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+=head1 NAME
+
+JSON::XS - JSON serialising/deserialising, done correctly and fast
+
+=encoding utf-8
+
+JSON::XS - 正しくて高速な JSON シリアライザ/デシリアライザ
+ (http://fleur.hio.jp/perldoc/mix/lib/JSON/XS.html)
+
+=head1 SYNOPSIS
+
+ use JSON::XS;
+
+ # exported functions, they croak on error
+ # and expect/generate UTF-8
+
+ $utf8_encoded_json_text = encode_json $perl_hash_or_arrayref;
+ $perl_hash_or_arrayref = decode_json $utf8_encoded_json_text;
+
+ # OO-interface
+
+ $coder = JSON::XS->new->ascii->pretty->allow_nonref;
+ $pretty_printed_unencoded = $coder->encode ($perl_scalar);
+ $perl_scalar = $coder->decode ($unicode_json_text);
+
+ # Note that JSON version 2.0 and above will automatically use JSON::XS
+ # if available, at virtually no speed overhead either, so you should
+ # be able to just:
+
+ use JSON;
+
+ # and do the same things, except that you have a pure-perl fallback now.
+
+=head1 DESCRIPTION
+
+This module converts Perl data structures to JSON and vice versa. Its
+primary goal is to be I<correct> and its secondary goal is to be
+I<fast>. To reach the latter goal it was written in C.
+
+Beginning with version 2.0 of the JSON module, when both JSON and
+JSON::XS are installed, then JSON will fall back on JSON::XS (this can be
+overridden) with no overhead due to emulation (by inheriting constructor
+and methods). If JSON::XS is not available, it will fall back to the
+compatible JSON::PP module as backend, so using JSON instead of JSON::XS
+gives you a portable JSON API that can be fast when you need it and
+doesn't require a C compiler when that is a problem.
+
+As this is the n-th-something JSON module on CPAN, what was the reason
+to write yet another JSON module? While it seems there are many JSON
+modules, none of them correctly handle all corner cases, and in most cases
+their maintainers are unresponsive, gone missing, or not listening to bug
+reports for other reasons.
+
+See MAPPING, below, on how JSON::XS maps perl values to JSON values and
+vice versa.
+
+=head2 FEATURES
+
+=over 4
+
+=item * correct Unicode handling
+
+This module knows how to handle Unicode, documents how and when it does
+so, and even documents what "correct" means.
+
+=item * round-trip integrity
+
+When you serialise a perl data structure using only data types supported
+by JSON and Perl, the deserialised data structure is identical on the Perl
+level. (e.g. the string "2.0" doesn't suddenly become "2" just because
+it looks like a number). There I<are> minor exceptions to this, read the
+MAPPING section below to learn about those.
+
+=item * strict checking of JSON correctness
+
+There is no guessing, no generating of illegal JSON texts by default,
+and only JSON is accepted as input by default (the latter is a security
+feature).
+
+=item * fast
+
+Compared to other JSON modules and other serialisers such as Storable,
+this module usually compares favourably in terms of speed, too.
+
+=item * simple to use
+
+This module has both a simple functional interface as well as an object
+oriented interface.
+
+=item * reasonably versatile output formats
+
+You can choose between the most compact guaranteed-single-line format
+possible (nice for simple line-based protocols), a pure-ASCII format
+(for when your transport is not 8-bit clean, still supports the whole
+Unicode range), or a pretty-printed format (for when you want to read that
+stuff). Or you can combine those features in whatever way you like.
+
+=back
+
+=cut
+
+package JSON::XS;
+
+use common::sense;
+
+our $VERSION = 3.04;
+our @ISA = qw(Exporter);
+
+our @EXPORT = qw(encode_json decode_json);
+
+use Exporter;
+use XSLoader;
+
+use Types::Serialiser ();
+
+=head1 FUNCTIONAL INTERFACE
+
+The following convenience methods are provided by this module. They are
+exported by default:
+
+=over 4
+
+=item $json_text = encode_json $perl_scalar
+
+Converts the given Perl data structure to a UTF-8 encoded, binary string
+(that is, the string contains octets only). Croaks on error.
+
+This function call is functionally identical to:
+
+ $json_text = JSON::XS->new->utf8->encode ($perl_scalar)
+
+Except being faster.
+
+=item $perl_scalar = decode_json $json_text
+
+The opposite of C<encode_json>: expects an UTF-8 (binary) string and tries
+to parse that as an UTF-8 encoded JSON text, returning the resulting
+reference. Croaks on error.
+
+This function call is functionally identical to:
+
+ $perl_scalar = JSON::XS->new->utf8->decode ($json_text)
+
+Except being faster.
+
+=back
+
+
+=head1 A FEW NOTES ON UNICODE AND PERL
+
+Since this often leads to confusion, here are a few very clear words on
+how Unicode works in Perl, modulo bugs.
+
+=over 4
+
+=item 1. Perl strings can store characters with ordinal values > 255.
+
+This enables you to store Unicode characters as single characters in a
+Perl string - very natural.
+
+=item 2. Perl does I<not> associate an encoding with your strings.
+
+... until you force it to, e.g. when matching it against a regex, or
+printing the scalar to a file, in which case Perl either interprets your
+string as locale-encoded text, octets/binary, or as Unicode, depending
+on various settings. In no case is an encoding stored together with your
+data, it is I<use> that decides encoding, not any magical meta data.
+
+=item 3. The internal utf-8 flag has no meaning with regards to the
+encoding of your string.
+
+Just ignore that flag unless you debug a Perl bug, a module written in
+XS or want to dive into the internals of perl. Otherwise it will only
+confuse you, as, despite the name, it says nothing about how your string
+is encoded. You can have Unicode strings with that flag set, with that
+flag clear, and you can have binary data with that flag set and that flag
+clear. Other possibilities exist, too.
+
+If you didn't know about that flag, just the better, pretend it doesn't
+exist.
+
+=item 4. A "Unicode String" is simply a string where each character can be
+validly interpreted as a Unicode code point.
+
+If you have UTF-8 encoded data, it is no longer a Unicode string, but a
+Unicode string encoded in UTF-8, giving you a binary string.
+
+=item 5. A string containing "high" (> 255) character values is I<not> a UTF-8 string.
+
+It's a fact. Learn to live with it.
+
+=back
+
+I hope this helps :)
+
+
+=head1 OBJECT-ORIENTED INTERFACE
+
+The object oriented interface lets you configure your own encoding or
+decoding style, within the limits of supported formats.
+
+=over 4
+
+=item $json = new JSON::XS
+
+Creates a new JSON::XS object that can be used to de/encode JSON
+strings. All boolean flags described below are by default I<disabled>.
+
+The mutators for flags all return the JSON object again and thus calls can
+be chained:
+
+ my $json = JSON::XS->new->utf8->space_after->encode ({a => [1,2]})
+ => {"a": [1, 2]}
+
+=item $json = $json->ascii ([$enable])
+
+=item $enabled = $json->get_ascii
+
+If C<$enable> is true (or missing), then the C<encode> method will not
+generate characters outside the code range C<0..127> (which is ASCII). Any
+Unicode characters outside that range will be escaped using either a
+single \uXXXX (BMP characters) or a double \uHHHH\uLLLLL escape sequence,
+as per RFC4627. The resulting encoded JSON text can be treated as a native
+Unicode string, an ascii-encoded, latin1-encoded or UTF-8 encoded string,
+or any other superset of ASCII.
+
+If C<$enable> is false, then the C<encode> method will not escape Unicode
+characters unless required by the JSON syntax or other flags. This results
+in a faster and more compact format.
+
+See also the section I<ENCODING/CODESET FLAG NOTES> later in this
+document.
+
+The main use for this flag is to produce JSON texts that can be
+transmitted over a 7-bit channel, as the encoded JSON texts will not
+contain any 8 bit characters.
+
+ JSON::XS->new->ascii (1)->encode ([chr 0x10401])
+ => ["\ud801\udc01"]
+
+=item $json = $json->latin1 ([$enable])
+
+=item $enabled = $json->get_latin1
+
+If C<$enable> is true (or missing), then the C<encode> method will encode
+the resulting JSON text as latin1 (or iso-8859-1), escaping any characters
+outside the code range C<0..255>. The resulting string can be treated as a
+latin1-encoded JSON text or a native Unicode string. The C<decode> method
+will not be affected in any way by this flag, as C<decode> by default
+expects Unicode, which is a strict superset of latin1.
