While snooping around the local network of EBHQ, you compile a list of IP addresses (they're IPv7, of course; IPv6 is much too limited). You'd like to figure out which IPs support TLS (transport-layer snooping).

An IP supports TLS if it has an Autonomous Bridge Bypass Annotation, or ABBA. An ABBA is any four-character sequence which consists of a pair of two different characters followed by the reverse of that pair, such as xyyx or abba. However, the IP also must not have an ABBA within any hypernet sequences, which are contained by square brackets.

Read the full puzzle.

using System.Collections.Generic;
using System.Linq;

namespace AdventOfCode.Y2016.Day07;

[ProblemName("Internet Protocol Version 7")]
class Solution : Solver {

    public object PartOne(string input) =>
        input.Split('\n').Count(TLS);

    public object PartTwo(string input) =>
        input.Split('\n').Count(SSL);


    bool TLS(string st) =>
        Classify(st).Any(c => !c.f && Abba(c.st).Any()) &&
        Classify(st).All(c => !c.f || !Abba(c.st).Any());

    bool SSL(string st) => (
        from c1 in Classify(st)
        from c2 in Classify(st)
        where !c1.f && c2.f
        from aba in Aba(c1.st)
        let bab = $"{aba[1]}{aba[0]}{aba[1]}"
        where c2.st.Contains(bab)
        select true
    ).Any();

    IEnumerable<(string st, bool f)> Classify(string st) {
        var part = "";
        for (var i = 0; i < st.Length; i++) {
            var ch = st[i];
            if (ch == '[') {
                yield return (part, false);
                part = "";
            } else if (ch == ']') {
                yield return (part, true);
                part = "";
            } else {
                part += ch;
            }
        }
        if (part != "")
            yield return (part, false);
    }

    IEnumerable<string> Abba(string st) {
        for (var i = 0; i < st.Length - 3; i++) {
            if(st[i + 2] == st[i + 1] && st[i] == st[i + 3] && st[i] != st[i + 2])
                yield return st.Substring(i, 4);
        }
    }
    
    IEnumerable<string> Aba(string st) {
        for (var i = 0; i < st.Length -2; i++) {
            if(st[i] == st[i + 2] && st[i] != st[i + 1])
                yield return st.Substring(i, 3);
        }
    }
}

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