78            version += 1;
79            positive = false;
80        }
81    }
82
83    if positive { count + 1 } else { count }
84}
aoc/year2016/day07.rs

aoc/year2016/
day07.rs