+
+If C<$enable> is false, then the C<encode> method will not escape Unicode
+characters unless required by the JSON syntax or other flags.
+
+See also the section I<ENCODING/CODESET FLAG NOTES> later in this
+document.
+
+The main use for this flag is efficiently encoding binary data as JSON
+text, as most octets will not be escaped, resulting in a smaller encoded
+size. The disadvantage is that the resulting JSON text is encoded
+in latin1 (and must correctly be treated as such when storing and
+transferring), a rare encoding for JSON. It is therefore most useful when
+you want to store data structures known to contain binary data efficiently
+in files or databases, not when talking to other JSON encoders/decoders.
+
+ JSON::XS->new->latin1->encode (["\x{89}\x{abc}"]
+ => ["\x{89}\\u0abc"] # (perl syntax, U+abc escaped, U+89 not)
+
+=item $json = $json->utf8 ([$enable])
+
+=item $enabled = $json->get_utf8
+
+If C<$enable> is true (or missing), then the C<encode> method will encode
+the JSON result into UTF-8, as required by many protocols, while the
+C<decode> method expects to be handled an UTF-8-encoded string. Please
+note that UTF-8-encoded strings do not contain any characters outside the
+range C<0..255>, they are thus useful for bytewise/binary I/O. In future
+versions, enabling this option might enable autodetection of the UTF-16
+and UTF-32 encoding families, as described in RFC4627.
+
+If C<$enable> is false, then the C<encode> method will return the JSON
+string as a (non-encoded) Unicode string, while C<decode> expects thus a
+Unicode string. Any decoding or encoding (e.g. to UTF-8 or UTF-16) needs
+to be done yourself, e.g. using the Encode module.
+
+See also the section I<ENCODING/CODESET FLAG NOTES> later in this
+document.
+
+Example, output UTF-16BE-encoded JSON:
+
+ use Encode;
+ $jsontext = encode "UTF-16BE", JSON::XS->new->encode ($object);
+
+Example, decode UTF-32LE-encoded JSON:
+
+ use Encode;
+ $object = JSON::XS->new->decode (decode "UTF-32LE", $jsontext);
+
+=item $json = $json->pretty ([$enable])
+
+This enables (or disables) all of the C<indent>, C<space_before> and
+C<space_after> (and in the future possibly more) flags in one call to
+generate the most readable (or most compact) form possible.
+
+Example, pretty-print some simple structure:
+
+ my $json = JSON::XS->new->pretty(1)->encode ({a => [1,2]})
+ =>
+ {
+ "a" : [
+ 1,
+ 2
+ ]
+ }
+
+=item $json = $json->indent ([$enable])
+
+=item $enabled = $json->get_indent
+
+If C<$enable> is true (or missing), then the C<encode> method will use a multiline
+format as output, putting every array member or object/hash key-value pair
+into its own line, indenting them properly.
+
+If C<$enable> is false, no newlines or indenting will be produced, and the
+resulting JSON text is guaranteed not to contain any C<newlines>.
+
+This setting has no effect when decoding JSON texts.
+
+=item $json = $json->space_before ([$enable])
+
+=item $enabled = $json->get_space_before
+
+If C<$enable> is true (or missing), then the C<encode> method will add an extra
+optional space before the C<:> separating keys from values in JSON objects.
+
+If C<$enable> is false, then the C<encode> method will not add any extra
+space at those places.
+
+This setting has no effect when decoding JSON texts. You will also
+most likely combine this setting with C<space_after>.
+
+Example, space_before enabled, space_after and indent disabled:
+
+ {"key" :"value"}
+
+=item $json = $json->space_after ([$enable])
+
+=item $enabled = $json->get_space_after
+
+If C<$enable> is true (or missing), then the C<encode> method will add an extra
+optional space after the C<:> separating keys from values in JSON objects
+and extra whitespace after the C<,> separating key-value pairs and array
+members.
+
+If C<$enable> is false, then the C<encode> method will not add any extra
+space at those places.
+
+This setting has no effect when decoding JSON texts.
+
+Example, space_before and indent disabled, space_after enabled:
+
+ {"key": "value"}
+
+=item $json = $json->relaxed ([$enable])
+
+=item $enabled = $json->get_relaxed
+
+If C<$enable> is true (or missing), then C<decode> will accept some
+extensions to normal JSON syntax (see below). C<encode> will not be
+affected in anyway. I<Be aware that this option makes you accept invalid
+JSON texts as if they were valid!>. I suggest only to use this option to
+parse application-specific files written by humans (configuration files,
+resource files etc.)
+
+If C<$enable> is false (the default), then C<decode> will only accept
+valid JSON texts.
+
+Currently accepted extensions are:
+
+=over 4
+
+=item * list items can have an end-comma
+
+JSON I<separates> array elements and key-value pairs with commas. This
+can be annoying if you write JSON texts manually and want to be able to
+quickly append elements, so this extension accepts comma at the end of
+such items not just between them:
+
+ [
+ 1,
+ 2, <- this comma not normally allowed
+ ]
+ {
+ "k1": "v1",
+ "k2": "v2", <- this comma not normally allowed
+ }
+
+=item * shell-style '#'-comments
+
+Whenever JSON allows whitespace, shell-style comments are additionally
+allowed. They are terminated by the first carriage-return or line-feed
+character, after which more white-space and comments are allowed.
+
+ [
+ 1, # this comment not allowed in JSON
+ # neither this one...
+ ]
+
+=item * literal ASCII TAB characters in strings
+
+Literal ASCII TAB characters are now allowed in strings (and treated as
+C<\t>).
+
+ [
+ "Hello\tWorld",
+ "Hello<TAB>World", # literal <TAB> would not normally be allowed
+ ]
+
+=back
+
+=item $json = $json->canonical ([$enable])
+
+=item $enabled = $json->get_canonical
+
+If C<$enable> is true (or missing), then the C<encode> method will output JSON objects
+by sorting their keys. This is adding a comparatively high overhead.
+
+If C<$enable> is false, then the C<encode> method will output key-value
+pairs in the order Perl stores them (which will likely change between runs
+of the same script, and can change even within the same run from 5.18
+onwards).
+
+This option is useful if you want the same data structure to be encoded as
+the same JSON text (given the same overall settings). If it is disabled,
+the same hash might be encoded differently even if contains the same data,
+as key-value pairs have no inherent ordering in Perl.
+
+This setting has no effect when decoding JSON texts.
+
+This setting has currently no effect on tied hashes.
+
+=item $json = $json->allow_nonref ([$enable])
+
+=item $enabled = $json->get_allow_nonref
+
+If C<$enable> is true (or missing), then the C<encode> method can convert a
+non-reference into its corresponding string, number or null JSON value,
+which is an extension to RFC4627. Likewise, C<decode> will accept those JSON
+values instead of croaking.
+
+If C<$enable> is false, then the C<encode> method will croak if it isn't
+passed an arrayref or hashref, as JSON texts must either be an object
+or array. Likewise, C<decode> will croak if given something that is not a
+JSON object or array.
+
+Example, encode a Perl scalar as JSON value with enabled C<allow_nonref>,
+resulting in an invalid JSON text:
+
+ JSON::XS->new->allow_nonref->encode ("Hello, World!")
+ => "Hello, World!"
+
+=item $json = $json->allow_unknown ([$enable])
+
+=item $enabled = $json->get_allow_unknown
+
+If C<$enable> is true (or missing), then C<encode> will I<not> throw an
+exception when it encounters values it cannot represent in JSON (for
+example, filehandles) but instead will encode a JSON C<null> value. Note
+that blessed objects are not included here and are handled separately by
+c<allow_nonref>.
+
+If C<$enable> is false (the default), then C<encode> will throw an
+exception when it encounters anything it cannot encode as JSON.
+
+This option does not affect C<decode> in any way, and it is recommended to
+leave it off unless you know your communications partner.
+
+=item $json = $json->allow_blessed ([$enable])
+
+=item $enabled = $json->get_allow_blessed
+
+See L<OBJECT SERIALISATION> for details.
+
+If C<$enable> is true (or missing), then the C<encode> method will not
+barf when it encounters a blessed reference that it cannot convert
+otherwise. Instead, a JSON C<null> value is encoded instead of the object.
