StackOverflow THAT

posts.xml

<?xml version="1.0" encoding="utf-8" ?>
<rows>
  <row Id="4" 
       PostTypeId="1"
       Body="This is the body of question 4"
       Title="When setting a form's opacity should I use a decimal or double"
       AnswerCount="5"
       />
  
  <row Id="6"
       PostTypeId="1"
       Body="This is the body of question 6"
       Title="Percentage width child element in absolutely positioned parent on Internet Explorer 7"
       AnswerCount="5"
       />
  <row Id="7"
     PostTypeId="1"
     ParentId="4"
     Body="This is the body of answer 7 of question 4"
       />
</rows>

program.cs

using System;
using System.Collections.Generic;
using System.IO;
using System.Xml;
using System.Text;
using System.Xml.Linq;
using System.IO.Compression;

namespace StackOverflowThat
{
    class Program
    {
        static void Main(string[] args)
        {
            PreparationPhase();
            OutputPhase();
        }

        static void PreparationPhase()
        {
            var postsFilePath = Path.Combine(AppDomain.CurrentDomain.BaseDirectory, "Posts.xml");
            using (var textReader = new StreamReader(postsFilePath, Encoding.UTF8))
            using (var xmlReader = new XmlTextReader(textReader))
            {
                foreach (var row in EnumerateRows(xmlReader))
                {
                    ProcessRow(row);
                }
            }
        }

        static IEnumerable<XElement> EnumerateRows(XmlReader reader)
        {
            reader.MoveToContent();
            while (reader.Read())
            {
                if (reader.NodeType == XmlNodeType.Element)
                {
                    yield return XNode.ReadFrom(reader) as XElement;
                }
            }
        }

        static void ProcessRow(XElement row)
        {
            var parentId = (string)row.Attribute("ParentId");
            if (parentId == null)
            {
                var filePath = AppDomain.CurrentDomain.BaseDirectory + GetFilePath((string)row.Attribute("Id"));
                var folderPath = Path.GetDirectoryName(filePath);
                if (!Directory.Exists(folderPath)) Directory.CreateDirectory(folderPath);
                row.Name = "question";
                File.WriteAllText(filePath, row.ToString(), Encoding.UTF8);
            }
            else
            {
                var filePath = AppDomain.CurrentDomain.BaseDirectory + GetFilePath(parentId);
                XElement question;
                using (var reader = new StreamReader(filePath, Encoding.UTF8))
                {
                    question = XElement.Load(reader);
                }
                row.Name = "answer";
                question.Add(row);
                File.WriteAllText(filePath, question.ToString(), Encoding.UTF8);
            }
        }

        /// <summary>
        /// Gets the file path given the row id. 
        /// Files are organized in folders, each folder can have up to 1000 files
        /// Large id's produce deeper paths.
        /// For example: id = 1234567, path = 1\234\567.xml
        /// I do this to avoid to have too many files in a single folder.
        /// </summary>
        /// <param name="id"></param>
        /// <returns></returns>
        static string GetFilePath(string id)
        {
            string path = ".xml";
            string remainingString = id;

            while (remainingString.Length > 0)
            {
                var startIndex = remainingString.Length - 3;
                if (startIndex < 0) startIndex = 0;
                path = "\\" + remainingString.Substring(startIndex) + path;
                remainingString = remainingString.Substring(0, startIndex);
            }
            return path;
        }

        static void OutputPhase()
        {
            var outputFilePath = Path.Combine(AppDomain.CurrentDomain.BaseDirectory, "Ouput.gzip");
            using (var stream = File.Open(outputFilePath, FileMode.Create, FileAccess.Write))
            using (var gzip = new GZipStream(stream, CompressionMode.Compress))
            {
                AddXmlFilesToStream(AppDomain.CurrentDomain.BaseDirectory, gzip);
            }
        }

        static void AddXmlFilesToStream(string folderPath, Stream output)
        {
            foreach (var filePath in Directory.EnumerateFiles(folderPath, "*.xml"))
            {
                using (var file = File.OpenRead(filePath))
                {
                    file.CopyTo(output);
                }
            }
            foreach (var subDirectory in Directory.EnumerateDirectories(folderPath))
            {
                AddXmlFilesToStream(subDirectory, output);
            }
        }
    }
}

