Closest isoweight bit string

ClosestIsoweight.java

package closestisoweight;

public class ClosestIsoweight {

    private ClosestIsoweight() {
    }

    /**
     * Find the nearest isoweight word in linear time and constant space.
     *
     * @param word a 64-bit word
     * @return the nearest isoweight word
     */
    public static long isoweight(long word) {
        int i = 0;
        while (i < 63) {
            if (((word >> i) & 0x1) != ((word >> (i + 1)) & 0x1)) {
                return word ^ ((1L << i) | (1L << i + 1));
            }
            i++;
        }
        throw new IllegalArgumentException("All bits are set (-1) or not set (0)");
    }


    /**
     * Find the nearest isoweight word in constant time and space.
     *
     * @param word a 64-bit word
     * @return the nearest isoweight word
     */
    public static long isoweightOptimized(long word) throws IllegalArgumentException {
        long swapMask = getAdjacentPolarBits(word);
        return word ^ swapMask;
    }

    private static long getAdjacentPolarBits(long word) {
        long leastSignificantSetBit = ~word | (word - 1);
        if (leastSignificantSetBit == word) throw new IllegalArgumentException("Bit string is all zeroes (0)");
        else leastSignificantSetBit = ~leastSignificantSetBit; // properly isolate bit, e.g. 0b..11110111 -> 0b..00001000
        long leastSignificantUnsetBit = ~word & (word + 1);
        if (leastSignificantUnsetBit == 0) throw new IllegalArgumentException("Bit string is all ones (-1)");

        if (leastSignificantSetBit > leastSignificantUnsetBit) {
            return leastSignificantSetBit | leastSignificantSetBit >>> 1;
        }
        return leastSignificantUnsetBit | leastSignificantUnsetBit >>> 1;
    }
}