+
+If C<$enable> is false (the default), then C<encode> will throw an
+exception when it encounters a blessed object that it cannot convert
+otherwise.
+
+This setting has no effect on C<decode>.
+
+=item $json = $json->convert_blessed ([$enable])
+
+=item $enabled = $json->get_convert_blessed
+
+See L<OBJECT SERIALISATION> for details.
+
+If C<$enable> is true (or missing), then C<encode>, upon encountering a
+blessed object, will check for the availability of the C<TO_JSON> method
+on the object's class. If found, it will be called in scalar context and
+the resulting scalar will be encoded instead of the object.
+
+The C<TO_JSON> method may safely call die if it wants. If C<TO_JSON>
+returns other blessed objects, those will be handled in the same
+way. C<TO_JSON> must take care of not causing an endless recursion cycle
+(== crash) in this case. The name of C<TO_JSON> was chosen because other
+methods called by the Perl core (== not by the user of the object) are
+usually in upper case letters and to avoid collisions with any C<to_json>
+function or method.
+
+If C<$enable> is false (the default), then C<encode> will not consider
+this type of conversion.
+
+This setting has no effect on C<decode>.
+
+=item $json = $json->allow_tags ([$enable])
+
+=item $enabled = $json->allow_tags
+
+See L<OBJECT SERIALISATION> for details.
+
+If C<$enable> is true (or missing), then C<encode>, upon encountering a
+blessed object, will check for the availability of the C<FREEZE> method on
+the object's class. If found, it will be used to serialise the object into
+a nonstandard tagged JSON value (that JSON decoders cannot decode).
+
+It also causes C<decode> to parse such tagged JSON values and deserialise
+them via a call to the C<THAW> method.
+
+If C<$enable> is false (the default), then C<encode> will not consider
+this type of conversion, and tagged JSON values will cause a parse error
+in C<decode>, as if tags were not part of the grammar.
+
+=item $json = $json->filter_json_object ([$coderef->($hashref)])
+
+When C<$coderef> is specified, it will be called from C<decode> each
+time it decodes a JSON object. The only argument is a reference to the
+newly-created hash. If the code references returns a single scalar (which
+need not be a reference), this value (i.e. a copy of that scalar to avoid
+aliasing) is inserted into the deserialised data structure. If it returns
+an empty list (NOTE: I<not> C<undef>, which is a valid scalar), the
+original deserialised hash will be inserted. This setting can slow down
+decoding considerably.
+
+When C<$coderef> is omitted or undefined, any existing callback will
+be removed and C<decode> will not change the deserialised hash in any
+way.
+
+Example, convert all JSON objects into the integer 5:
+
+ my $js = JSON::XS->new->filter_json_object (sub { 5 });
+ # returns [5]
+ $js->decode ('[{}]')
+ # throw an exception because allow_nonref is not enabled
+ # so a lone 5 is not allowed.
+ $js->decode ('{"a":1, "b":2}');
+
+=item $json = $json->filter_json_single_key_object ($key [=> $coderef->($value)])
+
+Works remotely similar to C<filter_json_object>, but is only called for
+JSON objects having a single key named C<$key>.
+
+This C<$coderef> is called before the one specified via
+C<filter_json_object>, if any. It gets passed the single value in the JSON
+object. If it returns a single value, it will be inserted into the data
+structure. If it returns nothing (not even C<undef> but the empty list),
+the callback from C<filter_json_object> will be called next, as if no
+single-key callback were specified.
+
+If C<$coderef> is omitted or undefined, the corresponding callback will be
+disabled. There can only ever be one callback for a given key.
+
+As this callback gets called less often then the C<filter_json_object>
+one, decoding speed will not usually suffer as much. Therefore, single-key
+objects make excellent targets to serialise Perl objects into, especially
+as single-key JSON objects are as close to the type-tagged value concept
+as JSON gets (it's basically an ID/VALUE tuple). Of course, JSON does not
+support this in any way, so you need to make sure your data never looks
+like a serialised Perl hash.
+
+Typical names for the single object key are C<__class_whatever__>, or
+C<$__dollars_are_rarely_used__$> or C<}ugly_brace_placement>, or even
+things like C<__class_md5sum(classname)__>, to reduce the risk of clashing
+with real hashes.
+
+Example, decode JSON objects of the form C<< { "__widget__" => <id> } >>
+into the corresponding C<< $WIDGET{<id>} >> object:
+
+ # return whatever is in $WIDGET{5}:
+ JSON::XS
+ ->new
+ ->filter_json_single_key_object (__widget__ => sub {
+ $WIDGET{ $_[0] }
+ })
+ ->decode ('{"__widget__": 5')
+
+ # this can be used with a TO_JSON method in some "widget" class
+ # for serialisation to json:
+ sub WidgetBase::TO_JSON {
+ my ($self) = @_;
+
+ unless ($self->{id}) {
+ $self->{id} = ..get..some..id..;
+ $WIDGET{$self->{id}} = $self;
+ }
+
+ { __widget__ => $self->{id} }
+ }
+
+=item $json = $json->shrink ([$enable])
+
+=item $enabled = $json->get_shrink
+
+Perl usually over-allocates memory a bit when allocating space for
+strings. This flag optionally resizes strings generated by either
+C<encode> or C<decode> to their minimum size possible. This can save
+memory when your JSON texts are either very very long or you have many
+short strings. It will also try to downgrade any strings to octet-form
+if possible: perl stores strings internally either in an encoding called
+UTF-X or in octet-form. The latter cannot store everything but uses less
+space in general (and some buggy Perl or C code might even rely on that
+internal representation being used).
+
+The actual definition of what shrink does might change in future versions,
+but it will always try to save space at the expense of time.
+
+If C<$enable> is true (or missing), the string returned by C<encode> will
+be shrunk-to-fit, while all strings generated by C<decode> will also be
+shrunk-to-fit.
+
+If C<$enable> is false, then the normal perl allocation algorithms are used.
+If you work with your data, then this is likely to be faster.
+
+In the future, this setting might control other things, such as converting
+strings that look like integers or floats into integers or floats
+internally (there is no difference on the Perl level), saving space.
+
+=item $json = $json->max_depth ([$maximum_nesting_depth])
+
+=item $max_depth = $json->get_max_depth
+
+Sets the maximum nesting level (default C<512>) accepted while encoding
+or decoding. If a higher nesting level is detected in JSON text or a Perl
+data structure, then the encoder and decoder will stop and croak at that
+point.
+
+Nesting level is defined by number of hash- or arrayrefs that the encoder
+needs to traverse to reach a given point or the number of C<{> or C<[>
+characters without their matching closing parenthesis crossed to reach a
+given character in a string.
+
+Setting the maximum depth to one disallows any nesting, so that ensures
+that the object is only a single hash/object or array.
+
+If no argument is given, the highest possible setting will be used, which
+is rarely useful.
+
+Note that nesting is implemented by recursion in C. The default value has
+been chosen to be as large as typical operating systems allow without
+crashing.
+
+See SECURITY CONSIDERATIONS, below, for more info on why this is useful.
+
+=item $json = $json->max_size ([$maximum_string_size])
+
+=item $max_size = $json->get_max_size
+
+Set the maximum length a JSON text may have (in bytes) where decoding is
+being attempted. The default is C<0>, meaning no limit. When C<decode>
+is called on a string that is longer then this many bytes, it will not
+attempt to decode the string but throw an exception. This setting has no
+effect on C<encode> (yet).
+
+If no argument is given, the limit check will be deactivated (same as when
+C<0> is specified).
+
+See SECURITY CONSIDERATIONS, below, for more info on why this is useful.
+
+=item $json_text = $json->encode ($perl_scalar)
+
+Converts the given Perl value or data structure to its JSON
+representation. Croaks on error.
+
+=item $perl_scalar = $json->decode ($json_text)
+
+The opposite of C<encode>: expects a JSON text and tries to parse it,
+returning the resulting simple scalar or reference. Croaks on error.
+
+=item ($perl_scalar, $characters) = $json->decode_prefix ($json_text)
+
+This works like the C<decode> method, but instead of raising an exception
+when there is trailing garbage after the first JSON object, it will
+silently stop parsing there and return the number of characters consumed
+so far.
