How to parse packed bcd bytes in golang?

[di3go.b...@gmail.com]

hi guys!

I need to implement a tracker protocol and in one of the field i have to do this:

The id is made up of numerical digits and is encoded in this field as packed BCD. The most significant digit is placed in the upper four bits of the first byte. In the case where an odd number of digits are used, a 0x0F is used to pad the lower 4 bits of the last byte.

How can i parse this []byte to a string in golang?

Thanks!!

[Matt Harden]

http://godoc.org/encoding/hex#EncodeToString. If you actually want an int, you can remove any trailing 'f' from the string (if the length is odd) and use http://godoc.org/strconv#Atoi.

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[Skip Tavakkolian]

maybe something like this: http://play.golang.org/p/qzqVtCR0C4

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[chris dollin]

On 27 November 2014 at 18:55, chris dollin <ehog....@googlemail.com> wrote
> You just pick the bytes apart into 4-bit chunks and do a shift-and-add
> to build up a number.

MULTIPLY (by 10) and add.

Grrr, argh.

Chris

--
Chris "allusive" Dollin

[chris dollin]

On 27 November 2014 at 18:44, Matt Harden <matt....@gmail.com> wrote:
> http://godoc.org/encoding/hex#EncodeToString. If you actually want an int,
> you can remove any trailing 'f' from the string (if the length is odd) and
> use http://godoc.org/strconv#Atoi.

packed BCD isn't hex.

> On Thu Nov 27 2014 at 12:08:40 PM <di3go.b...@gmail.com> wrote:
>>
>> hi guys!
>> I need to implement a tracker protocol and in one of the field i have to
>> do this:
>>
>> The id is made up of numerical digits and is encoded in this field as
>> packed BCD. The most significant digit is placed in the upper four bits of
>> the first byte. In the case where an odd number of digits are used, a 0x0F
>> is used to pad the lower 4 bits of the last byte.
>>
>> How can i parse this []byte to a string in golang?

Are you sure you want to parse it to a string and not, say, an int64?

You just pick the bytes apart into 4-bit chunks and do a shift-and-add
to build up a number.

[Matt Harden]

Packed BCD isn't hex, but it is equivalent to taking a decimal expansion then encoding as hex. Try it; it works.

[Matt Harden]

If you don't believe me check decodeBcd2 in http://play.golang.org/p/kZf3xWMjR8.

[di3go.b...@gmail.com]

Thanks guys!

Both solutions work, but bcd is faster than bcd2.

go test -bench=. 

PASS

BenchmarkBCD 100000000        23.8 ns/op

BenchmarkBCD2 5000000       326 ns/op

[Matt Harden]

Yes, but bcd2 is simpler and more obviously correct if you know what the underlying functions do.

--

[Matt Harden]

decodeBcd was handling overflows incorrectly.

http://play.golang.org/p/xCJabAJZRQ

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[Skip Tavakkolian]

ok what i sent is clearly not correct. i shouldn't try to code and cook a turkey at the same time.

to redeem myself here's a version that is correct (i think): http://play.golang.org/p/ChmTmCtkHq

-Skip