[DFDL-WG] more xpath functions: bitwise operations

[Cranford, Jonathan W.]

Mike,

Have you looked at the EXPath:Binary module?  Look at expath.org, I think (composing email offline at the moment).

It supports several bit-wise manipulations on base64Binary types.  DFDL uses hexBinary, but XPath allows a cast between the two types.

FYI,

Jonathan Cranford

>-----Original Message-----
>From: dfdl-wg-bounces at ogf.org [mailto:dfdl-wg-bounces at ogf.org] On Behalf
>Of Mike Beckerle
>Sent: Tuesday, July 29, 2014 8:10 AM
>To: dfdl-wg at ogf.org
>Subject: [DFDL-WG] more xpath functions: bitwise operations
>
>
>There seems to be no reasonable way to deal with binary data in our XPath
>subset.
>
>
>Ex: This formula, for unsigned binary integer n: (C-programming language)
>
> (n >> 1) ^ (-(n & 1))
>
>
>This is the decoder expression for what are called "zig zag integers" which is a
>way of encoding signed integers into unsigned integers so that there is no sign-bit
>that must be extended to the full length. (A trick used in Google Protocol Buffers -
>an increasingly popular format). There is no reason DFDL cannot encode and
>decode these zig-zag integers, except that there is no bit-wise arithmetic to do
>the calculation on inputValueCalc and outputValueCalc.
>
>
>
>The shift right isn't hard, and the & 1 becomes a simple enough if-then-else and a
>division. But there is no reasonable way to do a bitwise XOR (The ^ operator).
>
>
>I suspect we need a library of bitwise operations that interpret their arguments as
>binary integers:
>
>
>My first cut would be:
>
>
>arithmetic-shift-right(width, count)
>
>arithmetic-shift-left (width, count)
>
>logical-shift-right(width, count)
>
>logical-shift-left(width, count)
>
>bitwise-and(width, n1, n2)
>
>bitwise-or(width, n1, n2)
>
>bitwise-not(width, n1)
>
>bitwise-xor(width, n1, n2)
>
>
>Where width is 8, 16, 32, or 64.
>
>
>The arithmetic shifts and the logical shift left are for symmetry only. They can
>easily be implemented by multiplies and divides by 2.
>
>
>
>Mike Beckerle | OGF DFDL Workgroup Co-Chair | Tresys Technology |
>www.tresys.com
>
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>