The proposed change is to allow lengthUnits 'bits' for hexBinary data. This turned out to be more complex to describe than I originally suspected because of the need to deal with XSD minLength and maxLength facets, which are always measured in bytes. Those in conjunction with dfdl:lengthKind='explicit' and dfdl:lengthUnits='bits' create some minor complexities.
The below changes match the Daffodil implementation of this proposed feature.
These sentences in the description of dfdl:lengthUnits in Section 12.3 must change.
'bits' may only be used for xs:boolean, xs:byte, xs:short,
xs:int, xs:long, xs:unsignedByte, xs:unsignedShort, xs:unsignedInt, and xs:unsignedLong
simple types with binary representation.
'bytes' must be used for type xs:hexBinary.
The text should read
'bits' may only be used for xs:hexBinary and for xs:boolean, xs:byte, xs:short,
xs:int, xs:long, xs:unsignedByte, xs:unsignedShort, xs:unsignedInt, and xs:unsignedLong
simple types with binary representation.
Later in the section, the paragraph:
When unparsing a simple element with binary representation,
then for hexBinary the length is the number of bytes in the infoset value
padded to the XSD minLength facet value using dfdl:fillByte, and for the other types
the length is the minimum number of bytes to represent the value and any sign.
Must change to:
When unparsing a simple element with binary representation,
then for types other than hexBinary
the length is the minimum number of bytes to represent the value and any sign.
For type hexBinary when the dfdl:lengthUnits is 'bytes' then the length is the number of bytes in the infoset value
padded to the XSD minLength facet value using dfdl:fillByte.
For type hexBinary when the dfdl:lengthUnits is 'bits':
- First the data is padded to XSD minLength bytes as if the dfdl:lengthUnits was 'bytes'.
- When dfdl:lengthKind is other than 'explicit', the length is the number of bytes times 8.
- When the dfdl:lengthKind is 'explicit' then the value is further padded or truncated to fit the target length, in bits.
- if the data does not have sufficient bytes to supply the target length in bits it is a processing error.
- if the data is longer than the minimum number of bytes needed to supply the target length in bits, it is a processing error.
- If the explicit length in bits is not a multiple of 8, then the final byte is only partially unparsed according to the current dfdl:byteOrder.
Section 12.3.7.2.7 Length
of Binary Opaque Elements, the first sentence must be modified from:
"The dfdl:lengthUnits property must be 'bytes'. It is a
schema definition error otherwise."
to
"The dfdl:lengthUnits property must be 'bytes' or 'bits'. It is a schema definition error otherwise. Note that even when the dfdl:lengthUnits property is 'bits', the values of the XSD minLength and XSD maxLength facets are still always interpreted as constraining the length in units of bytes.
That's the end of the actual proposed language.
Note about alternatives: I considered the alternative to make it a schema definition error when dfdl:lengthUnits is 'bits' type is 'xs:hexBinary', and the XSD:minLength or XSD:maxLength facets are defined. I decided to go with the description above to support the use case where a data item is, in hex, some number of bytes long for a valid XML infoset, but the representation is explicitly a partial byte smaller. E.g., in a hexBinary with 8 bytes, but fewer than 64 bits in the representation. Ex: 63 bits as the explicit target length, so 1 bit will be unused from the xs:hexBinary logical value, but the above rules insure no more than 7 bits go unused from the final byte of the hexBinary logical value.
This is trying to be consistent with the notion that we do not truncate data to fit into the available length except for xs:string when properties explicitly allow it.
It is also trying to be consistent with our treatment of binary integers where a xs:long value may be output into an element having room for any number of bits, and any extra bits in the logical value are ignored.