Validate Encodable๏
Validation is the way to verify a given sequence of input can have a specific action performed on it. Particularly, we check here if the input of code points can be turned into code units of the given encoding. The way it does this, however, is two-fold:
it first encodes the input code units, to see if it can do the transformation without loss of information; then,
it decodes the output from the last step, to see if the final output is equivalent to the input.
The algorithm for this is as follows:
โฉ Is the
input
value empty? If so, is thestate
finished and have nothing to output? If both are true, return the current results with the the emptyinput
,valid
set totrue
andstate
s, everything is okay โ !โฉ Otherwise,
Set up an
intermediate
storage location ofcode_unit
s, using themax_code_units
of the input encoding, for the next operations.Set up an
intermediate_checked_output
storage location ofcode_point
s, using themax_code_points
of the input encoding, for the next operations.Do the
encode_one
step frominput
(using itsbegin()
andend()
) into theintermediate
code_unit
storage location.๐ If it failed, return with the current
input
(unmodified from before this iteration, if possible),valid
set to false, andstate
s.
Do the
decode_one
step from theintermediate
into theintermediate_checked_output
.๐ If it failed, return with the current
input
(unmodified from before this iteration, if possible),valid
set to false, andstate
s.
Compare the
code_point
s of theinput
sequentially against thecode_point
s within theintermediate_checked_output
.๐ If it failed, return with the current
input
(unmodified from before this iteration, if possible),valid
set to false, andstate
s.
โฉ Update
input
โsbegin()
value to point to after what was read by thedecode_one
step.โคด๏ธ Go back to the start.
This fundamental process works for all encoding objects, provided they implement the basic Lucky 7. The reason for checking if it can be turned back is to ensure that the input code units actually match up with the output code units. If an encoding performs a lossy transformation in one direction or the other, then validation will fail if it cannot reproduce the input exactly. And, you will know the exact place in the input
that caused such a failure.
There are extension points used in the API that allow certain encodings to get around the limitation of having to do both the encode_one
step and the decode_one
step, giving individual encodings control over the verification of a single unit of input and of bulk validation as well.
Check out the API documentation for ztd::text::validate_encodable_as to learn more.