NAME

Data::UUID::NCName - Make valid NCName tokens which are also UUIDs

VERSION

Version 0.05

SYNOPSIS

    use Data::UUID::NCName qw(:all);

    my $uuid  = '1ff916f3-6ed7-443a-bef5-f4c85f18cd10';
    my $ncn   = to_ncname($uuid, version => 1);
    my $ncn32 = to_ncname($uuid, version => 1, radix => 32);

    # $ncn is now "EH_kW827XQ6719MhfGM0QL".
    # $ncn32 is "Ed74rn43o25b255puzbprrtiql" and case-insensitive.

    # from Test::More, this will output 'ok':
    is(from_ncname($ncn, version => 1),
        $uuid, 'Decoding result matches original');

DESCRIPTION

The purpose of this module is to devise an alternative representation of the UUID which conforms to the constraints of various other identifiers such as NCName, and create an isomorphic mapping between them.

FORMAT DEPRECATION NOTICE

After careful consideration, I have decided to change the UUID-NCName format in a minor yet incompatible way. In particular, I have moved the quartet containing the variant to the very end of the identifier, whereas it previously was mixed into the middle somewhere.

This can be considered an application of Postel's Law, based on the assumption that these identifiers will be generated through other methods, and potentially naïvely. Like the version field, the variant field has a limited acceptable range of values. If, for example, one were to attempt to generate a conforming identifier by simply generating a random Base32 or Base64 string, it will be difficult to ensure that the variant field will indeed conform when the identifier is converted to a standard UUID. By moving the variant field out to the end of the identifier, everything between the version and variant bookends can be generated randomly without any further consideration, like so:

    our @B64_ALPHA = ('A'..'Z', 'a'..'z', 0..9, qw(- _));

    sub make_cheapo_b64_uuid_ncname () {
        my @vals = map { int rand 64 } (1..20); # generate content
        push @vals, 8 + int rand 4;             # last digit is special
        'E' . join '', map { $B64_ALPHA[$_] } @vals; # 'E' for UUID V4
    }

    # voilà:
    my $cheap = make_cheapo_b64_uuid_ncname;
    # EPrakcT1o2arqWSOuIMGSK or something

    # as expected, we can decode it (version 1, naturally)
    my $uu = Data::UUID::NCName::from_ncname($cheap, version => 1);
    # 3eb6a471-3d68-4d9a-aaea-5923ae20c192 - UUID is valid

Furthermore, since the default behaviour is to align the bits of the last byte to the size of the encoding symbol, and since the variant bits are masked, a compliant RFC4122 UUID will always end with I, J, K, or L, in both Base32 (case-insensitive) and Base64 variants.

Since I have already released this module prior to this format change, I have added a version parameter to both "to_ncname" and "from_ncname". The version currently defaults to 0, the old one, but will issue a warning if not explicitly set. Later I will change the default to 1, while keeping the warning, then later still, finally remove the warning with 1 as the default. This should ensure that any code written during the transition produces the correct results.

    Unless you have to support identifiers generated from version 0.04 or older, you should be running these functions with version => 1.

RATIONALE & METHOD

The UUID is a generic identifier which is large enough to be globally unique. This makes it useful as a canonical name for data objects in distributed systems, especially those that cross administrative jurisdictions, such as the World-Wide Web. The representation, however, of the UUID, precludes it from being used in many places where it would be useful to do so.

In particular, there are grammars for many types of identifiers which must not begin with a digit. Others are case-insensitive, or prohibited from containing hyphens (present in both the standard notation and Base64URL), or indeed anything outside of ^[A-Za-z_][0-9A-Za-z_]*$.

The hexadecimal notation of the UUID has a 5/8 chance of beginning with a digit, Base64 has a 5/32 chance, and Base32 has a 3/16 chance. As such, the identifier must be modified in such a way as to guarantee beginning with an alphabetic letter (or underscore _, but some grammars even prohibit that, so we omit it as well).

While it is conceivable to simply add a padding character, there are a few considerations which make it more appealing to derive the initial character from the content of the UUID itself:

  • UUIDs are large (128-bit) identifiers as it is, and it is undesirable to add meaningless syntax to them if we can avoid doing so.

  • 128 bits is an inconvenient number for aligning to both Base32 (130) and Base64 (132), though 120 divides cleanly into 5, 6 and 8.

  • The 13th quartet, or higher four bits of the time_hi_and_version_field of the UUID is constant, as it indicates the UUID's version. If we encode this value using the scheme common to both Base64 and Base32, we get values between A and P, with the valid subset between B and F.

Therefore: extract the UUID's version quartet, shift all subsequent data 4 bits to the left, zero-pad to the octet, encode with either base64url or base32, truncate, and finally prepend the encoded version character. Voilà, one token-safe UUID.

