Decimal Numbers

Remo via Digitalmars-d-announce digitalmars-d-announce at puremagic.com
Fri Jul 4 08:32:07 PDT 2014


On Thursday, 3 July 2014 at 21:55:42 UTC, Paul D Anderson wrote:
> A candidate implementation of decimal numbers 
> (arbitrary-precision
> floating-point numbers) is available for review at
> https://github.com/andersonpd/eris/tree/master/eris/decimal. 
> This is a
> substantial rework of an earlier implementation which was 
> located at
> https://github.com/andersonpd/decimal.
>
> This is a D language implementation of the General Decimal 
> Arithmetic
> Specification 
> (http://www.speleotrove.com/decimal/decarith.pdf), which is
> compliant with IEEE-754 and other standards as noted in the 
> specification.
>
> The current implementation is not complete; there are a lot of 
> TODOs and NOTEs
> scattered throughout the code, but all the arithmetic and 
> miscellaneous
> operations listed in the spec are working, along with decimal 
> versions of most
> of the functions and constants in std.math. I think it is far 
> enough along for
> effective review.
>
> Briefly, this software adds the capability of properly rounded
> arbitrary-precision floating-point arithmetic to the D 
> language. All arithmetic
> operations are governed by a "context", which specifies the 
> precision (number of
> decimal digits) and rounding mode for the operations. This same 
> functionality
> exists in most modern computer languages (for example, 
> java.math.BigDecimal).
> Unlike Java, however, which uses function syntax for arithmetic 
> ops
> (add(BigDecimal, BigDecimal), etc.), in D the same arithmetic 
> operators that
> work for floats or doubles work for decimal numbers. (Of 
> course!)
>
> In this implementation decimal numbers having different 
> contexts are different
> types. The types are specified using template parameters for 
> the precision,
> maximum exponent value and rounding mode. This means that
> Decimal!(9,99,Rounding.HALF_EVEN) is a different type than
> Decimal!(19,199,Rounding.HALF_DOWN). They are largely 
> interoperable, however.
> Different decimal types can be cast to and from each
> other.
>
> There are three standard decimal structs which fit into 32-, 
> 64- and 128-bits of
> memory, with 7, 16 and 34 digit precision, respectively. These 
> are used for
> compact storage; they are converted to their corresponding 
> decimal numbers for
> calculation. They bear the same relation to decimal numbers as 
> Walter's
> half-float type does to floats.
> (http://www.drdobbs.com/cpp/implementing-half-floats-in-d/240146674).
> Implementation of these still needs a little work, and will be 
> added to github
> very shortly.
>
> Major TODO items:
>
> 1) The current underlying integer type uses my own big integer 
> struct
> (eris.integer.extended) rather than std.bigint. This was mainly 
> due to problems
> with constness and CTFE of BigInts. These problems have since 
> been resolved, but
> I didn't want to switch over to BigInts until everything was 
> working for fear of
> introducing new bugs.
>
> 2) Integration of Decimal32, Decimal64 and Decimal128 structs 
> are not complete.
> (See above.)
>
> 3) Conversion to and from floats, doubles and reals is 
> currently working but it
> is slow. (Conversion is through strings: double to string to 
> decimal and vice
> versa.)
>
> 4) Still incomplete implementations of some functions in 
> decimal.math: expm1,
> acosh, atanh, possibly others.
>
> 5) More unit tests (always!).


This is looking very promising!


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