[DFDL-WG] proposal: functions to allow reasonable syntax for hex constants in DFDL expression language

[Mike Beckerle]

Rationale: in writing DFDL schemas for binary data formats, there is a
strong need to express binary data constants in hexadecimal.

Proposed new functions:

dfdl:hByte
dfdl:hUByte
dfdl:hShort
dfdl:hUShort
dfdl:hInt
dfdl:hUInt
dfdl:hLong
dfdl:hULong

The prefix 'h' is a reminder that the function converts from hexadecimal.
The prefix 'U' on the name denotes that the function creates an unsigned
type result. Normally one might like to write this out fully, but in this
case brevity is helpful given the expected usage of these functions to
construct literal constants in DFDL expressions.

All the functions take a single string argument. The string must begin with
"0x" and contain at least one hex digit after that.
The string must contain only hexadecimal digits, that is, the characters
0-9a-zA-Z. It is a schema definition error otherwise.

The hex digits represent a big endian twos complement representation of a
binary number.
Each function has a limit on the number of hex digits, with no more digits
than 2, 4, 8, or 16 for the byte, short, int, and long versions. That is to
say for dfdl:byte and dfdl:uByte, there can be at most 2 hex digits. For
dfdl:short and dfdl:uShort, there can be at most 4 hex digits, and so on.
It is a schema definition error if more digits are encountered than are
suitable for the type being created.

Examples:

dfdl:hUInt("0xa1b2c3d4") is the value 2712847316. Note that in the first
byte 'a1' the most significant bit is set, but since this is an unsigned
type, this is not interpreted as a sign bit.
dfdl:hInt("0xFFFFFFFF") is the int value -1. The sign bit indicates that
the number is negative, and this twos complement value represents -1.
dfdl:hUByte("0xFF") is the unsigned byte value 255
dfdl:hByte("0xff") is the byte value -1.
dfdl:hByte("0x7F") is the byte value 127
dfdl:hByte("0x80") is the byte value -128
dfdl:hUByte("0x80") is the unsigned byte value 128
dfdl:hByte("0x0A3") is a schema definition error as the leading zero is not
allowed because at most 2 digits are allowed for byte types.
dfdl:hShort("0x0A3") is short value 208. Leading zero causes no issue here
because up to 4 digits are allowed.

Example of usage in expressions:

 <xs:element name="magic_number" type="ex:uint32"
   dfdl:byteOrder="bigEndian">
   <xs:annotation>
     <xs:appinfo source="http://www.ogf.org/dfdl/dfdl-1.0/">
       <dfdl:setVariable ref="ex:bOrd">
           {
            if (xs:unsignedInt(.) eq *dfdl:hUInt('0xa1b2c3d4')*) then
'bigEndian'
            else if (xs:unsignedInt(.) eq *dfdl:hUInt('0xd4c3b2a1')*) then
'littleEndian'
            else fn:error(ex:magic_number, "Magic number was not 0xA1B2C3D4
(for bigEndian) or 0xD4C3B2A1 (for littleEndian).")
            }
       </dfdl:setVariable>
     </xs:appinfo>
   </xs:annotation>
</xs:element>

--
Mike Beckerle | OGF DFDL Workgroup Co-Chair | Tresys Technology |
www.tresys.com
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