Question

I have a double value d and would like a way to nudge it very slightly larger (or smaller) to get a new value that will be as close as possible to the original but still strictly greater than (or less than) the original.

It doesn’t have to be close down to the last bit—it’s more important that whatever change I make is guaranteed to produce a different value and not round back to the original.

(This question has been asked and answered for C, C++)

The reason I need this, is that I’m mapping from Double to (something), and I may have multiple items with the save double ‘value’, but they all need to go individually into the map.

My current code (which does the job) looks like this:

private void putUniqueScoreIntoMap(TreeMap map, Double score,
            A entry) {

        int exponent = 15;
        while (map.containsKey(score)) {
            Double newScore = score;
            while (newScore.equals(score) && exponent != 0) {
                newScore = score + (1.0d / (10 * exponent));
                exponent--;
            }
            if (exponent == 0) {
                throw new IllegalArgumentException("Failed to find unique new double value");
            }
            score = newScore;
        }
        map.put(score, entry);
    }

Answer 1

Use Double.doubleToRawLongBits and Double.longBitsToDouble:

double d = // your existing value;
long bits = Double.doubleToLongBits(d);
bits++;
d = Double.longBitsToDouble(bits);

The way IEEE-754 works, that will give you exactly the next viable double, i.e. the smallest amount greater than the existing value.

(Eventually it’ll hit NaN and probably stay there, but it should work for sensible values.)