+
+This is useful if your JSON texts are not delimited by an outer protocol
+and you need to know where the JSON text ends.
+
+ JSON::XS->new->decode_prefix ("[1] the tail")
+ => ([1], 3)
+
+=back
+
+
+=head1 INCREMENTAL PARSING
+
+In some cases, there is the need for incremental parsing of JSON
+texts. While this module always has to keep both JSON text and resulting
+Perl data structure in memory at one time, it does allow you to parse a
+JSON stream incrementally. It does so by accumulating text until it has
+a full JSON object, which it then can decode. This process is similar to
+using C<decode_prefix> to see if a full JSON object is available, but
+is much more efficient (and can be implemented with a minimum of method
+calls).
+
+JSON::XS will only attempt to parse the JSON text once it is sure it
+has enough text to get a decisive result, using a very simple but
+truly incremental parser. This means that it sometimes won't stop as
+early as the full parser, for example, it doesn't detect mismatched
+parentheses. The only thing it guarantees is that it starts decoding as
+soon as a syntactically valid JSON text has been seen. This means you need
+to set resource limits (e.g. C<max_size>) to ensure the parser will stop
+parsing in the presence if syntax errors.
+
+The following methods implement this incremental parser.
+
+=over 4
+
+=item [void, scalar or list context] = $json->incr_parse ([$string])
+
+This is the central parsing function. It can both append new text and
+extract objects from the stream accumulated so far (both of these
+functions are optional).
+
+If C<$string> is given, then this string is appended to the already
+existing JSON fragment stored in the C<$json> object.
+
+After that, if the function is called in void context, it will simply
+return without doing anything further. This can be used to add more text
+in as many chunks as you want.
+
+If the method is called in scalar context, then it will try to extract
+exactly I<one> JSON object. If that is successful, it will return this
+object, otherwise it will return C<undef>. If there is a parse error,
+this method will croak just as C<decode> would do (one can then use
+C<incr_skip> to skip the erroneous part). This is the most common way of
+using the method.
+
+And finally, in list context, it will try to extract as many objects
+from the stream as it can find and return them, or the empty list
+otherwise. For this to work, there must be no separators (other than
+whitespace) between the JSON objects or arrays, instead they must be
+concatenated back-to-back. If an error occurs, an exception will be
+raised as in the scalar context case. Note that in this case, any
+previously-parsed JSON texts will be lost.
+
+Example: Parse some JSON arrays/objects in a given string and return
+them.
+
+ my @objs = JSON::XS->new->incr_parse ("[5][7][1,2]");
+
+=item $lvalue_string = $json->incr_text
+
+This method returns the currently stored JSON fragment as an lvalue, that
+is, you can manipulate it. This I<only> works when a preceding call to
+C<incr_parse> in I<scalar context> successfully returned an object. Under
+all other circumstances you must not call this function (I mean it.
+although in simple tests it might actually work, it I<will> fail under
+real world conditions). As a special exception, you can also call this
+method before having parsed anything.
+
+That means you can only use this function to look at or manipulate text
+before or after complete JSON objects, not while the parser is in the
+middle of parsing a JSON object.
+
+This function is useful in two cases: a) finding the trailing text after a
+JSON object or b) parsing multiple JSON objects separated by non-JSON text
+(such as commas).
+
+=item $json->incr_skip
+
+This will reset the state of the incremental parser and will remove
+the parsed text from the input buffer so far. This is useful after
+C<incr_parse> died, in which case the input buffer and incremental parser
+state is left unchanged, to skip the text parsed so far and to reset the
+parse state.
+
+The difference to C<incr_reset> is that only text until the parse error
+occurred is removed.
+
+=item $json->incr_reset
+
+This completely resets the incremental parser, that is, after this call,
+it will be as if the parser had never parsed anything.
+
+This is useful if you want to repeatedly parse JSON objects and want to
+ignore any trailing data, which means you have to reset the parser after
+each successful decode.
+
+=back
+
+=head2 LIMITATIONS
+
+All options that affect decoding are supported, except
+C<allow_nonref>. The reason for this is that it cannot be made to work
+sensibly: JSON objects and arrays are self-delimited, i.e. you can
+concatenate them back to back and still decode them perfectly. This does
+not hold true for JSON numbers, however.
+
+For example, is the string C<1> a single JSON number, or is it simply the
+start of C<12>? Or is C<12> a single JSON number, or the concatenation
+of C<1> and C<2>? In neither case you can tell, and this is why JSON::XS
+takes the conservative route and disallows this case.
+
+=head2 EXAMPLES
+
+Some examples will make all this clearer. First, a simple example that
+works similarly to C<decode_prefix>: We want to decode the JSON object at
+the start of a string and identify the portion after the JSON object:
+
+ my $text = "[1,2,3] hello";
+
+ my $json = new JSON::XS;
+
+ my $obj = $json->incr_parse ($text)
+ or die "expected JSON object or array at beginning of string";
+
+ my $tail = $json->incr_text;
+ # $tail now contains " hello"
+
+Easy, isn't it?
+
+Now for a more complicated example: Imagine a hypothetical protocol where
+you read some requests from a TCP stream, and each request is a JSON
+array, without any separation between them (in fact, it is often useful to
+use newlines as "separators", as these get interpreted as whitespace at
+the start of the JSON text, which makes it possible to test said protocol
+with C<telnet>...).
+
+Here is how you'd do it (it is trivial to write this in an event-based
+manner):
+
+ my $json = new JSON::XS;
+
+ # read some data from the socket
+ while (sysread $socket, my $buf, 4096) {
+
+ # split and decode as many requests as possible
+ for my $request ($json->incr_parse ($buf)) {
+ # act on the $request
+ }
+ }
+
+Another complicated example: Assume you have a string with JSON objects
+or arrays, all separated by (optional) comma characters (e.g. C<[1],[2],
+[3]>). To parse them, we have to skip the commas between the JSON texts,
+and here is where the lvalue-ness of C<incr_text> comes in useful:
+
+ my $text = "[1],[2], [3]";
+ my $json = new JSON::XS;
+
+ # void context, so no parsing done
+ $json->incr_parse ($text);
+
+ # now extract as many objects as possible. note the
+ # use of scalar context so incr_text can be called.
+ while (my $obj = $json->incr_parse) {
+ # do something with $obj
+
+ # now skip the optional comma
+ $json->incr_text =~ s/^ \s* , //x;
+ }
+
+Now lets go for a very complex example: Assume that you have a gigantic
+JSON array-of-objects, many gigabytes in size, and you want to parse it,
+but you cannot load it into memory fully (this has actually happened in
+the real world :).
+
+Well, you lost, you have to implement your own JSON parser. But JSON::XS
+can still help you: You implement a (very simple) array parser and let
+JSON decode the array elements, which are all full JSON objects on their
+own (this wouldn't work if the array elements could be JSON numbers, for
+example):
+
+ my $json = new JSON::XS;
+
+ # open the monster
+ open my $fh, "<bigfile.json"
+ or die "bigfile: $!";
+
+ # first parse the initial "["
+ for (;;) {
+ sysread $fh, my $buf, 65536
+ or die "read error: $!";
+ $json->incr_parse ($buf); # void context, so no parsing
+
+ # Exit the loop once we found and removed(!) the initial "[".
+ # In essence, we are (ab-)using the $json object as a simple scalar
+ # we append data to.
+ last if $json->incr_text =~ s/^ \s* \[ //x;
+ }
+
+ # now we have the skipped the initial "[", so continue
+ # parsing all the elements.
+ for (;;) {
+ # in this loop we read data until we got a single JSON object
+ for (;;) {
+ if (my $obj = $json->incr_parse) {
+ # do something with $obj
+ last;
+ }
+
+ # add more data
+ sysread $fh, my $buf, 65536
+ or die "read error: $!";
+ $json->incr_parse ($buf); # void context, so no parsing
+ }
+
+ # in this loop we read data until we either found and parsed the
+ # separating "," between elements, or the final "]"
+ for (;;) {
+ # first skip whitespace
+ $json->incr_text =~ s/^\s*//;
+
+ # if we find "]", we are done
+ if ($json->incr_text =~ s/^\]//) {
+ print "finished.\n";
+ exit;
+ }
+
+ # if we find ",", we can continue with the next element
+ if ($json->incr_text =~ s/^,//) {
+ last;
+ }
+
+ # if we find anything else, we have a parse error!
