Cast long to double with non-default rounding mode

Question

In Java, when a long value is cast to double value, the IEEE754 round-to-nearest rounding mode is applied when lossless conversion is not possible.

For example, cast from 1L << 55 or (1L << 55) + 8 to double are lossless. However, any number in between cannot be accurately represented as double. The IEEE-754 round-to-nearest (ties to even) rounding mode is applied by default. As a result, (1L << 55) + 4) or below will round down where as (1L << 55) + 5 or up will round up.

Is there any function in standard library or guava that allows me to cast a long to a double using Round toward +∞ or Round toward -∞ mode (chosen on a per-callsite basis)? Assuming there isn't one, how do I write such two functions myself efficiently?

Answer 1

How about this:

static double longRoundUp(long lng)
{
    double d = (double)lng;     
    return lng <= (long)d ? d : Math.nextUp(d);
}

static double longRoundDown(long lng)
{
    double d = (double)lng;     
    return lng >= (long)d ? d : Math.nextDown(d);
}

Test:

long l1 = (1L << 55);
long l2 = (1L << 55) + 8;
long l3 = (1L << 55) + 4;
long l4 = (1L << 55) + 5;

System.out.format("Up %d %.0f%n", l1, longRoundUp(l1));
System.out.format("Up %d %.0f%n", l2, longRoundUp(l2));
System.out.format("Up %d %.0f%n", l3, longRoundUp(l3));
System.out.format("Up %d %.0f%n", l4, longRoundUp(l4));

System.out.println();
System.out.format("Down %d %.0f%n", l1, longRoundDown(l1));
System.out.format("Down %d %.0f%n", l2, longRoundDown(l2));
System.out.format("Down %d %.0f%n", l3, longRoundDown(l3));
System.out.format("Down %d %.0f%n", l4, longRoundDown(l4));

Output:

Up 36028797018963968 36028797018963968
Up 36028797018963976 36028797018963976
Up 36028797018963972 36028797018963976
Up 36028797018963973 36028797018963976

Down 36028797018963968 36028797018963968
Down 36028797018963976 36028797018963976
Down 36028797018963972 36028797018963968
Down 36028797018963973 36028797018963968