在 java 中处理四倍精度浮点（128 位）数

Question

I need to make use of numbers coming from another system that are 128-bit (quadruple-precision) floating point numbers in java.

Considering that there is no equivalent type in java, I would like to reduce the precision of the numbers using java code so they can be stored in a java double. This can be done fairly easily in c or using assembly but I would like to do it purely in java.

It is fair to assume that the quadruple-precision number is stored in a 128-bit byte array in java.

Is there a good solution, using only java? Thanks.

Answer 1

I was so intrigued by this question that I was compelled to write a library to handle IEEE-754 floating point numbers. With the library, you can use the following:

byte[] quadBytes; // your quad-floating point number in 16 bytes
IEEE754 quad = IEEE754.decode(IEEE754Format.QUADRUPLE, 
        BitUtils.wrapSource(quadBytes));
// IEEE754 holds the number in a 'lossless' format

From there, you can:

ByteBuffer doubleBuffer = ByteBuffer.allocateDirect(8);
quad.toBits(IEEE754Format.DOUBLE, BitUtils.wrapSink(doubleBuffer));
doubleBuffer.rewind();
double converted = doubleBuffer.asDoubleBuffer().get();

But the above snippet is just to illustrate general usage... a shorthand is provided for double:

double converted = quad.doubleValue();

The code is available at kerbaya.com/ieee754lib .

Answer 2

Depending on the size of the data set BigDecimal instantiated from an imported String representation might be an easy and accurate option. I assume one can export string representations of those numbers from any programming language.

Answer 3

Although the question was asked rather long ago, perhaps it may still be of interest for someone. There is a Java class for 128-bit floating point arithmetic, that has methods for converting 128-bit IEEE-754 floating-point values into its own internal representation without any loss of precision. It can perform arithmetic operations on such values, and convert them back to IEEE-754 binary128 , as well as to other common numeric types like BidDecimal , double and long . It can also parse strings containing decimal representations of such values and convert them back to strings. Internally, it stores 128 bits of the mantissa, so that the relative error of the calculations does not exceed 1.47e-39.