Problem of the Day -- Markdown Parser

Markdown Parser

Happy Monday! Hope you had a good weekend. Over the weekend Github Flavored Markdown and permalinking were added to comments. Hopefully this makes posting and sharing easier for all. Other cool new features are on the way!

In honor of the Markdown parser upgrade today's problem will be somewhat related. We'll be building a very basic Markdown compiler of our own. Our Markdown parser will parse the following:

Headers
Single depth lists
Paragraphs

Sample input:

## Monday March 24th 2014 - Todo List

* Solve the Problem of the Day
* Eat breakfast
* Do work
* Party

Today was a good day

Sample output:

<h2>Monday March 24th 2014 - Todo List</h2>
<ul>
    <li>Solve the Problem of the Day</li>
    <li>Eat breakfast</li>
    <li>Do work</li>
    <li>Party</li>
</ul>
<p>Today was a good day</p>

Permalink: http://problemotd.com/problem/markdown-parser/

Comments:

Carlos - 10 years, 9 months ago
Well, parsers are just hard work, so here's a simple solution that might work for most cases:

public static void parse(String text) { boolean bullets = false; String[] lines = text.split("\r\n|\r|\n"); String result = ""; int size = lines.length; for(int i=0; i<size; i++) { if(lines[i].compareTo("\n") == 0) { result += ""; } else if(lines[i].startsWith("##")) { result += "<h2>" + lines[i].substring(3, lines[i].length()) + "</h2>\n"; } else if(lines[i].startsWith("") && !bullets) { result += "<ul>\n"; result += "\t<li>" + lines[i].substring(2, lines[i].length()) + "</li>\n"; bullets = true; } else if(lines[i].startsWith("") && bullets) { result += "\t<li>" + lines[i].substring(2, lines[i].length()) + "</li>\n"; if(i+1 < size) { if(!lines[i+1].startsWith("*")) { result += "</ul>\n"; bullets = false; }
} } else { result += "<p>" + lines[i] + "</p>\n"; } }
```
    System.out.println(result);
}
```
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Carlos - 10 years, 9 months ago
Well, the code got f*cked up, have no idea what went wrong, it's about time you some accounts into this site and give us the ability to edit OR a preview button.

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Max Burstein - 10 years, 9 months ago

Accounts are on the way. I realize this is a major point so it should hopefully be done sometime within the next week or 2. The site now supports github flavored markdown though so posting code is as easy as placing it between 3 backticks. https://help.github.com/articles/github-flavored-markdown#fenced-code-blocks

Thanks for your submission!

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Carlos - 10 years, 9 months ago
Thanks for the reply and the understanding, sorry if i sounded pedantic and uneducated, had a bit of a rough morning.

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Apanatshka - 10 years, 9 months ago
I was going to implement this in SDF3 and Stratego, but then I realised that Markdown has some pretty specific whitespace rules in its syntax and SDF3 doesn't have a nice way of expressing that yet :(

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Hueho - 10 years, 9 months ago

I went with the easiest way to just get the output for the input, without cheating:

def header(line)
  size = line.each_char.reduce(0) { |p, n| if n == '#' then p + 1 else break p end }
  content = (line.gsub /[#]+/, '').strip
  "<h#{size}>#{content}<h#{size}>\n"
end

def list(lines)
  size = lines.reduce(0) { |p, n| if n.start_with? '*' then p + 1 else break p end }
  content = lines[0, size].map { |l| 
    stripped = l.gsub(/[\*]+/, '').strip
    "\t<li>#{stripped}</li>" 
  }.join("\n")
  return "<ul>\n#{content}\n</ul>\n", size
end

def paragraph(lines)
  size = lines.reduce(0) { |p, n| 
    unless n.strip.empty? or n.start_with? '#', '*' then p + 1 else break p end 
  }
  return "<p>#{lines[0, size].join.strip}</p>\n", size
end

def parse(lines)
  idx, len = 0, lines.size
  result = ''

  while idx < len
    case lines[idx][0]
    when '#' then 
      result << header(lines[idx])
      idx += 1
    when '*' then
      content, skip = list(lines[idx..-1])
      result << content
      idx += skip
    else
      unless lines[idx].strip.empty? then
        content, skip = paragraph(lines[idx..-1])
        result << content
        idx += skip
      else
        idx += 1
      end
    end
  end

  result
end

print(parse ARGF.readlines)

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Max Burstein - 10 years, 9 months ago

