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Extension that performs abbreviation detection like in python markdown
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package io.femo.commonmark.ext;

import org.commonmark.node.*;
import org.commonmark.parser.PostProcessor;

import java.util.regex.Matcher;
import java.util.regex.Pattern;

/**
 * Created by felix on 8/1/16.
 */
public class AbbreviationDictionaryPostprocessor implements PostProcessor {

    private static String regexAbbr = "\\*\\[(.*)\\]:(.*)";
    private static Pattern pattern = Pattern.compile(regexAbbr);

    @Override
    public Node process(Node node) {
        AbbreviationDictionaryVisitor visitor = new AbbreviationDictionaryVisitor();
        node.accept(visitor);
        return node;
    }

    private class AbbreviationDictionaryVisitor extends AbstractVisitor {

        int inLink = 0;

        @Override
        public void visit(Link link) {
            inLink++;
            super.visit(link);
            inLink--;
        }

        @Override
        public void visit(Text text) {
            if (inLink == 0) {
                handle(text);
            }
        }
    }

    private class AbbreviationReplacerVisitor extends AbstractVisitor {

        private String abbr;
        private String descr;

        int inLink = 0;

        public AbbreviationReplacerVisitor(String abbr, String descr) {
            this.abbr = abbr;
            this.descr = descr;
        }

        @Override
        public void visit(Link link) {
            inLink++;
            super.visit(link);
            inLink--;
        }

        @Override
        public void visit(Text text) {
            if(inLink == 0) {
                String literal = text.getLiteral();
                if(literal.contains(abbr)) {
                    int index;
                    while ((index = literal.indexOf(abbr)) > 0) {
                        Abbreviation abbreviation = new Abbreviation(descr);
                        abbreviation.appendChild(new Text(abbr));
                        if(index > 0) {
                            String value = literal.substring(0, index);
                            text.insertBefore(new Text(value));
                        }
                        text.insertBefore(abbreviation);
                        literal = literal.substring(index + abbr.length());
                    }
                    text.insertBefore(new Text(literal));
                    text.unlink();
                }
            }
        }
    }

    private void handle(Text text) {
        String literal = text.getLiteral();
        Matcher matcher = pattern.matcher(literal);
        if(matcher.find()) {
            String abbr, descr;
            abbr = matcher.group(1).trim();
            descr = matcher.group(2).trim();
            Document document = document(text);
            text.unlink();
            document.accept(new AbbreviationReplacerVisitor(abbr, descr));
        }
    }

    private Document document(Node node) {
        Node parent = node;
        while (!(parent instanceof Document)) parent = parent.getParent();
        return (Document) parent;
    }
}