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Re: Corner case for encoding delta value
Dale Wilson / Object Computing, Inc
8 Jun 2010 10:28AM ET 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
Dale Wilson / Object Computing, Inc 8 Jun 2010 10:28AM ET
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