|
FAST Protocol
< Previous Next >
Re: Corner case for encoding delta value
Fred Malabre / CME Group <>
8 Jun 2010 10:31AM ET Thanks Dale, your explanation makes sense.
> Hi Fred,
>
> > Let's say I have a field defined as U64 to encode with delta value.
> > The reference value is 0. I want to send the value 12178 (then as a signed value because of delta encoding).
> > The corresponding binary value is 0000 ... 0000 0010 1111 1001 0010.
> > Encoding with FAST, I would end up with:
> > (0)101 1111 (1)001 0010
> > => Continuation byte is in parenthesis in the example above.
> > => Only 2 bytes are sent according to the rule truncating most significant bits set to zero.
>
> This is where the misunderstanding arises. There is a significant leading zero bit on this number because it is a positive signed number. The encoding you have shown discards this significant bit. The correct FAST encoding takes 3 bytes:
>
> (0)... ...0 (0)101 1111 (1)001 0010
>
> I am using a dot to represent insignificant bits which are actually transmitted as zero bits. Even though the first byte is zero, this is not an over-long encoding. An overlong encoding would have all insignificant bits in the first byte.
>
> HTH,
>
> Dale
> --
> Principal Software Engineer
> Object Computing, Inc.
> Lead Developer of the QuickFAST open source implementation of FAST.
Re: Corner case for encoding delta value
Fred Malabre / CME Group 8 Jun 2010 10:31AM ET
|