+ if (length $json->incr_text) {
+ die "parse error near ", $json->incr_text;
+ }
+
+ # else add more data
+ sysread $fh, my $buf, 65536
+ or die "read error: $!";
+ $json->incr_parse ($buf); # void context, so no parsing
+ }
+
+This is a complex example, but most of the complexity comes from the fact
+that we are trying to be correct (bear with me if I am wrong, I never ran
+the above example :).
+
+
+
+=head1 MAPPING
+
+This section describes how JSON::XS maps Perl values to JSON values and
+vice versa. These mappings are designed to "do the right thing" in most
+circumstances automatically, preserving round-tripping characteristics
+(what you put in comes out as something equivalent).
+
+For the more enlightened: note that in the following descriptions,
+lowercase I<perl> refers to the Perl interpreter, while uppercase I<Perl>
+refers to the abstract Perl language itself.
+
+
+=head2 JSON -> PERL
+
+=over 4
+
+=item object
+
+A JSON object becomes a reference to a hash in Perl. No ordering of object
+keys is preserved (JSON does not preserve object key ordering itself).
+
+=item array
+
+A JSON array becomes a reference to an array in Perl.
+
+=item string
+
+A JSON string becomes a string scalar in Perl - Unicode codepoints in JSON
+are represented by the same codepoints in the Perl string, so no manual
+decoding is necessary.
+
+=item number
+
+A JSON number becomes either an integer, numeric (floating point) or
+string scalar in perl, depending on its range and any fractional parts. On
+the Perl level, there is no difference between those as Perl handles all
+the conversion details, but an integer may take slightly less memory and
+might represent more values exactly than floating point numbers.
+
+If the number consists of digits only, JSON::XS will try to represent
+it as an integer value. If that fails, it will try to represent it as
+a numeric (floating point) value if that is possible without loss of
+precision. Otherwise it will preserve the number as a string value (in
+which case you lose roundtripping ability, as the JSON number will be
+re-encoded to a JSON string).
+
+Numbers containing a fractional or exponential part will always be
+represented as numeric (floating point) values, possibly at a loss of
+precision (in which case you might lose perfect roundtripping ability, but
+the JSON number will still be re-encoded as a JSON number).
+
+Note that precision is not accuracy - binary floating point values cannot
+represent most decimal fractions exactly, and when converting from and to
+floating point, JSON::XS only guarantees precision up to but not including
+the least significant bit.
+
+=item true, false
+
+These JSON atoms become C<Types::Serialiser::true> and
+C<Types::Serialiser::false>, respectively. They are overloaded to act
+almost exactly like the numbers C<1> and C<0>. You can check whether
+a scalar is a JSON boolean by using the C<Types::Serialiser::is_bool>
+function (after C<use Types::Serialier>, of course).
+
+=item null
+
+A JSON null atom becomes C<undef> in Perl.
+
+=item shell-style comments (C<< # I<text> >>)
+
+As a nonstandard extension to the JSON syntax that is enabled by the
+C<relaxed> setting, shell-style comments are allowed. They can start
+anywhere outside strings and go till the end of the line.
+
+=item tagged values (C<< (I<tag>)I<value> >>).
+
+Another nonstandard extension to the JSON syntax, enabled with the
+C<allow_tags> setting, are tagged values. In this implementation, the
+I<tag> must be a perl package/class name encoded as a JSON string, and the
+I<value> must be a JSON array encoding optional constructor arguments.
+
+See L<OBJECT SERIALISATION>, below, for details.
+
+=back
+
+
+=head2 PERL -> JSON
+
+The mapping from Perl to JSON is slightly more difficult, as Perl is a
+truly typeless language, so we can only guess which JSON type is meant by
+a Perl value.
+
+=over 4
+
+=item hash references
+
+Perl hash references become JSON objects. As there is no inherent
+ordering in hash keys (or JSON objects), they will usually be encoded
+in a pseudo-random order. JSON::XS can optionally sort the hash keys
+(determined by the I<canonical> flag), so the same datastructure will
+serialise to the same JSON text (given same settings and version of
+JSON::XS), but this incurs a runtime overhead and is only rarely useful,
+e.g. when you want to compare some JSON text against another for equality.
+
+=item array references
+
+Perl array references become JSON arrays.
+
+=item other references
+
+Other unblessed references are generally not allowed and will cause an
+exception to be thrown, except for references to the integers C<0> and
+C<1>, which get turned into C<false> and C<true> atoms in JSON.
+
+Since C<JSON::XS> uses the boolean model from L<Types::Serialiser>, you
+can also C<use Types::Serialiser> and then use C<Types::Serialiser::false>
+and C<Types::Serialiser::true> to improve readability.
+
+ use Types::Serialiser;
+ encode_json [\0, Types::Serialiser::true] # yields [false,true]
+
+=item Types::Serialiser::true, Types::Serialiser::false
+
+These special values from the L<Types::Serialiser> module become JSON true
+and JSON false values, respectively. You can also use C<\1> and C<\0>
+directly if you want.
+
+=item blessed objects
+
+Blessed objects are not directly representable in JSON, but C<JSON::XS>
+allows various ways of handling objects. See L<OBJECT SERIALISATION>,
+below, for details.
+
+=item simple scalars
+
+Simple Perl scalars (any scalar that is not a reference) are the most
+difficult objects to encode: JSON::XS will encode undefined scalars as
+JSON C<null> values, scalars that have last been used in a string context
+before encoding as JSON strings, and anything else as number value:
+
+ # dump as number
+ encode_json [2] # yields [2]
+ encode_json [-3.0e17] # yields [-3e+17]
+ my $value = 5; encode_json [$value] # yields [5]
+
+ # used as string, so dump as string
+ print $value;
+ encode_json [$value] # yields ["5"]
+
+ # undef becomes null
+ encode_json [undef] # yields [null]
+
+You can force the type to be a JSON string by stringifying it:
+
+ my $x = 3.1; # some variable containing a number
+ "$x"; # stringified
+ $x .= ""; # another, more awkward way to stringify
+ print $x; # perl does it for you, too, quite often
+
+You can force the type to be a JSON number by numifying it:
+
+ my $x = "3"; # some variable containing a string
+ $x += 0; # numify it, ensuring it will be dumped as a number
+ $x *= 1; # same thing, the choice is yours.
+
+You can not currently force the type in other, less obscure, ways. Tell me
+if you need this capability (but don't forget to explain why it's needed
+:).
+
+Note that numerical precision has the same meaning as under Perl (so
+binary to decimal conversion follows the same rules as in Perl, which
+can differ to other languages). Also, your perl interpreter might expose
+extensions to the floating point numbers of your platform, such as
+infinities or NaN's - these cannot be represented in JSON, and it is an
+error to pass those in.
+
+=back
+
+=head2 OBJECT SERIALISATION
+
+As JSON cannot directly represent Perl objects, you have to choose between
+a pure JSON representation (without the ability to deserialise the object
+automatically again), and a nonstandard extension to the JSON syntax,
+tagged values.
+
+=head3 SERIALISATION
+
+What happens when C<JSON::XS> encounters a Perl object depends on the
+C<allow_blessed>, C<convert_blessed> and C<allow_tags> settings, which are
+used in this order:
+
+=over 4
+
+=item 1. C<allow_tags> is enabled and the object has a C<FREEZE> method.
+
+In this case, C<JSON::XS> uses the L<Types::Serialiser> object
+serialisation protocol to create a tagged JSON value, using a nonstandard
+extension to the JSON syntax.
+
+This works by invoking the C<FREEZE> method on the object, with the first
+argument being the object to serialise, and the second argument being the
+constant string C<JSON> to distinguish it from other serialisers.
+
+The C<FREEZE> method can return any number of values (i.e. zero or
+more). These values and the paclkage/classname of the object will then be
+encoded as a tagged JSON value in the following format:
+
+ ("classname")[FREEZE return values...]
+
+e.g.:
+
+ ("URI")["http://www.google.com/"]
+ ("MyDate")[2013,10,29]
+ ("ImageData::JPEG")["Z3...VlCg=="]
+
+For example, the hypothetical C<My::Object> C<FREEZE> method might use the
+objects C<type> and C<id> members to encode the object:
+
+ sub My::Object::FREEZE {
+ my ($self, $serialiser) = @_;
+
+ ($self->{type}, $self->{id})
+ }
+
+=item 2. C<convert_blessed> is enabled and the object has a C<TO_JSON> method.
