A Simple Markdown Parser

package lab;

import lombok.Builder;
import org.apache.commons.lang3.StringUtils;
import org.apache.commons.lang3.tuple.Pair;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.StringJoiner;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class MarkdownParser {

    private static String createIndentation(int level) {
        return " ".repeat(level * 4);
    }

    private static String decorate(final String line) {
        // First test image
        final Pattern imagePattern = Pattern.compile("!\\[(?<alt>.+?)\\]\\((?<src>.+?)\\)");
        final Matcher m1 = imagePattern.matcher(line);
        String value = m1.replaceAll("<img src=\"${src}\" alt=\"${alt}\">");

        // Second, replace links
        final Pattern urlPattern = Pattern.compile("\\[(?<text>.+?)\\]\\((?<href>.+?)\\)");
        final Matcher m2 = urlPattern.matcher(value);
        value = m2.replaceAll("<a href=\"${href}\">${text}</a>");

        // Finally, replace bold
        final Pattern boldPattern = Pattern.compile("\\*\\*(.+?)\\*\\*");
        final Matcher m3 = boldPattern.matcher(value);
        value = m3.replaceAll("<b>$1</b>");
        return value;
    }

    private interface HtmlGenerator {
        String generate();
    }

    private static class RawLines implements HtmlGenerator {
        final List<String> lines;

        RawLines() {
            this.lines = new ArrayList<>();
        }
        public void add(final String line) {
            this.lines.add(line);
        }

        @Override
        public String generate() {
            final StringJoiner sj = new StringJoiner("\n");
            for (var line : this.lines) {
                sj.add(line);
            }
            return sj.toString();
        }
    }

    @Builder
    private static class Paragraph implements HtmlGenerator {
        private final String content;

        @Override
        public String generate() {
            final StringJoiner sj = new StringJoiner("\n");
            final String startTag = "<div class=\"paragraph\">";
            final String endTag = "</div>";
            sj.add(startTag);
            sj.add(decorate(this.content));
            sj.add(endTag);
            return sj.toString();
        }
    }

    private static class ListItems implements HtmlGenerator {

        private final List<Pair<Integer, String>> contents;

        public ListItems() {
            this.contents = new ArrayList<>();
        }

        private int count(String line) {
            int k = 0;
            while (k < line.length() && line.charAt(k) == ' ') {
                ++k;
            }
            return k;
        }

        public void add(final String line) {
            int c = this.count(line);
            this.contents.add(Pair.of(c, line.substring(c + 1)));
        }

        @Override
        public String generate() {
            final String ul_start = "<ul>";
            final String ul_end = "</ul>";
            final String li_start = "<li>";
            final String li_end = "</li>";
            final StringJoiner sj = new StringJoiner("\n");

            sj.add("<div class=\"listing\">");

            int prevLevel = -1;
            for (var p : contents) {
                final int level = p.getLeft();
                final String content = p.getRight();

                if (level > prevLevel) {
                    sj.add(ul_start);
                    prevLevel = level;
                    sj.add(li_start + decorate(content) + li_end);
                } else if (level == prevLevel) {
                    sj.add(li_start + decorate(content) + li_end);
                } else {
                    sj.add(ul_end);
                    sj.add(li_start + decorate(content) + li_end);
                    prevLevel = level;
                }
            }
            sj.add(ul_end);
            sj.add("</div>");
            return sj.toString();
        }
    }

    @Builder
    private static class Header implements HtmlGenerator {
        private final int level;
        private final String content;

        @Override
        public String generate() {
            final StringJoiner sj = new StringJoiner("\n");
            final String startTag = "<h" + this.level + ">";
            final String endTag = "</h" + this.level + ">";
            sj.add(startTag);
            sj.add(this.content);
            sj.add(endTag);
            return sj.toString();
        }
    }


    public static String toHtml(final String content) {
            final List<String> lines = Arrays.asList(content.split("\n"));
            return toHtml(lines);
        }

    private static String toHtml(final List<String> lines) {
        int k = 0;

        final List<HtmlGenerator> results = new ArrayList<>();

        while (k < lines.size()) {
            String line = lines.get(k);
            String trimmedLine = line.trim();

            if (trimmedLine.startsWith("#")) {
                int j = 0;
                while (j < trimmedLine.length() && trimmedLine.charAt(j) == '#') {
                    j++;
                }
                results.add(Header.builder()
                                .level(j)
                                .content(trimmedLine.substring(j + 1))
                                .build());
            } else if (trimmedLine.startsWith("-")) {
                final ListItems listItems = new ListItems();
                listItems.add(line);
                ++k;
                // Go through the list
                while (k < lines.size() && lines.get(k).trim().startsWith("-")) {
                    listItems.add(lines.get(k));
                    ++k;
                }
                results.add(listItems);
                --k;

            } else if (trimmedLine.startsWith("\\begin{html}")) {
                final RawLines rl = new RawLines();
                ++k;
                while (k < lines.size() && !lines.get(k).trim().startsWith("\\end{html}")) {
                    rl.add(lines.get(k));
                    ++k;
                }
                results.add(rl);
            } else if (StringUtils.isNotEmpty(trimmedLine)) {
                results.add(Paragraph.builder()
                        .content(trimmedLine)
                        .build());
            }
            ++k;
        }

        final StringJoiner sj = new StringJoiner("\n\n");

        for (var gen : results) {
            sj.add(gen.generate());
        }

        return sj.toString();
    }
}