APPLICATIONS

XML IDs

The ID production appears to have been constricted, inadvertently or otherwise, from Name in both the XML 1.0 and 1.1 specifications, to NCName by XML Schema Part 2. This removes the colon character : from the grammar. The net effect is that

    <foo id="urn:uuid:b07caf81-baae-449d-8a2e-48c0f5fa5538"/>

while being a well-formed ID and valid under DTD validation, is not valid per XML Schema Part 2 or anything that uses it (e.g. Relax NG).

RDF blank node identifiers

Blank node identifiers in RDF are intended for serialization, to act as a handle so that multiple RDF statements can refer to the same blank node. The RDF abstract syntax specifies that the validity constraints of blank node identifiers be delegated to the concrete syntax specifications. The RDF/XML syntax specification lists the blank node identifier as NCName. However, according to the Turtle spec, this is a valid blank node identifier:

    _:42df00ec-30a2-431f-be9e-e3a612b325db

despite an older version listing a production equivalent to the more conservative NCName. NTriples syntax is even more constrained, given as ^[A-Za-z][0-9A-Za-z]*$.

Generated symbols

    There are only two hard things in computer science: cache invalidation and naming things [and off-by-one errors].

    -- Phil Karlton [extension of unknown origin]

Suppose you wanted to create a literate programming system (I do). One of your (my) stipulations is that the symbols get defined in the prose, rather than the code. However, you (I) still want to be able to validate the code's syntax, and potentially even run the code, without having to commit to naming anything. You are (I am) also interested in creating a global map of classes, datatypes and code fragments, which can be operated on and tested in isolation, ported to other languages, or transplanted into the more conventional packages of programs, libraries and frameworks. The Base32 UUID NCName representation should be adequate for placeholder symbols in just about any programming language, save for those which do not permit identifiers as long as 26 characters (which are extremely scarce).

EXPORT

No subroutines are exported by default. Be sure to include at least one of the following in your use statement:

:all

Import all functions.

:decode

Import decode-only functions.

:encode

Import encode-only functions.

:32

Import base32-only functions.

:64

Import base64-only functions.

SUBROUTINES

to_ncname $UUID [, $RADIX ] [, %PARAMS ]

Turn $UUID into an NCName. The UUID can be in the canonical (hyphenated) hexadecimal form, non-hyphenated hexadecimal, Base64 (regular and base64url), or binary. The function returns a legal NCName equivalent to the UUID, in either Base32 or Base64 (url), given a specified $RADIX of 32 or 64. If the radix is omitted, Base64 is assumed.

The following keyword parameters are also accepted, and override the positional parameters where applicable:

radix 32|64

Either 32 or 64 to explicitly specify Base32 or Base64 output. Defaults to 64.

version 0|1

Version 0 will generate the original version of NCName identifiers, prior to the changes noted above. Version 1 is the new version, which is not backwards-compatible. The default, for a transitional period, is to generate version 0, but complain about it. Set the version explicitly (to 1, or to 0 if you need backwards compatibility) to eliminate the warning messages.

align $FALSY|$TRUTHY

Align the last 4 bits to the Base32/Base64 symbol size. You almost certainly want this, so the default is true.

from_ncname $NCNAME [, $FORMAT [, $RADIX] ] [, %PARAMS ]

Turn an appropriate $NCNAME back into a UUID, where appropriate, unless overridden by $RADIX, is defined beginning with one initial alphabetic letter (A to Z, case-insensitive) followed by either:

25 Base32 characters, or
21 Base64URL characters.

The function will return undef immediately if it cannot match either of these patterns. Input past the 21-character mark (for Base64) or 25-character mark (for Base32) is ignored.

This function returns a UUID of type $FORMAT, which if left undefined, must be one of the following:

str

The canonical UUID format, like so: 33fcc995-5d10-477e-a9b4-c9cc405bbf04. This is the default.

hex

The same thing, minus the hyphens.

b64

Base64.

bin

A binary string.

This function also takes the new keyword-style parameters:

format

As above.

radix

As above.

version

Sets the identifier version. Defaults to version 0 with a warning. See the note about setting an explicit version parameter in "to_ncname".

align

Assume the last few bits are aligned to the symbol, as in "to_ncname".

to_ncname_64 $UUID [, %PARAMS ]

Shorthand for Base64 NCNames.

from_ncname_64 $NCNAME [, $FORMAT | %PARAMS ]

Ditto.

to_ncname_32 $UUID [, %PARAMS ]

Shorthand for Base32 NCNames.

from_ncname_32 $NCNAME [, $FORMAT | %PARAMS ]

Ditto.

AUTHOR

Dorian Taylor, <dorian at cpan.org>

BUGS

Please report bugs/issues/etc in GitHub.

SEE ALSO

This module lives under the Data:: namespace for the purpose of namespace hygiene. The main module does not depend on Data::UUID, howevever the script uuid-ncname does depend on UUID::Tiny to generate UUIDs.

LICENSE AND COPYRIGHT

Copyright 2012-2018 Dorian Taylor.

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0 .

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.