Unsigned long to double and back

[David Lloyd]

I encountered this issue as well; for now I'm using the following
transformation:

long tmp = input >>> 1;
double output = ((double) tmp) * 2.0 + (tmp & 1);

I... *think* it's correct but I'm not 100% sure and have a long-standing
TODO to try and figure it out...

On Sat, Nov 5, 2022 at 7:17 PM Johannes Kuhn <info at j-kuhn.de> wrote:

> When I tried to implement an WASM transpiler, I noticed some missing
> conversion methods from unsigned types to floating point, for example
> from unsigned long to a double.
>
> For the meaning of unsigned long, see Long.toUnsignedString(long i).
>
> Converting between unsigned long and floating point is not a trivial
> task, as it probably requires some bit manipulation.
>
> In particular, I would love to see the following methods added*:
>
> - double Double.fromUnsignedLong(long i)
> - long Double.toUnsignedLong(double d)
> - float Float.fromUnsignedLong(long i)
> - long Float.toUnsignedLong(float f)
>
> * Subject to bikeshedding - I don't care about the name, or if it is
> added to the Long class.
>
> Currently, I don't think that additional methods for unsigned int are
> necessary - as it is possible to cast between long and int, but feel
> free to correct me.
>
> In WASM, the specification for those methods can be found here:
>
> https://www.w3.org/TR/wasm-core-1/#op-trunc-u
> https://www.w3.org/TR/wasm-core-1/#op-convert-u
>
> Note that the WASM specification is undefined for some values, notably
> NaN, infinities, and values that fall out of the range.
>
> As *I* want to use it to implement WASM instructions, I do not have any
> strong opinion on the undefined cases - for example, returning the
> nearest unsigned long value or throwing an exception is fine for me.
>
> What do you think?
>
> - Johannes
>
>

-- 
- DML • he/him
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