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Open HTML to PDF is a CSS 2.1 renderer written in Java. This artifact contains the core rendering and layout code.
There is a newer version: 1.1.4
package com.openhtmltopdf.util;

import java.io.ByteArrayOutputStream;
import java.io.Closeable;
import java.io.IOException;
import java.io.InputStream;
import java.io.Reader;
import java.nio.charset.StandardCharsets;
import java.util.Objects;
import java.util.function.Consumer;
import java.util.function.Function;

public class OpenUtil {

	private OpenUtil() {}

	/**
	 * Checks if a code point is printable. If false, it can be safely discarded at the 
	 * rendering stage, else it should be replaced with the replacement character,
	 * if a suitable glyph can not be found.
	 * 
	 * NOTE: This should only be called after a character has been shown to be
	 * NOT present in the font. It can not be called beforehand because some fonts
	 * contain private area characters and so on. Issue#588.
	 * 
	 * @param codePoint
	 * @return whether codePoint is printable
	 */
	public static boolean isCodePointPrintable(int codePoint) {
		if (Character.isISOControl(codePoint))
			return false;
		
		int category = Character.getType(codePoint);
		
		return !(category == Character.CONTROL ||
				 category == Character.FORMAT ||
				 category == Character.UNASSIGNED ||
				 category == Character.PRIVATE_USE ||
				 category == Character.SURROGATE);
	}

    /**
     * Whether the code point should be passed through to the font
     * for rendering. It effectively filters out characters that
     * have been shown to be problematic in some (broken) fonts such
     * as visible soft-hyphens.
     */
    public static boolean isSafeFontCodePointToPrint(int codePoint) {
        switch (codePoint) {
        case 0xAD:        // Soft hyphen, PR#550, FALLTHRU
        case 0xFFFC:      // Object replacement character, Issue#564.
            return false;

        default:
            return true;
        }
    }

	/**
	 * Returns true, when all characters of the given string are printable.
	 * @param str a non-null string to test
	 * @return whether all characters are printable
	 */
	public static boolean areAllCharactersPrintable(String str) {
		Objects.requireNonNull(str, "str");
		return str.codePoints().allMatch(OpenUtil::isSafeFontCodePointToPrint);
	}

	public static Integer parseIntegerOrNull(String possibleInteger) {
	        try {
	            return Integer.parseInt(possibleInteger);
	        } catch (NumberFormatException e) {
	            return null;
	        }
	}

    /**
     * First non-zero value or zero if none.
     */
    public static int firstNonZero(int... values) {
        for (int value : values) {
            if (value != 0) {
                return value;
            }
        }
        return 0;
    }

    public static void closeQuietly(Closeable resource) {
        if (resource != null) {
            try {
                resource.close();
            } catch (IOException e) {
            }
        }
    }

    public static void tryQuietly(Runnable r) {
        try {
            r.run();
        } catch (Throwable e) {
            // Swallow.
        }
    }

    public static byte[] readAll(InputStream is) throws IOException {
        ByteArrayOutputStream result = new ByteArrayOutputStream(512);

        byte[] buf = new byte[10240];
        int i;

        while ((i = is.read(buf)) != -1) {
            result.write(buf, 0, i);
        }

        return result.toByteArray();
    }

    public static String readAll(Reader reader) throws IOException {
        StringBuilder buffer = new StringBuilder(256);

        char[] arr = new char[10240];
        int numCharsRead;

        while ((numCharsRead = reader.read(arr, 0, arr.length)) != -1) {
            buffer.append(arr, 0, numCharsRead);
        }

        return buffer.toString();
    }

    /**
     * Reads a stream to a string using UTF-8 encoding.
     */
    public static String readString(InputStream is) throws IOException {
        byte[] bytes = readAll(is);
        return new String(bytes, StandardCharsets.UTF_8);
    }

    /**
     * Reads a resource to a string using UTF-8.
     */
    public static String readString(Class clazz, String resource) throws IOException {
        try (InputStream is = clazz.getResourceAsStream(resource)) {
            return readString(is);
        }
    }

    @FunctionalInterface
    public interface ThrowableFunction {
        R apply(T arg) throws Exception;
    }

    @FunctionalInterface
    public interface ThrowableConsumer {
        void accept(T arg) throws Exception;
    }

    /**
     * Given a {@link ThrowableFunction} (which can throw checked exceptions) returns
     * a standard {@link Function} (which can't throw checked exceptions).
     * 


     * In the returned function, checked exceptions are wrapped in {@link RuntimeException}
     * and rethrown.
     */
    public static  Function rethrowingFunction(ThrowableFunction func) {
        return arg -> {
            try {
                return func.apply(arg);
            } catch (Exception e) {
                throw new RuntimeException(e);
            }
        };
    }

    /**
     * Same as {@link #rethrowingFunction(ThrowableFunction)} for consumers.
     */
    public static  Consumer rethrowingConsumer(ThrowableConsumer consumer) {
        return arg -> {
            try {
                consumer.accept(arg);
            } catch (Exception e) {
                throw new RuntimeException(e);
            }
        };
    }

}