A large stream blocks your path. According to the locals, it's not safe to cross the stream at the moment because it's full of garbage. You look down at the stream; rather than water, you discover that it's a stream of characters.

You sit for a while and record part of the stream (your puzzle input). The characters represent groups - sequences that begin with { and end with }. Within a group, there are zero or more other things, separated by commas: either another group or garbage. Since groups can contain other groups, a } only closes the most-recently-opened unclosed group - that is, they are nestable. Your puzzle input represents a single, large group which itself contains many smaller ones.

Read the full puzzle.

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

namespace AdventOfCode.Y2017.Day09;

[ProblemName("Stream Processing")]
class Solution : Solver {

    public object PartOne(string input) => BlockScores(input).Sum();
    public object PartTwo(string input) => Classify(input).Where((x) => x.garbage).Count();

    IEnumerable<int> BlockScores(string input) {
        var score = 0;
        foreach (var ch in Classify(input).Where((x) => !x.garbage).Select(x => x.ch)) {
            if (ch == '}') {
                score--;
            } else if (ch == '{') {
                score++;
                yield return score;
            }
        }
    }

    IEnumerable<(char ch, bool garbage)> Classify(string input) {
        var skip = false;
        var garbage = false;
        foreach (var ch in input) {
            if (garbage) {
                if (skip) {
                    skip = false;
                } else {
                    if (ch == '>') {
                        garbage = false;
                    } else if (ch == '!') {
                        skip = true;
                    } else {
                        yield return (ch, garbage);
                    }
                }
            } else {
                if (ch == '<') {
                    garbage = true;
                } else {
                    yield return (ch, garbage);
                }
            }
        }
    }
}

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