+
+In this case, the C<TO_JSON> method of the object is invoked in scalar
+context. It must return a single scalar that can be directly encoded into
+JSON. This scalar replaces the object in the JSON text.
+
+For example, the following C<TO_JSON> method will convert all L<URI>
+objects to JSON strings when serialised. The fatc that these values
+originally were L<URI> objects is lost.
+
+ sub URI::TO_JSON {
+ my ($uri) = @_;
+ $uri->as_string
+ }
+
+=item 3. C<allow_blessed> is enabled.
+
+The object will be serialised as a JSON null value.
+
+=item 4. none of the above
+
+If none of the settings are enabled or the respective methods are missing,
+C<JSON::XS> throws an exception.
+
+=back
+
+=head3 DESERIALISATION
+
+For deserialisation there are only two cases to consider: either
+nonstandard tagging was used, in which case C<allow_tags> decides,
+or objects cannot be automatically be deserialised, in which
+case you can use postprocessing or the C<filter_json_object> or
+C<filter_json_single_key_object> callbacks to get some real objects our of
+your JSON.
+
+This section only considers the tagged value case: I a tagged JSON object
+is encountered during decoding and C<allow_tags> is disabled, a parse
+error will result (as if tagged values were not part of the grammar).
+
+If C<allow_tags> is enabled, C<JSON::XS> will look up the C<THAW> method
+of the package/classname used during serialisation (it will not attempt
+to load the package as a Perl module). If there is no such method, the
+decoding will fail with an error.
+
+Otherwise, the C<THAW> method is invoked with the classname as first
+argument, the constant string C<JSON> as second argument, and all the
+values from the JSON array (the values originally returned by the
+C<FREEZE> method) as remaining arguments.
+
+The method must then return the object. While technically you can return
+any Perl scalar, you might have to enable the C<enable_nonref> setting to
+make that work in all cases, so better return an actual blessed reference.
+
+As an example, let's implement a C<THAW> function that regenerates the
+C<My::Object> from the C<FREEZE> example earlier:
+
+ sub My::Object::THAW {
+ my ($class, $serialiser, $type, $id) = @_;
+
+ $class->new (type => $type, id => $id)
+ }
+
+
+=head1 ENCODING/CODESET FLAG NOTES
+
+The interested reader might have seen a number of flags that signify
+encodings or codesets - C<utf8>, C<latin1> and C<ascii>. There seems to be
+some confusion on what these do, so here is a short comparison:
+
+C<utf8> controls whether the JSON text created by C<encode> (and expected
+by C<decode>) is UTF-8 encoded or not, while C<latin1> and C<ascii> only
+control whether C<encode> escapes character values outside their respective
+codeset range. Neither of these flags conflict with each other, although
+some combinations make less sense than others.
+
+Care has been taken to make all flags symmetrical with respect to
+C<encode> and C<decode>, that is, texts encoded with any combination of
+these flag values will be correctly decoded when the same flags are used
+- in general, if you use different flag settings while encoding vs. when
+decoding you likely have a bug somewhere.
+
+Below comes a verbose discussion of these flags. Note that a "codeset" is
+simply an abstract set of character-codepoint pairs, while an encoding
+takes those codepoint numbers and I<encodes> them, in our case into
+octets. Unicode is (among other things) a codeset, UTF-8 is an encoding,
+and ISO-8859-1 (= latin 1) and ASCII are both codesets I<and> encodings at
+the same time, which can be confusing.
+
+=over 4
+
+=item C<utf8> flag disabled
+
+When C<utf8> is disabled (the default), then C<encode>/C<decode> generate
+and expect Unicode strings, that is, characters with high ordinal Unicode
+values (> 255) will be encoded as such characters, and likewise such
+characters are decoded as-is, no changes to them will be done, except
+"(re-)interpreting" them as Unicode codepoints or Unicode characters,
+respectively (to Perl, these are the same thing in strings unless you do
+funny/weird/dumb stuff).
+
+This is useful when you want to do the encoding yourself (e.g. when you
+want to have UTF-16 encoded JSON texts) or when some other layer does
+the encoding for you (for example, when printing to a terminal using a
+filehandle that transparently encodes to UTF-8 you certainly do NOT want
+to UTF-8 encode your data first and have Perl encode it another time).
+
+=item C<utf8> flag enabled
+
+If the C<utf8>-flag is enabled, C<encode>/C<decode> will encode all
+characters using the corresponding UTF-8 multi-byte sequence, and will
+expect your input strings to be encoded as UTF-8, that is, no "character"
+of the input string must have any value > 255, as UTF-8 does not allow
+that.
+
+The C<utf8> flag therefore switches between two modes: disabled means you
+will get a Unicode string in Perl, enabled means you get an UTF-8 encoded
+octet/binary string in Perl.
+
+=item C<latin1> or C<ascii> flags enabled
+
+With C<latin1> (or C<ascii>) enabled, C<encode> will escape characters
+with ordinal values > 255 (> 127 with C<ascii>) and encode the remaining
+characters as specified by the C<utf8> flag.
+
+If C<utf8> is disabled, then the result is also correctly encoded in those
+character sets (as both are proper subsets of Unicode, meaning that a
+Unicode string with all character values < 256 is the same thing as a
+ISO-8859-1 string, and a Unicode string with all character values < 128 is
+the same thing as an ASCII string in Perl).
+
+If C<utf8> is enabled, you still get a correct UTF-8-encoded string,
+regardless of these flags, just some more characters will be escaped using
+C<\uXXXX> then before.
+
+Note that ISO-8859-1-I<encoded> strings are not compatible with UTF-8
+encoding, while ASCII-encoded strings are. That is because the ISO-8859-1
+encoding is NOT a subset of UTF-8 (despite the ISO-8859-1 I<codeset> being
+a subset of Unicode), while ASCII is.
+
+Surprisingly, C<decode> will ignore these flags and so treat all input
+values as governed by the C<utf8> flag. If it is disabled, this allows you
+to decode ISO-8859-1- and ASCII-encoded strings, as both strict subsets of
+Unicode. If it is enabled, you can correctly decode UTF-8 encoded strings.
+
+So neither C<latin1> nor C<ascii> are incompatible with the C<utf8> flag -
+they only govern when the JSON output engine escapes a character or not.
+
+The main use for C<latin1> is to relatively efficiently store binary data
+as JSON, at the expense of breaking compatibility with most JSON decoders.
+
+The main use for C<ascii> is to force the output to not contain characters
+with values > 127, which means you can interpret the resulting string
+as UTF-8, ISO-8859-1, ASCII, KOI8-R or most about any character set and
+8-bit-encoding, and still get the same data structure back. This is useful
+when your channel for JSON transfer is not 8-bit clean or the encoding
+might be mangled in between (e.g. in mail), and works because ASCII is a
+proper subset of most 8-bit and multibyte encodings in use in the world.
+
+=back
+
+
+=head2 JSON and ECMAscript
+
+JSON syntax is based on how literals are represented in javascript (the
+not-standardised predecessor of ECMAscript) which is presumably why it is
+called "JavaScript Object Notation".
+
+However, JSON is not a subset (and also not a superset of course) of
+ECMAscript (the standard) or javascript (whatever browsers actually
+implement).
+
+If you want to use javascript's C<eval> function to "parse" JSON, you
+might run into parse errors for valid JSON texts, or the resulting data
+structure might not be queryable:
+
+One of the problems is that U+2028 and U+2029 are valid characters inside
+JSON strings, but are not allowed in ECMAscript string literals, so the
+following Perl fragment will not output something that can be guaranteed
+to be parsable by javascript's C<eval>:
+
+ use JSON::XS;
+
+ print encode_json [chr 0x2028];
+
+The right fix for this is to use a proper JSON parser in your javascript
+programs, and not rely on C<eval> (see for example Douglas Crockford's
+F<json2.js> parser).
+
+If this is not an option, you can, as a stop-gap measure, simply encode to
+ASCII-only JSON:
+
+ use JSON::XS;
+
+ print JSON::XS->new->ascii->encode ([chr 0x2028]);
+
+Note that this will enlarge the resulting JSON text quite a bit if you
+have many non-ASCII characters. You might be tempted to run some regexes
+to only escape U+2028 and U+2029, e.g.:
+
+ # DO NOT USE THIS!
