aosp12/external/libcppbor/README.md

LibCppBor: A Modern C++ CBOR Parser and Generator

LibCppBor provides a natural and easy-to-use syntax for constructing and parsing CBOR messages. It does not (yet) support all features of CBOR, nor (yet) support validation against CDDL schemata, though both are planned. CBOR features that aren't supported include:

• Indefinite length values
• Semantic tagging
• Floating point

LibCppBor requires C++-17.

CBOR representation

LibCppBor represents CBOR data items as instances of the Item class or, more precisely, as instances of subclasses of Item, since Item is a pure interface. The subclasses of Item correspond almost one-to-one with CBOR major types, and are named to match the CDDL names to which they correspond. They are:

• Uint corresponds to major type 0, and can hold unsigned integers up through (2^64 - 1).
• Nint corresponds to major type 1. It can only hold values from -1 to -(2^63 - 1), since it's internal representation is an int64_t. This can be fixed, but it seems unlikely that applications will need the omitted range from -(2^63) to (2^64 - 1), since it's inconvenient to represent them in many programming languages.
• Int is an abstract base of Uint and Nint that facilitates working with all signed integers representable with int64_t.
• Bstr corresponds to major type 2, a byte string.
• Tstr corresponds to major type 3, a text string.
• Array corresponds to major type 4, an Array. It holds a variable-length array of Items.
• Map corresponds to major type 5, a Map. It holds a variable-length array of pairs of Items.
• Simple corresponds to major type 7. It's an abstract class since items require more specific type.
• Bool is the only currently-implemented subclass of Simple.

Note that major type 6, semantic tag, is not yet implemented.

In practice, users of LibCppBor will rarely use most of these classes when generating CBOR encodings. This is because LibCppBor provides straightforward conversions from the obvious normal C++ types. Specifically, the following conversions are provided in appropriate contexts:

• Signed and unsigned integers convert to Uint or Nint, as appropriate.
• std::string, std::string_view, const char* and std::pair<char iterator, char iterator> convert to Tstr.
• std::vector<uint8_t>, std::pair<uint8_t iterator, uint8_t iterator> and std::pair<uint8_t*, size_t> convert to Bstr.
• bool converts to Bool.

CBOR generation

Complete tree generation

The set of encode methods in Item provide the interface for producing encoded CBOR. The basic process for "complete tree" generation (as opposed to "incremental" generation, which is discussed below) is to construct an Item which models the data to be encoded, and then call one of the encode methods, whichever is convenient for the encoding destination. A trivial example:

cppbor::Uint val(0);
std::vector<uint8_t> encoding = val.encode();

It's relatively rare that single values are encoded as above.  More often, the
"root" data item will be an `Array` or `Map` which contains a more complex structure.For example
:

using cppbor::Array;

std::vector<uint8_t> vec =  // ...
    Map val("key1", Array(Map("key_a", 99 "key_b", vec), "foo"), "key2", true);
std::vector<uint8_t> encoding = val.encode();

This creates a map with two entries, with Tstr keys "Outer1" and "Outer2", respectively. The "Outer1" entry has as its value an Array containing a Map and a Tstr. The "Outer2" entry has a Bool value.

This example demonstrates how automatic conversion of C++ types to LibCppBor Item subclass instances is done. Where the caller provides a C++ or C string, a Tstr entry is added. Where the caller provides an integer literal or variable, a Uint or Nint is added, depending on whether the value is positive or negative.

As an alternative, a more fluent-style API is provided for building up structures. For example:

using cppbor::Map;
using cppbor::Array;

std::vector<uint8_t> vec =  // ...
    Map val();
val.add("key1", Array().add(Map().add("key_a", 99).add("key_b", vec)).add("foo")).add("key2", true);
std::vector<uint8_t> encoding = val.encode();

An advantage of this interface over the constructor -
based creation approach above is that it need not be done all at once.
The `add` methods return a reference to the object added to to allow calls to be chained,
but chaining is not necessary; calls can be made
sequentially, as the data to add is available.

encode methods

There are several variations of Item::encode, all of which accomplish the same task but output the encoded data in different ways, and with somewhat different performance characteristics. The provided options are:

• bool encode(uint8\_t** pos, const uint8\_t* end) encodes into the buffer referenced by the range [*pos, end). *pos is moved. If the encoding runs out of buffer space before finishing, the method returns false. This is the most efficient way to encode, into an already-allocated buffer.
• void encode(EncodeCallback encodeCallback) calls encodeCallback for each encoded byte. It's the responsibility of the implementor of the callback to behave safely in the event that the output buffer (if applicable) is exhausted. This is less efficient than the prior method because it imposes an additional function call for each byte.
• template </*...*/> void encode(OutputIterator i) encodes into the provided iterator. SFINAE ensures that the template doesn't match for non-iterators. The implementation actually uses the callback-based method, plus has whatever overhead the iterator adds.
• std::vector<uint8_t> encode() creates a new std::vector, reserves sufficient capacity to hold the encoding, and inserts the encoded bytes with a std::pushback_iterator and the previous method.
• std::string toString() does the same as the previous method, but returns a string instead of a vector.

Incremental generation

Incremental generation requires deeper understanding of CBOR, because the library can't do as much to ensure that the output is valid. The basic tool for intcremental generation is the encodeHeader function. There are two variations, one which writes into a buffer, and one which uses a callback. Both simply write out the bytes of a header. To construct the same map as in the above examples, incrementally, one might write:

using namespace cppbor;  // For example brevity

std::vector encoding;
auto iter = std::back_inserter(result);
encodeHeader(MAP, 2 /* # of map entries */, iter);
std::string s = "key1";
encodeHeader(TSTR, s.size(), iter);
std::copy(s.begin(), s.end(), iter);
encodeHeader(ARRAY, 2 /* # of array entries */, iter);
Map().add("key_a", 99).add("key_b", vec).encode(iter)
s = "foo";
encodeHeader(TSTR, foo.size(), iter);
std::copy(s.begin(), s.end(), iter);
s = "key2";
encodeHeader(TSTR, foo.size(), iter);
std::copy(s.begin(), s.end(), iter);
encodeHeader(SIMPLE, TRUE, iter);

As the above example demonstrates, the styles can be mixed -- Note the creation and encoding of the inner Map using the fluent style.

Parsing

LibCppBor also supports parsing of encoded CBOR data, with the same feature set as encoding. There are two basic approaches to parsing, "full" and "stream"

Full parsing

Full parsing means completely parsing a (possibly-compound) data item from a byte buffer. The parse functions that do not take a ParseClient pointer do this. They return a ParseResult which is a tuple of three values:

• std::unique_ptr that points to the parsed item, or is nullptr if there was a parse error.
• const uint8_t* that points to the byte after the end of the decoded item, or to the first unparseable byte in the event of an error.
• std::string that is empty on success or contains an error message if a parse error occurred.

Assuming a successful parse, you can then use Item::type() to discover the type of the parsed item (e.g. MAP), and then use the appropriate Item::as*() method (e.g. Item::asMap()) to get a pointer to an interface which allows you to retrieve specific values.

Stream parsing

Stream parsing is more complex, but more flexible. To use StreamParsing, you must create your own subclass of ParseClient and call one of the parse functions that accepts it. See the ParseClient methods docstrings for details.

One unusual feature of stream parsing is that the ParseClient callback methods not only provide the parsed Item, but also pointers to the portion of the buffer that encode that Item. This is useful if, for example, you want to find an element inside of a structure, and then copy the encoding of that sub-structure, without bothering to parse the rest.

The full parser is implemented with the stream parser.

Disclaimer

This is not an officially supported Google product