program2.cs

using System;
using System.Collections.Generic;
using System.IO;
using System.Xml;
using System.Text;
using System.Xml.Linq;
using System.IO.Compression;

namespace StackOverflowThat
{
    class Program
    {
        static void Main(string[] args)
        {
            PreparationPhase();
            OutputPhase();
        }

        static void PreparationPhase()
        {
            WriteQuestionFiles();
            AddAnswersToQuestions();
        }

        static void WriteQuestionFiles()
        {
            foreach (var row in EnumerateRows())
            {
                var parentId = (string)row.Attribute("ParentId");
                if (parentId == null)
                {
                    var filePath = AppDomain.CurrentDomain.BaseDirectory + GetFilePath((string)row.Attribute("Id"));
                    var folderPath = Path.GetDirectoryName(filePath);
                    if (!Directory.Exists(folderPath)) Directory.CreateDirectory(folderPath);
                    row.Name = "question";
                    File.WriteAllText(filePath, row.ToString(), Encoding.UTF8);
                }
            }
        }

        static void AddAnswersToQuestions()
        {
            foreach (var row in EnumerateRows())
            {
                var parentId = (string)row.Attribute("ParentId");
                if (parentId != null)
                {
                    var filePath = AppDomain.CurrentDomain.BaseDirectory + GetFilePath(parentId);
                    XElement question;
                    using (var reader = new StreamReader(filePath, Encoding.UTF8))
                    {
                        question = XElement.Load(reader);
                    }
                    row.Name = "answer";
                    question.Add(row);
                    File.WriteAllText(filePath, question.ToString(), Encoding.UTF8);
                }
            }
        }

        static IEnumerable<XElement> EnumerateRows()
        {
            var postsFilePath = Path.Combine(AppDomain.CurrentDomain.BaseDirectory, "Posts.xml");
            using (var textReader = new StreamReader(postsFilePath, Encoding.UTF8))
            using (var reader = new XmlTextReader(textReader))
            {
                reader.MoveToContent();
                while (reader.Read())
                {
                    if (reader.NodeType == XmlNodeType.Element)
                    {
                        yield return XNode.ReadFrom(reader) as XElement;
                    }
                }
            }
        }

        /// <summary>
        /// Gets the file path given the row id. 
        /// Files are organized in folders, each folder can have up to 1000 files
        /// Large id's produce deeper paths.
        /// For example: id = 1234567, path = 1\234\567.xml
        /// I do this to avoid to have too many files in a single folder.
        /// </summary>
        /// <param name="id"></param>
        /// <returns></returns>
        static string GetFilePath(string id)
        {
            string path = ".xml";
            string remainingString = id;

            while (remainingString.Length > 0)
            {
                var startIndex = remainingString.Length - 3;
                if (startIndex < 0) startIndex = 0;
                path = "\\" + remainingString.Substring(startIndex) + path;
                remainingString = remainingString.Substring(0, startIndex);
            }
            return path;
        }

        static void OutputPhase()
        {
            var outputFilePath = Path.Combine(AppDomain.CurrentDomain.BaseDirectory, "Ouput.gzip");
            using (var stream = File.Open(outputFilePath, FileMode.Create, FileAccess.Write))
            using (var gzip = new GZipStream(stream, CompressionMode.Compress))
            {
                AddXmlFilesToStream(AppDomain.CurrentDomain.BaseDirectory, gzip);
            }
        }

        static void AddXmlFilesToStream(string folderPath, Stream output)
        {
            foreach (var filePath in Directory.EnumerateFiles(folderPath, "*.xml"))
            {
                using (var file = File.OpenRead(filePath))
                {
                    file.CopyTo(output);
                }
            }
            foreach (var subDirectory in Directory.EnumerateDirectories(folderPath))
            {
                AddXmlFilesToStream(subDirectory, output);
            }
        }
    }
}

This is the response to this interview question from ayende https://ayende.com/blog/175617/interview-question-stackoverflow-that

Program.cs assumes answers come later than questions in posts.xml. Program2.cs doesn't, therefore it needs to read posts.xml twice.