+ my $json = JSON::XS->new->utf8->encode ([chr 0x2028]);
+ $json =~ s/\xe2\x80\xa8/\\u2028/g; # escape U+2028
+ $json =~ s/\xe2\x80\xa9/\\u2029/g; # escape U+2029
+ print $json;
+
+Note that I<this is a bad idea>: the above only works for U+2028 and
+U+2029 and thus only for fully ECMAscript-compliant parsers. Many existing
+javascript implementations, however, have issues with other characters as
+well - using C<eval> naively simply I<will> cause problems.
+
+Another problem is that some javascript implementations reserve
+some property names for their own purposes (which probably makes
+them non-ECMAscript-compliant). For example, Iceweasel reserves the
+C<__proto__> property name for its own purposes.
+
+If that is a problem, you could parse try to filter the resulting JSON
+output for these property strings, e.g.:
+
+ $json =~ s/"__proto__"\s*:/"__proto__renamed":/g;
+
+This works because C<__proto__> is not valid outside of strings, so every
+occurrence of C<"__proto__"\s*:> must be a string used as property name.
+
+If you know of other incompatibilities, please let me know.
+
+
+=head2 JSON and YAML
+
+You often hear that JSON is a subset of YAML. This is, however, a mass
+hysteria(*) and very far from the truth (as of the time of this writing),
+so let me state it clearly: I<in general, there is no way to configure
+JSON::XS to output a data structure as valid YAML> that works in all
+cases.
+
+If you really must use JSON::XS to generate YAML, you should use this
+algorithm (subject to change in future versions):
+
+ my $to_yaml = JSON::XS->new->utf8->space_after (1);
+ my $yaml = $to_yaml->encode ($ref) . "\n";
+
+This will I<usually> generate JSON texts that also parse as valid
+YAML. Please note that YAML has hardcoded limits on (simple) object key
+lengths that JSON doesn't have and also has different and incompatible
+unicode character escape syntax, so you should make sure that your hash
+keys are noticeably shorter than the 1024 "stream characters" YAML allows
+and that you do not have characters with codepoint values outside the
+Unicode BMP (basic multilingual page). YAML also does not allow C<\/>
+sequences in strings (which JSON::XS does not I<currently> generate, but
+other JSON generators might).
+
+There might be other incompatibilities that I am not aware of (or the YAML
+specification has been changed yet again - it does so quite often). In
+general you should not try to generate YAML with a JSON generator or vice
+versa, or try to parse JSON with a YAML parser or vice versa: chances are
+high that you will run into severe interoperability problems when you
+least expect it.
+
+=over 4
+
+=item (*)
+
+I have been pressured multiple times by Brian Ingerson (one of the
+authors of the YAML specification) to remove this paragraph, despite him
+acknowledging that the actual incompatibilities exist. As I was personally
+bitten by this "JSON is YAML" lie, I refused and said I will continue to
+educate people about these issues, so others do not run into the same
+problem again and again. After this, Brian called me a (quote)I<complete
+and worthless idiot>(unquote).
+
+In my opinion, instead of pressuring and insulting people who actually
+clarify issues with YAML and the wrong statements of some of its
+proponents, I would kindly suggest reading the JSON spec (which is not
+that difficult or long) and finally make YAML compatible to it, and
+educating users about the changes, instead of spreading lies about the
+real compatibility for many I<years> and trying to silence people who
+point out that it isn't true.
+
+Addendum/2009: the YAML 1.2 spec is still incompatible with JSON, even
+though the incompatibilities have been documented (and are known to Brian)
+for many years and the spec makes explicit claims that YAML is a superset
+of JSON. It would be so easy to fix, but apparently, bullying people and
+corrupting userdata is so much easier.
+
+=back
+
+
+=head2 SPEED
+
+It seems that JSON::XS is surprisingly fast, as shown in the following
+tables. They have been generated with the help of the C<eg/bench> program
+in the JSON::XS distribution, to make it easy to compare on your own
+system.
+
+First comes a comparison between various modules using
+a very short single-line JSON string (also available at
+L<http://dist.schmorp.de/misc/json/short.json>).
+
+ {"method": "handleMessage", "params": ["user1",
+ "we were just talking"], "id": null, "array":[1,11,234,-5,1e5,1e7,
+ 1, 0]}
+
+It shows the number of encodes/decodes per second (JSON::XS uses
+the functional interface, while JSON::XS/2 uses the OO interface
+with pretty-printing and hashkey sorting enabled, JSON::XS/3 enables
+shrink. JSON::DWIW/DS uses the deserialise function, while JSON::DWIW::FJ
+uses the from_json method). Higher is better:
+
+ module | encode | decode |
+ --------------|------------|------------|
+ JSON::DWIW/DS | 86302.551 | 102300.098 |
+ JSON::DWIW/FJ | 86302.551 | 75983.768 |
+ JSON::PP | 15827.562 | 6638.658 |
+ JSON::Syck | 63358.066 | 47662.545 |
+ JSON::XS | 511500.488 | 511500.488 |
+ JSON::XS/2 | 291271.111 | 388361.481 |
+ JSON::XS/3 | 361577.931 | 361577.931 |
+ Storable | 66788.280 | 265462.278 |
+ --------------+------------+------------+
+
+That is, JSON::XS is almost six times faster than JSON::DWIW on encoding,
+about five times faster on decoding, and over thirty to seventy times
+faster than JSON's pure perl implementation. It also compares favourably
+to Storable for small amounts of data.
+
+Using a longer test string (roughly 18KB, generated from Yahoo! Locals
+search API (L<http://dist.schmorp.de/misc/json/long.json>).
+
+ module | encode | decode |
+ --------------|------------|------------|
+ JSON::DWIW/DS | 1647.927 | 2673.916 |
+ JSON::DWIW/FJ | 1630.249 | 2596.128 |
+ JSON::PP | 400.640 | 62.311 |
+ JSON::Syck | 1481.040 | 1524.869 |
+ JSON::XS | 20661.596 | 9541.183 |
+ JSON::XS/2 | 10683.403 | 9416.938 |
+ JSON::XS/3 | 20661.596 | 9400.054 |
+ Storable | 19765.806 | 10000.725 |
+ --------------+------------+------------+
+
+Again, JSON::XS leads by far (except for Storable which non-surprisingly
+decodes a bit faster).
+
+On large strings containing lots of high Unicode characters, some modules
+(such as JSON::PC) seem to decode faster than JSON::XS, but the result
+will be broken due to missing (or wrong) Unicode handling. Others refuse
+to decode or encode properly, so it was impossible to prepare a fair
+comparison table for that case.
+
+
+=head1 SECURITY CONSIDERATIONS
+
+When you are using JSON in a protocol, talking to untrusted potentially
+hostile creatures requires relatively few measures.
+
+First of all, your JSON decoder should be secure, that is, should not have
+any buffer overflows. Obviously, this module should ensure that and I am
+trying hard on making that true, but you never know.
+
+Second, you need to avoid resource-starving attacks. That means you should
+limit the size of JSON texts you accept, or make sure then when your
+resources run out, that's just fine (e.g. by using a separate process that
+can crash safely). The size of a JSON text in octets or characters is
+usually a good indication of the size of the resources required to decode
+it into a Perl structure. While JSON::XS can check the size of the JSON
+text, it might be too late when you already have it in memory, so you
+might want to check the size before you accept the string.
+
+Third, JSON::XS recurses using the C stack when decoding objects and
+arrays. The C stack is a limited resource: for instance, on my amd64
+machine with 8MB of stack size I can decode around 180k nested arrays but
+only 14k nested JSON objects (due to perl itself recursing deeply on croak
+to free the temporary). If that is exceeded, the program crashes. To be
+conservative, the default nesting limit is set to 512. If your process
+has a smaller stack, you should adjust this setting accordingly with the
+C<max_depth> method.
+
+Something else could bomb you, too, that I forgot to think of. In that
+case, you get to keep the pieces. I am always open for hints, though...
+
+Also keep in mind that JSON::XS might leak contents of your Perl data
+structures in its error messages, so when you serialise sensitive
+information you might want to make sure that exceptions thrown by JSON::XS
+will not end up in front of untrusted eyes.