Looks good and easy to read. I like it

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vick - 10 years, 9 months ago
My humble solution would be: public static String render(String markdownText) throws Exception { BufferedReader br = new BufferedReader(new StringReader(markdownText)); StringBuilder sb = new StringBuilder(); String line; boolean inList = false; while ((line = br.readLine()) != null) { if (line.startsWith("*")) { if (!inList) { inList = true; sb.append("<ul>").append("\n"); } else { sb.append("<li>").append(line.substring(1).trim()).append("</li>").append("\n"); } } else { if (inList) { inList = false; sb.append("</ul>").append("\n"); } if (line.startsWith("##")) { sb.append("<h2>").append(line.substring(2).trim()).append("</h2>").append("\n"); } else if (!line.trim().isEmpty()) { sb.append("<p>").append(line.trim()).append("</p>").append("\n"); } } } return sb.toString(); } ```
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vick - 10 years, 9 months ago

public class MarkdownParser {

    public static String render(String markdownText) throws Exception {
        BufferedReader br = new BufferedReader(new StringReader(markdownText));
        StringBuilder sb = new StringBuilder();
        String line;
        boolean inList = false;
        while ((line = br.readLine()) != null) {
            if (line.startsWith("*")) {
                if (!inList) {
                    inList = true;
                    sb.append("<ul>").append("\n");
                } else {
                    sb.append("<li>").append(line.substring(1).trim()).append("</li>").append("\n");
                }
            } else {
                if (inList) {
                    inList = false;
                    sb.append("</ul>").append("\n");
                }
                if (line.startsWith("##")) {
                    sb.append("<h2>").append(line.substring(2).trim()).append("</h2>").append("\n");
                } else if (!line.trim().isEmpty()) {
                    sb.append("<p>").append(line.trim()).append("</p>").append("\n");
                }
            }
        }
        return sb.toString();
    }
}

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Efraya - 10 years, 9 months ago

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

namespace Markdown_Parser { class Program { static void Main(string[] args) { if(args.Any() == false || args.Count() > 1 || args[0].EndsWith(".txt") == false) { Console.WriteLine("Supply a .txt file to get a markdown parser"); Console.ReadKey(); }

        List<string> inFile = File.ReadAllLines(args[0]).ToList();
        List<string> outFile = new List<string>();

        bool startedList = false;

        foreach(string line in inFile)
        {
            if (startedList && line.StartsWith("* ") == false)
            {
                startedList = false;
                outFile.Add("</ul>");
            }

            if (line.StartsWith("## "))
            {
                outFile.Add(string.Format("<h2>{0}</h2>", line.TrimStart("## ".ToCharArray())));
            }
            else if(string.IsNullOrEmpty(line))
            {
                outFile.Add(string.Empty);
            }
            else if(line.StartsWith("* "))
            {
                if(startedList == false)
                {
                    startedList = true;
                    outFile.Add("<ul>");
                }
                else
                {
                    outFile.Add(string.Format("\t<li>{0}</li>", line.TrimStart("* ".ToCharArray())));
                }
            }
            else
            {
                outFile.Add(string.Format("<p>{0}</p>", line));
            }
        }

        outFile.ForEach(Console.WriteLine);
        Console.ReadKey();
    }
}

}

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Evgeny Tataurov - 10 years, 9 months ago
My very first Elixir code ```elixir defmodule Markdown do def parse(filename) do {:ok, file} = File.open(filename, [:read, :utf8]) IO.puts parse_line(IO.read(file, :line), file, []) end

defp parse_line(line, file, list) do case line do << "## ", rest :: binary >> -> parse_line(readline(file), file, ["<h2>"<>String.strip(rest)<>"</h2>" | list]) << "* ", _ :: binary >> -> {new_line, li_list} = parse_list(line, file, []) parse_line(new_line, file, ["</ul>"] ++ li_list ++ ["<ul>"] ++ list) "\n" -> parse_line(readline(file), file, list) :eof -> Enum.join(Enum.reverse(list), "\n") other -> parse_line(readline(file), file, ["<p>"<>String.strip(other)<>"</p>" | list]) end end

defp parse_list(line, file, list) do case line do << "* ", rest :: binary >> -> parse_list(readline(file), file, ["\t<li>"<>String.strip(rest)<>"</li>" | list]) _ -> {line, list} end end

defp readline(file) do IO.read(file, :line) end end

Markdown.parse(hd System.argv) ```

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Evgeny Tataurov - 10 years, 9 months ago
Another try ``` defmodule Markdown do def parse(filename) do {:ok, file} = File.open(filename, [:read, :utf8]) IO.puts parse_line(IO.read(file, :line), file, []) end

defp parse_line(line, file, list) do case line do << "## ", rest :: binary >> -> parse_line(readline(file), file, ["<h2>"<>String.strip(rest)<>"</h2>" | list]) << "* ", _ :: binary >> -> {new_line, li_list} = parse_list(line, file, []) parse_line(new_line, file, ["</ul>"] ++ li_list ++ ["<ul>"] ++ list) "\n" -> parse_line(readline(file), file, list) :eof -> Enum.join(Enum.reverse(list), "\n") other -> parse_line(readline(file), file, ["<p>"<>String.strip(other)<>"</p>" | list]) end end

defp parse_list(line, file, list) do case line do << "* ", rest :: binary >> -> parse_list(readline(file), file, ["\t<li>"<>String.strip(rest)<>"</li>" | list]) _ -> {line, list} end end

defp readline(file) do IO.read(file, :line) end end

Markdown.parse(hd System.argv) ``` I think commenting form lacks previewer

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Problem of the Day A new programming or logic puzzle every Mon-Fri

Markdown Parser

Comments:

Problem of the Day
A new programming or logic puzzle every Mon-Fri