Add json_document and json_view: node index, parser, and document

Add json_document and json_view, a read-only, zero-copy index of a
JSON text, as the first public slice of the zero-copy view (#5295).

A parse produces a flat array of 16-byte nodes in document order,
one per value and one per object key. Strings stay in the source
text; escaped strings are decoded into an arena. Integers are
converted while their digits are in the cache; floats keep only
their digit layout and are converted on read. Containers store the
size of their subtree, so a reader can step over one in constant
time. A document makes a handful of allocations, however many
values it has.

The parser accepts exactly what json::parse accepts, with every
combination of ignore_comments and ignore_trailing_commas, with and
without a trailing NUL, and under JSON_STRICT_NUL_HANDLING. It is
portable C++11 and does not depend on byte order.

basic_json_document adds parse, parse_copy, accept, read (reuses a
document's memory), root, is_discarded, source, owns_source,
node_count, memory_usage, and shrink_to_fit. basic_json_view adds
type, the is_* queries, operator bool, size, empty, materialize,
and source_offset. A parse error throws the same exception
basic_json::parse would throw for the same input, message and
position included.

detail::abi_config keeps JSON_STRICT_NUL_HANDLING and
JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON readable after json.hpp
undefines them, in the ABI namespace so they always match the
basic_json in use.

A NUL byte that ends a // comment is the end of the input, as in
parse() since #5696.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/api/basic_json/empty.md

@@ -64,6 +64,7 @@ itself is empty which is `#!cpp false` in the case of a string.
 
 - [size](size.md) returns the number of elements
 - [clear](clear.md) clears the content and resets the value to the default value
+- [basic_json_view::empty](../basic_json_view/empty.md) - the same check on a zero-copy view
 
 ## Version history
 

docs/mkdocs/docs/api/basic_json/is_array.md

@@ -40,6 +40,7 @@ Constant.
 - [is_structured](is_structured.md) checks whether the JSON value is structured (array or object)
 - [type](type.md) returns the type of the JSON value
 - [array_t](array_t.md) the type used to store JSON arrays
+- [basic_json_view::is_array](../basic_json_view/is_array.md) - the same check on a zero-copy view
 
 ## Version history
 

docs/mkdocs/docs/api/basic_json/is_binary.md

@@ -39,6 +39,7 @@ Constant.
 - [is_primitive](is_primitive.md) checks whether the JSON value is primitive
 - [binary_t](binary_t.md) the type used to store binary values
 - [get_binary](get_binary.md) returns a reference to the stored binary value
+- [basic_json_view::is_binary](../basic_json_view/is_binary.md) - the same check on a zero-copy view
 
 ## Version history
 

docs/mkdocs/docs/api/basic_json/is_boolean.md

@@ -38,6 +38,7 @@ Constant.
 
 - [boolean_t](boolean_t.md) the type used to store JSON booleans
 - [is_primitive](is_primitive.md) checks whether the JSON value is primitive
+- [basic_json_view::is_boolean](../basic_json_view/is_boolean.md) - the same check on a zero-copy view
 
 ## Version history
 

docs/mkdocs/docs/api/basic_json/is_discarded.md

@@ -85,6 +85,11 @@ with `allow_exceptions` set to `#!cpp false`: a parse error then yields a discar
     --8<-- "examples/is_discarded__parse.output"
     ```
 
+## See also
+
+- [basic_json_view::is_discarded](../basic_json_view/is_discarded.md) - the corresponding check on a zero-copy view,
+  which is `#!cpp true` if the view refers to no value
+
 ## Version history
 
 - Added in version 1.0.0.

docs/mkdocs/docs/api/basic_json/is_null.md

@@ -40,6 +40,7 @@ Constant.
 - [is_object](is_object.md) checks whether the JSON value is an object
 - [type](type.md) returns the type of the JSON value
 - [value_t](value_t.md) the enumeration of JSON types
+- [basic_json_view::is_null](../basic_json_view/is_null.md) - the same check on a zero-copy view
 
 ## Version history
 

docs/mkdocs/docs/api/basic_json/is_number.md

@@ -49,6 +49,7 @@ constexpr bool is_number() const noexcept
 - [is_number_integer()](is_number_integer.md) check if the value is an integer or unsigned integer number
 - [is_number_unsigned()](is_number_unsigned.md) check if the value is an unsigned integer number
 - [is_number_float()](is_number_float.md) check if the value is a floating-point number
+- [basic_json_view::is_number](../basic_json_view/is_number.md) - the same check on a zero-copy view
 
