Reddit Daily Programmer 320 Hard

Hard.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Xml;
using System.Xml.Linq;
using System.Xml.XPath;

namespace Reddit
{
    internal static class Hard
    {
        internal static void Run()
        {
            Run("Molecule");
        }

        internal static void Run(string input)
        {
            var visited = new HashSet<string>();
            visited.Add("Philosophy");

            string value = input;
            while (value != null)
            {
                Console.WriteLine(value);
                if (visited.Contains(value))
                {
                    value = null;
                }
                else
                {
                    visited.Add(value);
                    value = Next(value);
                }
            }
        }

        internal static string Next(string input)
        {
            string content = Download(input);
            if (content != null)
            {
                var parentheses = new Parentheses();
                parentheses.Load(content);

                var root = XElement.Parse(content, LoadOptions.SetLineInfo);

                var items = root.Descendants("a").ToList();
                if (items.Count > 0)
                {
                    foreach (var item in items.Where(e =>
                        e.Attribute("title") != null &&
                        e.Attribute("href") != null))
                    {
                        int position = ((IXmlLineInfo)item).LinePosition;
                        if (parentheses.Surround(position) == false)
                        {
                            return item.Attribute("title").Value;
                        }
                    }
                }
            }
            return null;
        }

        internal static string Download(string input)
        {
            var uri = new Uri("https://en.wikipedia.org/wiki/" + input);
            using (var client = new WebClient())
            {
                string value = client.DownloadString(uri);

                var root = XElement.Parse(value);
                var content = root.XPathSelectElement(
                    "/body/div[@id='content']/div[@id='bodyContent']" +
                    "/descendant::p[1]");

                if (content != null)
                {
                    return content.ToString(SaveOptions.DisableFormatting);
                }
            }
            return null;
        }
    }

    internal class Parentheses
    {
        internal List<Node> Nodes;

        internal void Load(string input)
        {
            const char start = '(', end = ')';

            var root = new Node();
            root.Index = -1;
            root.Length = -1;

            var child = root;

            for (int i = 0; i < input.Length; i++)
            {
                if (input[i] == start)
                {
                    var node = new Node();
                    node.Index = i;
                    node.Length = -1;
                    node.Parent = child;

                    child.Add(node);
                    child = node;
                }
                else
                {
                    if (input[i] == end)
                    {
                        if (child.Index != -1)
                        {
                            child.Length = (i - child.Index) + 1;
                            child = child.Parent;
                        }
                    }
                }
            }

            Nodes = new List<Node>(root.Flatten());
        }

        internal bool Surround(int position)
        {
            foreach (var node in Nodes)
            {
                if (position >= node.Index &&
                    position <= node.Index + node.Length)
                {
                    return true;
                }
            }
            return false;
        }

        internal class Node
        {
            internal int Index;
            internal int Length;

            internal Node Parent;

            internal List<Node> Children = new List<Node>();

            internal void Add(Node node)
            {
                Children.Add(node);
            }

            internal IEnumerable<Node> Flatten()
            {
                if (Length != -1)
                {
                    yield return this;
                }
                else
                {
                    foreach (var child in Children)
                    {
                        foreach (var node in child.Flatten())
                        {
                            yield return node;
                        }
                    }
                }
            }
        }
    }
}