server/protocols/jmap-rfc-8621/src/main/java/org/apache/james/jmap/utils/JsoupHtmlTextExtractor.java

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package org.apache.james.jmap.utils;

import java.util.ArrayDeque;
import java.util.Deque;
import java.util.Optional;
import java.util.function.Predicate;
import java.util.stream.Collectors;
import java.util.stream.Stream;

import org.apache.commons.lang3.StringUtils;
import org.apache.james.util.html.HtmlTextExtractor;
import org.apache.james.util.streams.Iterators;
import org.jsoup.Jsoup;
import org.jsoup.nodes.Document;
import org.jsoup.nodes.Element;
import org.jsoup.nodes.Node;
import org.jsoup.nodes.TextNode;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import com.google.common.base.Strings;

public class JsoupHtmlTextExtractor implements HtmlTextExtractor {

    private static final Logger LOGGER = LoggerFactory.getLogger(JsoupHtmlTextExtractor.class);
    public static final String BR_TAG = "br";
    public static final String UL_TAG = "ul";
    public static final String OL_TAG = "ol";
    public static final String LI_TAG = "li";
    public static final String P_TAG = "p";
    public static final String IMG_TAG = "img";
    public static final String ALT_TAG = "alt";
    public static final int INITIAL_LIST_NESTED_LEVEL = 0;

    @Override
    public String toPlainText(String html) {
        try {
            Document document = Jsoup.parse(html);

            Element body = Optional.ofNullable(document.body()).orElse(document);

            return flatten(body)
                .map(this::convertNodeToText)
                .collect(Collectors.joining());
        } catch (Exception e) {
            LOGGER.warn("Failed extracting text from html", e);
            return html;
        }
    }

    private String convertNodeToText(HTMLNode htmlNode) {
        Node node = htmlNode.underlyingNode;
        if (node instanceof TextNode) {
            TextNode textNode = (TextNode) node;
            return textNode.getWholeText();
        }
        if (node instanceof Element) {
            Element element = (Element) node;
            if (element.tagName().equals(BR_TAG)) {
                return "\n";
            }
            if (isList(element)) {
                return convertListElement(htmlNode.listNestedLevel);
            }
            if (element.tagName().equals(OL_TAG)) {
                return "\n\n";
            }
            if (element.tagName().equals(LI_TAG)) {
                return "\n" + StringUtils.repeat(" ", htmlNode.listNestedLevel) + "- ";
            }
            if (element.tagName().equals(P_TAG)) {
                return "\n\n";
            }
            if (element.tagName().equals(IMG_TAG)) {
                return generateImageAlternativeText(element);
            }
        }
        return "";
    }

    private String generateImageAlternativeText(Element element) {
        return Optional.ofNullable(element.attributes().get(ALT_TAG))
            .map(StringUtils::normalizeSpace)
            .filter(Predicate.not(Strings::isNullOrEmpty))
            .map(s -> "[" + s + "]")
            .orElse("");
    }

    private String convertListElement(int nestedLevel) {
        if (nestedLevel == 0) {
            return "\n\n";
        } else {
            return "";
        }
    }

    Stream<HTMLNode> flatten(Node base) {
        Deque<HTMLNode> in = new ArrayDeque<>();
        in.addFirst(new HTMLNode(base, JsoupHtmlTextExtractor.INITIAL_LIST_NESTED_LEVEL));
        Deque<HTMLNode> out = new ArrayDeque<>();

        while (!in.isEmpty()) {
            HTMLNode node = in.removeFirst();
            if (node.isDone) {
                out.addLast(node);
                continue;
            }
            int nextElementLevel = getNewNestedLevel(node.listNestedLevel, node.underlyingNode);
            Position position = getPosition(node.underlyingNode);

            if (position == Position.SUFFIX) {
                node.underlyingNode.childNodes()
                    .forEach(child -> in.addFirst(new HTMLNode(child, nextElementLevel)));
                out.addLast(node);
            } else {
                in.addFirst(node.done());
                node.underlyingNode.childNodes()
                    .forEach(child -> in.addFirst(new HTMLNode(child, nextElementLevel)));
            }
        }
        return Iterators.toStream(out.descendingIterator());
    }

    private int getNewNestedLevel(int listNestedLevel, Node node) {
        if (node instanceof Element) {
            Element element = (Element) node;
            if (isList(element)) {
                return listNestedLevel + 1;
            }
        }
        return listNestedLevel;
    }

    private boolean isList(Element element) {
        return element.tagName().equals(UL_TAG) || element.tagName().equals(OL_TAG);
    }

    private enum Position {
        PREFIX,
        SUFFIX
    }

    private Position getPosition(Node node) {
        if (node instanceof Element) {
            Element element = (Element) node;
            if (element.tagName().equals(LI_TAG)) {
                return Position.PREFIX;
            }
        }
        return Position.SUFFIX;
    }

    private static class HTMLNode {
        private final Node underlyingNode;
        private final int listNestedLevel;
        private final boolean isDone;

        public HTMLNode(Node underlyingNode, int listNestedLevel, boolean isDone) {
            this.underlyingNode = underlyingNode;
            this.listNestedLevel = listNestedLevel;
            this.isDone = isDone;
        }

        public HTMLNode(Node underlyingNode, int listNestedLevel) {
            this.underlyingNode = underlyingNode;
            this.listNestedLevel = listNestedLevel;
            this.isDone = false;
        }

        public HTMLNode done() {
            return new HTMLNode(underlyingNode, listNestedLevel, true);
        }
    }

}