 ## Version history
 

docs/mkdocs/docs/api/basic_json/is_number_float.md

@@ -40,6 +40,7 @@ Constant.
 - [is_number()](is_number.md) check if the value is a number
 - [is_number_integer()](is_number_integer.md) check if the value is an integer or unsigned integer number
 - [is_number_unsigned()](is_number_unsigned.md) check if the value is an unsigned integer number
+- [basic_json_view::is_number_float](../basic_json_view/is_number_float.md) - the same check on a zero-copy view
 
 ## Version history
 

docs/mkdocs/docs/api/basic_json/is_number_integer.md

@@ -40,6 +40,7 @@ Constant.
 - [is_number()](is_number.md) check if the value is a number
 - [is_number_unsigned()](is_number_unsigned.md) check if the value is an unsigned integer number
 - [is_number_float()](is_number_float.md) check if the value is a floating-point number
+- [basic_json_view::is_number_integer](../basic_json_view/is_number_integer.md) - the same check on a zero-copy view
 
 ## Version history
 

docs/mkdocs/docs/api/basic_json/is_number_unsigned.md

@@ -40,6 +40,7 @@ Constant.
 - [is_number()](is_number.md) check if the value is a number
 - [is_number_integer()](is_number_integer.md) check if the value is an integer or unsigned integer number
 - [is_number_float()](is_number_float.md) check if the value is a floating-point number
+- [basic_json_view::is_number_unsigned](../basic_json_view/is_number_unsigned.md) - the same check on a zero-copy view
 
 ## Version history
 

docs/mkdocs/docs/api/basic_json/is_object.md

@@ -40,6 +40,7 @@ Constant.
 - [is_structured](is_structured.md) checks whether the JSON value is structured (array or object)
 - [type](type.md) returns the type of the JSON value
 - [object_t](object_t.md) the type used to store JSON objects
+- [basic_json_view::is_object](../basic_json_view/is_object.md) - the same check on a zero-copy view
 
 ## Version history
 

docs/mkdocs/docs/api/basic_json/is_primitive.md

@@ -62,6 +62,7 @@ This library extends primitive types to binary types, because binary types are r
 - [is_boolean()](is_boolean.md) returns whether the JSON value is a boolean
 - [is_number()](is_number.md) returns whether the JSON value is a number
 - [is_binary()](is_binary.md) returns whether the JSON value is a binary array
+- [basic_json_view::is_primitive](../basic_json_view/is_primitive.md) - the same check on a zero-copy view
 
 ## Version history
 

docs/mkdocs/docs/api/basic_json/is_string.md

@@ -39,6 +39,7 @@ Constant.
 - [is_primitive](is_primitive.md) checks whether the JSON value is primitive
 - [type](type.md) returns the type of the JSON value
 - [string_t](string_t.md) the type used to store JSON strings
+- [basic_json_view::is_string](../basic_json_view/is_string.md) - the same check on a zero-copy view
 
 ## Version history
 

docs/mkdocs/docs/api/basic_json/is_structured.md

@@ -57,6 +57,7 @@ Note that though strings are containers in C++, they are treated as primitive va
 - [is_primitive()](is_primitive.md) returns whether JSON value is primitive
 - [is_array()](is_array.md) returns whether the value is an array
 - [is_object()](is_object.md) returns whether the value is an object
+- [basic_json_view::is_structured](../basic_json_view/is_structured.md) - the same check on a zero-copy view
 
 ## Version history
 

docs/mkdocs/docs/api/basic_json/size.md

@@ -55,6 +55,7 @@ JSON value which is `1` in the case of a string.
 
 - [empty](empty.md) checks whether the JSON value has no elements
 - [max_size](max_size.md) returns the maximum possible number of elements
+- [basic_json_view::size](../basic_json_view/size.md) - the same function on a zero-copy view
 
 ## Version history
 

docs/mkdocs/docs/api/basic_json/type.md

@@ -52,6 +52,7 @@ Constant.
 - [operator value_t](operator_value_t.md) implicit conversion operator equivalent to this named member function
 - [type_name](type_name.md) returns the type as a string, for use in error messages
 - [value_t](value_t.md) the enumeration of JSON types
+- [basic_json_view::type](../basic_json_view/type.md) - the same function on a zero-copy view
 
 ## Version history