How to represent a Decimal for GoProtobuf ??

[Th3x0d3r]

Hello there !

I'm trying to communicate my go app using goprotobuf with an existing c# service that uses protobuf-net as serialization, but since there's no decimal definition on protocol buffers, protobuf-net uses this representation structure:

message Decimal {
  optional uint64 lo = 1; // the first 64 bits of the underlying value
  optional uint32 hi = 2; // the last 32 bis of the underlying value
  optional uint32 signScale = 3; // the number of decimal digits (bits 1-16), and the sign (bit 0)
}

Go code:

val := math.Float64bits(3.74)
pd.Lo = proto.Uint64(0x400deb851eb851ec) // Using 3.74 Float64bits hexa representation
pd.Hi = proto.Uint32(0x00000000)
pd.SignScale = proto.Uint32(0x000000020)

But, the C# server side doesn't get the 3.74  :(

Any help ?

Thanks !

[Michael Jones]

On Mon, Oct 27, 2014 at 9:41 AM, Th3x0d3r <xss...@gmail.com> wrote:

0x400deb851eb851ec

What do you get?

The significand (fraction) is 1.87 and the effective exponent is 2 so that could help decode your "C# gets the wrong thing" value if we new what that value is...

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Michael T. Jones | Chief Technology Advocate  | m...@google.com |  +1 650-335-5765

[Th3x0d3r]

The thing is that I'm clueless on how to generate the Lo, Hi and Sign Scale for the Decimal Structure. Any example would be great

[Daniel Eloff]

You're trying to stuff a binary floating point number into a decimal floating point type. That's all wrong.

Decimal in .NET is a decimal floating point number (base 10, not base 2 like binary floats.) It is of the form sign * mantissa / 10^exponent where mantissa is a 96 bit integer, and exponent is 0-28 (effectively all negative, so the smallest number is 10^-28.)

So to represent 3.74, you would store 374 in the mantissa, and use 2 for the exponent. If you need more than 19 digits, you can use math.big.Int for the mantissa, otherwise uint64 will suffice.

Cheers,

Dan

[Michael Jones]

Just realized this myself by looking at:

https://code.google.com/p/protobuf-net/source/browse/trunk/protobuf-net/ProtoBcl/ProtoDecimal.cs?r=155

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[Th3x0d3r]

Thanks ! , can you show some example ??

[Michael Jones]

This is an unknown encoding to me beyond your emails. Perhaps there are tests and examples at the website I linked to. You might study those.

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[Th3x0d3r]

Thanks ! but math it's not my field, and frankly I'm quite lost, can you show me some how to do that ?

[Michael Jones]

Daniel explained it. Try this:

pd.Lo = 374

pd.Hi = 0

pd.SignScale = 4

that (should/may/might) mean (0*(2**64)+374)/(10**2) == 374/100 == 3.74

If that works for you, come back for more. ;-)

On Mon, Oct 27, 2014 at 12:52 PM, Th3x0d3r <xss...@gmail.com> wrote:

Thanks ! but math it's not my field, and frankly I'm quite lost, can you show me some how to do that ?

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