Formatting Markdown to HTML

MarkdownFormatter.java

import java.util.Deque;
import java.util.Objects;
import java.util.Queue;
import java.util.Set;
import java.util.HashSet;
import java.util.LinkedList;
import java.util.Map;
import java.util.HashMap;

class MarkdownFormatter {
  
  static class FormatToken {
    private final String tag;

    private StringBuilder content = null;

    FormatToken(String tag) {
      this.tag = tag;
    }

    public String getTag() {
      return this.tag;
    }

    public int hashCode() {
      return this.tag.hashCode();
    }

    public boolean equals(Object o) {
      if (o == this) {
        return true;
      }
      if (o == null || o.getClass() != this.getClass()) {
        return false;
      }
      FormatToken that = (FormatToken) o;
      return this.tag.equals(that.tag);
    }

    public void appendContent(StringBuilder content) {
      if (this.content == null) {
        this.content = content;
      } else {
        this.content.append(content);
      }
    }

    public StringBuilder getContent() {
      return this.content;
    }
  }

  private static class HtmlTag {
    private final String open, close;

    public HtmlTag(String name) {
      this(String.format("<%s>", name), String.format("</%s>", name));
    }

    public HtmlTag(String open, String close) {
      this.open = open;
      this.close = close;
    }

    public String getOpenTag() {
      return this.open;
    }

    public String getCloseTag() {
      return this.close;
    }
  }

  private final Set<Character> special = new HashSet<>();

  private final Map<FormatToken, HtmlTag> html = new HashMap<>();

  public MarkdownFormatter() {
    this.registerTag("**", new HtmlTag("strong"));
    this.registerTag("_", new HtmlTag("em"));
    this.registerTag("~", new HtmlTag("s"));
    this.registerTag("`", new HtmlTag("code"));
  }

  private void registerTag(String tag, HtmlTag html) {
    this.special.add(tag.charAt(0));
    this.html.put(new FormatToken(tag), html);
  }

  public String toHTML(String input) {
    Deque<FormatToken> formattingStack = new LinkedList<>();
    Map<FormatToken, Integer> present = new HashMap<>();
    FormatToken head = null;
    StringBuilder result = new StringBuilder();
    int lm1 = input.length() - 1;
    for (int i = 0; i < input.length(); i++) {
      char c = input.charAt(i);
      if (special.contains(c)) {
        StringBuilder sb = new StringBuilder(3).append(c);
        while (i < lm1 && input.charAt(i + 1) == c) {
          sb.append(c);
          i++;
        }
        FormatToken token = new FormatToken(sb.toString());
        if (present.containsKey(token)) {
          sb = new StringBuilder();
          FormatToken open;
          while ((open = formattingStack.pollLast()) != null) {
            if (open.getContent() != null) {
              sb.insert(0, open.getContent());
            }

            present.computeIfPresent(open, ($, counter) -> counter != 1 ? counter - 1 : null);

            if (open.equals(token)) {
              // Matching but unknown tags should be treated as is
              HtmlTag tag = this.html.computeIfAbsent(open, o -> new HtmlTag(o.getTag(), token.getTag()));
              sb.insert(0, tag.getOpenTag());
              sb.append(tag.getCloseTag());
              break;
            } else {
              sb.insert(0, open.getTag());
            }
          }
          head = formattingStack.peekLast();
          if (head == null) {
            result.append(sb);
          } else {
            head.appendContent(sb);
          }
        } else {
          present.merge(token, 1, Integer::sum);
          head = token;
          formattingStack.offer(token);
        }
      } else {
        StringBuilder sb = new StringBuilder().append(c);
        while (i < lm1 && !special.contains(input.charAt(i + 1))) {
          sb.append(input.charAt(i + 1));
          i++;
        }
        if (head == null) {
          result.append(sb);
        } else {
          head.appendContent(sb);
        }
      }
    }
    return result.toString();
  }
}