+
+If you are using JSON::XS to return packets to consumption
+by JavaScript scripts in a browser you should have a look at
+L<http://blog.archive.jpsykes.com/47/practical-csrf-and-json-security/> to
+see whether you are vulnerable to some common attack vectors (which really
+are browser design bugs, but it is still you who will have to deal with
+it, as major browser developers care only for features, not about getting
+security right).
+
+
+=head1 "OLD" VS. "NEW" JSON (RFC 4627 VS. RFC 7159)
+
+TL;DR: Due to security concerns, JSON::XS will not allow scalar data in
+JSON texts by default - you need to create your own JSON::XS object and
+enable C<allow_nonref>:
+
+
+ my $json = JSON::XS->new->allow_nonref;
+
+ $text = $json->encode ($data);
+ $data = $json->decode ($text);
+
+The long version: JSON being an important and supposedly stable format,
+the IETF standardised it as RFC 4627 in 2006. Unfortunately, the inventor
+of JSON, Dougles Crockford, unilaterally changed the definition of JSON in
+javascript. Rather than create a fork, the IETF decided to standardise the
+new syntax (apparently, so Iw as told, without finding it very amusing).
+
+The biggest difference between thed original JSON and the new JSON is that
+the new JSON supports scalars (anything other than arrays and objects) at
+the toplevel of a JSON text. While this is strictly backwards compatible
+to older versions, it breaks a number of protocols that relied on sending
+JSON back-to-back, and is a minor security concern.
+
+For example, imagine you have two banks communicating, and on one side,
+trhe JSON coder gets upgraded. Two messages, such as C<10> and C<1000>
+might then be confused to mean C<101000>, something that couldn't happen
+in the original JSON, because niether of these messages would be valid
+JSON.
+
+If one side accepts these messages, then an upgrade in the coder on either
+side could result in this becoming exploitable.
+
+This module has always allowed these messages as an optional extension, by
+default disabled. The security concerns are the reason why the default is
+still disabled, but future versions might/will likely upgrade to the newer
+RFC as default format, so you are advised to check your implementation
+and/or override the default with C<< ->allow_nonref (0) >> to ensure that
+future versions are safe.
+
+
+=head1 INTEROPERABILITY WITH OTHER MODULES
+
+C<JSON::XS> uses the L<Types::Serialiser> module to provide boolean
+constants. That means that the JSON true and false values will be
+comaptible to true and false values of other modules that do the same,
+such as L<JSON::PP> and L<CBOR::XS>.
+
+
+=head1 INTEROPERABILITY WITH OTHER JSON DECODERS
+
+As long as you only serialise data that can be directly expressed in JSON,
+C<JSON::XS> is incapable of generating invalid JSON output (modulo bugs,
+but C<JSON::XS> has found more bugs in the official JSON testsuite (1)
+than the official JSON testsuite has found in C<JSON::XS> (0)).
+
+When you have trouble decoding JSON generated by this module using other
+decoders, then it is very likely that you have an encoding mismatch or the
+other decoder is broken.
+
+When decoding, C<JSON::XS> is strict by default and will likely catch all
+errors. There are currently two settings that change this: C<relaxed>
+makes C<JSON::XS> accept (but not generate) some non-standard extensions,
+and C<allow_tags> will allow you to encode and decode Perl objects, at the
+cost of not outputting valid JSON anymore.
+
+=head2 TAGGED VALUE SYNTAX AND STANDARD JSON EN/DECODERS
+
+When you use C<allow_tags> to use the extended (and also nonstandard and
+invalid) JSON syntax for serialised objects, and you still want to decode
+the generated When you want to serialise objects, you can run a regex
+to replace the tagged syntax by standard JSON arrays (it only works for
+"normal" package names without comma, newlines or single colons). First,
+the readable Perl version:
+
+ # if your FREEZE methods return no values, you need this replace first:
+ $json =~ s/\( \s* (" (?: [^\\":,]+|\\.|::)* ") \s* \) \s* \[\s*\]/[$1]/gx;
+
+ # this works for non-empty constructor arg lists:
+ $json =~ s/\( \s* (" (?: [^\\":,]+|\\.|::)* ") \s* \) \s* \[/[$1,/gx;
+
+And here is a less readable version that is easy to adapt to other
+languages:
+
+ $json =~ s/\(\s*("([^\\":,]+|\\.|::)*")\s*\)\s*\[/[$1,/g;
+
+Here is an ECMAScript version (same regex):
+
+ json = json.replace (/\(\s*("([^\\":,]+|\\.|::)*")\s*\)\s*\[/g, "[$1,");
+
+Since this syntax converts to standard JSON arrays, it might be hard to
+distinguish serialised objects from normal arrays. You can prepend a
+"magic number" as first array element to reduce chances of a collision:
+
+ $json =~ s/\(\s*("([^\\":,]+|\\.|::)*")\s*\)\s*\[/["XU1peReLzT4ggEllLanBYq4G9VzliwKF",$1,/g;
+
+And after decoding the JSON text, you could walk the data
+structure looking for arrays with a first element of
+C<XU1peReLzT4ggEllLanBYq4G9VzliwKF>.
+
+The same approach can be used to create the tagged format with another
+encoder. First, you create an array with the magic string as first member,
+the classname as second, and constructor arguments last, encode it as part
+of your JSON structure, and then:
+
+ $json =~ s/\[\s*"XU1peReLzT4ggEllLanBYq4G9VzliwKF"\s*,\s*("([^\\":,]+|\\.|::)*")\s*,/($1)[/g;
+
+Again, this has some limitations - the magic string must not be encoded
+with character escapes, and the constructor arguments must be non-empty.
+
+
+=head1 RFC7159
+
+Since this module was written, Google has written a new JSON RFC, RFC 7159
+(and RFC7158). Unfortunately, this RFC breaks compatibility with both the
+original JSON specification on www.json.org and RFC4627.
+
+As far as I can see, you can get partial compatibility when parsing by
+using C<< ->allow_nonref >>. However, consider the security implications
+of doing so.
+
+I haven't decided yet when to break compatibility with RFC4627 by default
+(and potentially leave applications insecure) and change the default to
+follow RFC7159, but application authors are well advised to call C<<
+->allow_nonref(0) >> even if this is the current default, if they cannot
+handle non-reference values, in preparation for the day when the default
+will change.
+
+
+=head1 (I-)THREADS
+
+This module is I<not> guaranteed to be ithread (or MULTIPLICITY-) safe
+and there are no plans to change this. Note that perl's builtin so-called
+theeads/ithreads are officially deprecated and should not be used.
+
+
+=head1 THE PERILS OF SETLOCALE
+
+Sometimes people avoid the Perl locale support and directly call the
+system's setlocale function with C<LC_ALL>.
+
+This breaks both perl and modules such as JSON::XS, as stringification of
+numbers no longer works correctly (e.g. C<$x = 0.1; print "$x"+1> might
+print C<1>, and JSON::XS might output illegal JSON as JSON::XS relies on
+perl to stringify numbers).
+
+The solution is simple: don't call C<setlocale>, or use it for only those
+categories you need, such as C<LC_MESSAGES> or C<LC_CTYPE>.
+
+If you need C<LC_NUMERIC>, you should enable it only around the code that
+actually needs it (avoiding stringification of numbers), and restore it
+afterwards.
+
+
+=head1 BUGS
+
+While the goal of this module is to be correct, that unfortunately does
+not mean it's bug-free, only that I think its design is bug-free. If you
+keep reporting bugs they will be fixed swiftly, though.
+
+Please refrain from using rt.cpan.org or any other bug reporting
+service. I put the contact address into my modules for a reason.
+
+=cut
+
+BEGIN {
+ *true = \$Types::Serialiser::true;
+ *true = \&Types::Serialiser::true;
+ *false = \$Types::Serialiser::false;
+ *false = \&Types::Serialiser::false;
+ *is_bool = \&Types::Serialiser::is_bool;
+
+ *JSON::XS::Boolean:: = *Types::Serialiser::Boolean::;
+}
+
+XSLoader::load "JSON::XS", $VERSION;
+
+=head1 SEE ALSO
+
+The F<json_xs> command line utility for quick experiments.
+
+=head1 AUTHOR
+
+ Marc Lehmann <schmorp@schmorp.de>
+ http://home.schmorp.de/
+
+=cut
+
+1
+