DoubleDabble.java

// https://en.wikipedia.org/wiki/Double_dabble
public class DoubleDabble {
    static int bcd(int value, int bits) {
        int res = value << (16 - bits);
        res <<= 3;
        for (int i = 3; i < bits; i++) {
            // find bcd digits > 4 and incremented them 3
            // either bit 4 is set or bit 3 and at least on of bits 2 or 1 is set
            // bit 1 in mask ist set if digit > 4
            // to get the add operand or the mask with mask<<1
            int mask = ((res & 0x88880000) >>> 3)
                    | (((res & 0x44440000) >>> 2)
                      & (((res & 0x22220000) >>> 1) | (res & 0x11110000)));
            mask |= mask << 1;
            res += mask;
            res <<= 1;
        }
        return res >>> 16;
    }

    static String toString(int value, int bits, int digits) {
        char[] chars = new char[digits];
        getChars(value, bits, digits, chars, 0);
        return new String(chars);
    }

    static void getChars(int value, int bits, int digits, char[] chars, int offset) {
        int bcd = bcd(value, bits);
        switch (digits) {
            case 1:
                chars[offset  ] = (char) ('0' + (bcd & 0x0F));
                break;
            case 2:
                chars[offset  ] = (char) ('0' + ((bcd & 0xF0) >>> 4));
                chars[offset+1] = (char) ('0' + ((bcd & 0x0F)));
                break;
            case 3:
                chars[offset  ] = (char) ('0' + ((bcd & 0xF00) >>> 8));
                chars[offset+1] = (char) ('0' + ((bcd & 0x0F0) >>> 4));
                chars[offset+2] = (char) ('0' + ((bcd & 0x00F)));
                break;
            case 4:
                chars[offset  ] = (char) ('0' + ((bcd & 0xF000) >>> 12));
                chars[offset+1] = (char) ('0' + ((bcd & 0x0F00) >>> 8));
                chars[offset+2] = (char) ('0' + ((bcd & 0x00F0) >>> 4));
                chars[offset+3] = (char) ('0' + ((bcd & 0x000F)));
                break;
        }
    }

}

DoubleDabbleTest.java

package net.jhorstmann.packedtime;

import org.junit.jupiter.api.Assertions;
import org.junit.jupiter.api.Test;

public class DoubleDabbleTest {


    @Test
    public void testBCD() {
        Assertions.assertEquals("36", Integer.toHexString(DoubleDabble.bcd(36, 7)));
        Assertions.assertEquals("55", Integer.toHexString(DoubleDabble.bcd(55, 7)));
        Assertions.assertEquals("123", Integer.toHexString(DoubleDabble.bcd(123, 7)));
        Assertions.assertEquals("243", Integer.toHexString(DoubleDabble.bcd(243, 8)));
        Assertions.assertEquals("432", Integer.toHexString(DoubleDabble.bcd(432, 10)));
        Assertions.assertEquals("543", Integer.toHexString(DoubleDabble.bcd(543, 10)));
        Assertions.assertEquals("765", Integer.toHexString(DoubleDabble.bcd(765, 10)));
        Assertions.assertEquals("876", Integer.toHexString(DoubleDabble.bcd(876, 10)));
        Assertions.assertEquals("976", Integer.toHexString(DoubleDabble.bcd(976, 10)));
        Assertions.assertEquals("9876", Integer.toHexString(DoubleDabble.bcd(9876, 15)));
        Assertions.assertEquals("9876", Integer.toHexString(DoubleDabble.bcd(9876, 16)));
    }

    @Test
    public void testToString() {
        Assertions.assertEquals("36", DoubleDabble.toString(36, 7, 2));
        Assertions.assertEquals("55", DoubleDabble.toString(55, 7, 2));
        Assertions.assertEquals("123", DoubleDabble.toString(123, 7, 3));
        Assertions.assertEquals("243", DoubleDabble.toString(243, 8, 3));
        Assertions.assertEquals("432", DoubleDabble.toString(432, 10, 3));
        Assertions.assertEquals("543", DoubleDabble.toString(543, 10, 3));
        Assertions.assertEquals("765", DoubleDabble.toString(765, 10, 3));
        Assertions.assertEquals("876", DoubleDabble.toString(876, 10, 3));
        Assertions.assertEquals("976", DoubleDabble.toString(976, 10, 3));
        Assertions.assertEquals("9876", DoubleDabble.toString(9876, 15, 4));
        Assertions.assertEquals("9876", DoubleDabble.toString(9876, 16, 4));
    }

}