FileDecoder.cpp

using namespace codecs;

/* Define a set of codecs for encoding/decoding VoodooPad's encrypted document header */
auto versionCodec = (
    // `|' associates an arbitrary string context with a codec; this is used
    // for error reporting, providing additional information on parse errors.
    ("record_compat_version"     | codecs::uint8  ) &
    ("record_feature_version"    | codecs::uint8  )
).as<VDERecordVersion>();

// `as` takes a set of Codecs over types <T...> and returns a new Codec<U> that
// 1) Extracts T... from U's fields when encoding, and
// 2) Applies T... to U's constructor to produce a well-typed object when decoding.

auto sectionCodec = (
    // The base uintXX codecs assume big endian serialization; the `L'
    // variants assume little endian serialization. */
    ("section_offset"            | codecs::uint64L  ) &
    ("section_length"            | codecs::uint64L  )
).as<VDESectionRecord>();

const auto fileMagic = ByteVector::Bytes("VPVDE", 5, false);
auto headerCodec = (
    // The `constant' codec always encodes the given constant, producing
    // no value, and on decoding, consumes sizeof(fileMagic) bytes
    // and produces a parse error if the consumed value does not match
    // the expected constant.
    //
    // The '>>' operator drops the produced/consumed constant from the
    // set of input/output of the combined codec; since it's a constant,
    // we don't need to save the value anywhere after we've parsed it,
    // and we don't need to supply the value when encoding.
    ("magic"                     | constant(fileMagic)    ) >>
    ("file_version"              | versionCodec           ) &
    ("vde_encrypted_sect"        | sectionCodec           ) &
    ("vde_session_sect"          | sectionCodec           )
).as<VDEFileHeader>();

/* XCTAssertRoundTrip() is a test assertion that encodes the given value,
 * decodes it, and asserts that the initial value and final decoded value
 * are identical */
auto header = VDEFileHeader(VDERecordVersion(1, 2), VDESectionRecord(10, 20), VDESectionRecord(30, 40));
XCTAssertRoundTrip(headerCodec, header);