Hi Zach
Thanks for the detailed response
On Wednesday, January 7, 2015 at 7:28:06 AM UTC+2, Zach Tellman wrote:
>
> Hey Tzach,
>
> If I understand what you're trying to do, you want something like this:
>
> [ :uint32
> :ubyte ;; or bit-seq, depending
> (delimited-block (prefix uint24 #(- % 5) #(+ % 5))) ]
>
> And then you'll need to parse the final block separately. I've left
> defining the uint24 as an exercise for the reader (sorry, long day), but
> using (compile-frame [:uint16 :uint8] ...) should be pretty straightforward.
>
This is what work for me, in case anyone is interested or want to comment
(defn valid-length [l]
(and (< l 16777216)
(>= l 0)))
(def uint24 (compile-frame [:ubyte :uint16]
(fn [u]
{:pre [(valid-length u)]}
(let [u8 (bit-shift-right u 16)
u16 (- u (bit-shift-left u8 16))]
[u8 u16]))
(fn [[u8 u16]]
{:post [(valid-length %)]}
(+ (bit-shift-left u8 16) u16))))
(def avp
(compile-frame
[:uint32
:ubyte
(repeated :int32 :prefix (prefix uint24 #(- % 5) #(+ % 5))) ]
))
My next step is to parse the bits from the header :ubyte and base on it,
read an extra uint32 before the repeated part.
The header is probably the answer.
>
> Hope that helps, happy to answer any followup questions.
>
It did help! Thanks
BTW, I spent too much time trying to use (compile-frame [:uint16 :uint8])
before realizing :uint8 is not defined.
The result is simply encoding of the :uint8 symbol.
I wonder if its a bug or a feature.
>
> Zach
>
> On Saturday, January 3, 2015 11:51:14 PM UTC-8, Tzach wrote:
>>
>> I'm trying to work with Gloss binary encoder/decoder, and need some help
>> to kick start.
>>
>> My first task is simple(not for me ;)
>> I have the following binary buffer to read/write:
>>
>> - 4 byte (32 bit) - code (uint)
>> - 8 bit - misc flags
>> - 3 byte (24 bit) - the entire buffer length
>> - 4 byte (32 bit) uint value - optional, depending on on of the
>> flags.
>> - List of 4 byte (uints) data elements - size dependent on the
>> overall size
>>
>> How should I represent this structure?
>> Clearly I need to use *prefix* and *repeated* for the last element, but
>> I failed to understand how.
>> I also struggle with the optional element and how to represent a 3 byte
>> element.
>>
>> Thanks!
>>
>
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