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

VERSION
    Version 0.07

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 <http://tools.ietf.org/html/rfc4122> which conforms to the
    constraints of various other identifiers such as NCName, and create an
    isomorphic <http://en.wikipedia.org/wiki/Isomorphism> 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"
    <https://tools.ietf.org/html/rfc4122#section-4.1.1> 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
    <https://en.wikipedia.org/wiki/Postel%27s_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 1, the new one, but will issue a
    warning if not explicitly set. Later I will 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
    <http://tools.ietf.org/html/rfc4122#section-3>, 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 <http://www.w3.org/TR/xml11/#NT-Name> in
        both the XML 1.0 and 1.1 specifications, to NCName
        <http://www.w3.org/TR/xml-names/#NT-NCName> by XML Schema Part 2
        <http://www.w3.org/TR/xmlschema-2/#ID>. 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
        <http://www.w3.org/TR/rdf-concepts/#section-URI-Vocabulary> that the
        validity constraints of blank node identifiers be delegated to the
        concrete syntax specifications. The RDF/XML syntax specification
        <http://www.w3.org/TR/rdf-syntax-grammar/#rdf-id> lists the blank
        node identifier as NCName. However, according to the Turtle spec
        <http://www.w3.org/TR/turtle/#BNodes>, this is a valid blank node
        identifier:

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

        despite an older version
        <http://www.w3.org/TeamSubmission/turtle/#nodeID> listing a
        production equivalent to the more conservative NCName. NTriples
        syntax is even more constrained
        <http://www.w3.org/TR/rdf-testcases/#ntriples>, 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
        <http://en.wikipedia.org/wiki/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.

    :58 Import base58-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, Base58, or Base64 (url), given a specified
    $RADIX of 32, 58, 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|58|64
        Either 32 or 64 to explicitly specify Base32, Base58, 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*. (Does not apply to
        Base58.)

  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_58 $UUID [, %PARAMS ]
    Shorthand for Base58 NCNames.

  from_ncname_58 $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
    <https://github.com/doriantaylor/p5-data-uuid-ncname/issues>.

    *   MetaCPAN

        <https://metacpan.org/release/Data-UUID-NCName>

    *   GitHub repository (bugs also go here)

        <https://github.com/doriantaylor/p5-data-uuid-ncname>

    *   AnnoCPAN: Annotated CPAN documentation

        <http://annocpan.org/dist/Data-UUID-NCName>

    *   CPAN Ratings

        <http://cpanratings.perl.org/d/Data-UUID-NCName>

SEE ALSO
    *   UUID::Tiny

    *   Data::UUID

    *   OSSP::uuid

    *   RFC 4122 <http://tools.ietf.org/html/rfc4122>

    *   RFC 4648 <http://tools.ietf.org/html/rfc4648>

    *   Namespaces in XML <http://www.w3.org/TR/xml-names/#NT-NCName>
        (NCName)

    *   W3C XML Schema Definition Language (XSD) 1.1 Part 2: Datatypes
        <http://www.w3.org/TR/xmlschema11-2/#ID> (ID)

    *   RDF/XML Syntax Specification (Revised)
        <http://www.w3.org/TR/rdf-syntax-grammar/#rdf-id>

    *   Turtle <http://www.w3.org/TR/turtle/#BNodes>

    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.

