import java.nio.charset.StandardCharsets;

import javax.xml.bind.DatatypeConverter;

import org.owasp.esapi.codecs.PercentCodec;
import org.owasp.esapi.codecs.PushbackSequence;
import org.owasp.esapi.codecs.PushbackString;

/**
 * The standard {@link PercentCodec} doesn't account for non BMP
 * characters which may be represented by more than 1 hexadecimal
 * value. The class below works around that.
 */
public class CustomPercentCodec extends PercentCodec
{
	@Override
	public String decode(String input) {
		StringBuilder sb = new StringBuilder();
		PushbackSequence<Character> pbs = new PushbackString(input);
		while (pbs.hasNext()) {
			String c = decodeHex(pbs);
			if (c != null) {
				sb.append(c);
			} else {
				sb.append(pbs.next());
			}
		}
		return sb.toString();
	}


	public String decodeHex( PushbackSequence<Character> input )
	{
		StringBuilder hexCode = new StringBuilder();

		while( hasNextEncodedCharacter( input ) )
		{
			appendHexDigits( input, hexCode );

			if( !isEvenNumberOfHexDigits( hexCode ) ) {
				input.reset();
				return null;
			}
		}

		if( !isEvenNumberOfHexDigits( hexCode ) ) {
			input.reset();
			return null;
		}

		return convertHexToString( hexCode.toString() );
	}


	private Boolean hasNextEncodedCharacter( PushbackSequence<Character> input )
	{
		input.mark();

		Character first = input.next();

		if ( first == null ) {
			input.reset();
			return false;
		}

		// if this is not an encoded character, return null
		if( first != '%' ) {
			input.reset();
			return false;
		}

		return true;
	}


	private boolean isEvenNumberOfHexDigits( StringBuilder hexCode ) {
		return isNotEmpty( hexCode ) && hexCode.toString().length() % 2 == 0;
	}


	private boolean isNotEmpty( StringBuilder hexCode ) {
		return hexCode.toString().length() > 0;
	}


	private String convertHexToString( String finalHexCode ) {
		return new String( DatatypeConverter.parseHexBinary( finalHexCode ), StandardCharsets.UTF_8 );
	}


	private void appendHexDigits( PushbackSequence<Character> input, StringBuilder hexCode )
	{
		for( int i = 0; i < 2; i++ ) {
			Character c = input.nextHex();
			if( c != null ) {
				hexCode.append( c );
			}
		}
	}
}