FBB::Field - sets and retrieves offset based number fields
Numbers may contain offset-based sub-fields. E.g., a value like 12345 might consist of fields 12 and 345. Such fields can be considered offset-based, using end- and begin offsets in the number representations. In this example 345 begins at digit position 0 (indicating the least significant digit of 12345) and at digit position 3 the next field starts. Likewise, the field 12 begins at digit position 3 and has ended at digit position 5.
The Field class template provides facilities for retrieving and assigning position based values of existing numeric values of (currently) at most 64 bits.
To represent such fields the following format is used:
where base specifies the number system’s base value, end specifies the (0-based) index position where the number field ends, and begin specifies the index position where the number field begins. Here are two examples, using the decimal number system:
Field<base, end, begin>::function(argument(s))
Field<10, 3, 0>::get(12345) // returns 345
Field<10, 5, 3>::get(12345) // returns 12
The decision to specify the end offset before (i.e., left of) the begin offset is based on the consideration that this corresponds to the standard way of looking at digit positions in numbers, where the end offset is found to the left of the begin offset.
Values of fields can be retrieved, but they can also be set: to set a field’s value the following format is used:
When values are assigned to fields the maximum width of the destination field is taken into account. When specifying 9999 instead of 99 in the above example the returned value will still be 12995, as the destination field has a width of two digit positions. Likewise, specifying a smaller value sets the remaining (more significant) digits to 0:
Field<10, 3, 1>::set(12345, 99) // returns 12995
Field<10, 1, 0>::set(12345, 0) // returns 12450
Field<10, 3, 1>::set(12345, 9) // returns 12095
The class templates themselves are unaware of bases of number systems. Since 0xdeaf equals the decimal value 57007 and 0xd equals 13, calling the above function as
returns the hexadecimal value 0xdead’.
Field<16, 1, 0>::set(76007, 13)
The Field class template requires three non-type numeric arguments:
- base, specifying the base of the number system;
- end, specifying the 0-based offset of the digit position where the field has ended;
- begin, specifying the 0-based offset of the digit position where the field begins;
The class template is specialized for situations where base is a mere power of 2 (like 2, 4, 8, 16, ...) because in those cases bit-operations can be used which are faster than multiplications, divisions and modulo computation which are required when other number system bases are used.
All constructors, members, operators and manipulators, mentioned in this man-page, are defined in the namespace FBB.
- uint64_t Field<base, end, begin>::get(uint64_t value):
value is interpreted as a value in the base number system, and using digit positions in that number system the value of the digits from offset begin to (but not including) offset end is returned;
- uint64_t Field<base, end, begin>::set(uint64_t value, uint64_t
value is interpreted as a value in the base number system, and using digit positions in that number system the digits from offset begin to (but not including) offset end are replaced by field’s value. When the number of fields’s digits (also using number system base) exceeds end - begin then those excess digits are ignored.
See the examples in the DESCRIPTION section
bobcat/field - defines the class interface
- https://fbb-git.gitlab.io/bobcat/: gitlab project page;
- bobcat_6.02.02-x.dsc: detached signature;
- bobcat_6.02.02-x.tar.gz: source archive;
- bobcat_6.02.02-x_i386.changes: change log;
- libbobcat1_6.02.02-x_*.deb: debian package containing the libraries;
- libbobcat1-dev_6.02.02-x_*.deb: debian package containing the libraries, headers and manual pages;
Bobcat is an acronym of `Brokken’s Own Base Classes And Templates’.
This is free software, distributed under the terms of the GNU General Public License (GPL).
Frank B. Brokken (email@example.com).