Smaller simpler (?) presentation of binary syntax
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@ -21,34 +21,72 @@ syntax](preserves-text.html) also exists.
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## Machine-Oriented Binary Syntax
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A `Repr` is a binary-syntax encoding, or representation, of a `Value`.
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For a value `v`, we write `«v»` for the `Repr` of v.
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### Type and Length representation.
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Each `Repr` starts with a tag byte, describing the kind of information
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represented.
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However, inspired by [argdata][], a `Repr` does *not* describe its own
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length. Instead, the expected length of the `Repr` is always available
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represented. The expected length of the `Repr` is always available
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from the surrounding context: either from a containing encoded value, or
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from the overall container of the data, which could be a file, an HTTP
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message, a UDP packet, etc.
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As a consequence, `Repr`s for `Compound` values store the lengths of
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their contained values. Each contained `Value` is represented as a
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length in bytes followed by its own `Repr`. Implementations use each
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stored length to decide when to stop reading the following `Repr`.
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### Atomic Values.
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**Booleans.** The "false" boolean's `Repr` is just tag `0xA0`; "true" is
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`0xA1`.
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**Floats and Doubles.** Both `Float` and `Double` values are represented
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as tag `0xA2` followed by big-endian 4- or 8-byte IEEE 754 binary
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representations of the values, respectively.
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**SignedIntegers.** A `SignedInteger` encodes as tag `0xA3` followed by
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a big-endian two's-complement binary representation of the value, taking
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at least as many whole bytes as needed to unambiguously identify the
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value and its sign. Zero may be represented as the tag alone, with no
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following bytes. The most-significant bit in the first byte after the
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tag is the sign bit.[^zero-intbytes] The shortest possible encoding
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*SHOULD* be used.[^overlong-signedinteger]
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[^zero-intbytes]: The value 0 needs zero bytes to identify the value,
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so `intbytes(0)` can be the empty byte string. Non-zero values need
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at least one byte.
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[^overlong-signedinteger]: **Implementation note.** The spec permits
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overlong `SignedInteger` encodings to allow e.g. construction of
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`Repr`s by filling in partially-completed templates, which can be
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useful in resource-constrained situations.
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**Strings.** A `String` encodes as tag `0xA4` followed by the UTF-8
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encoding of the string, with an additional trailing `NUL` (0) byte. The
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`NUL` byte *MUST NOT* be treated as part of the `String`: it exists to
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permit zero-copy C interoperability.[^zero-copy-c-string-interop]
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[^zero-copy-c-string-interop]: Some care must still be taken when
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passing `String` `Repr`s directly to a C-style ABI, since `String`s
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may contain the zero Unicode code point, which C library routines
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will usually misinterpret as an end-of-string marker.
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**ByteStrings.** A `ByteString` encodes as tag `0xA5` followed by the
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bytes themselves.
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**Symbols.** A `Symbol` encodes as tag `0xA6` followed by the UTF-8
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encoding of the symbol's code points.
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### Compound Values.
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`Repr`s for `Compound` values store the lengths of their contained
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values. Each contained `Value` is converted to a `Repr` and stored as
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the length of the `Repr` in bytes followed by the `Repr` itself.
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Implementations use each stored length to decide when to stop reading
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the associated `Repr`. Similarly, no sentinel marks the end of a
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sequence of length-prefixed `Repr`s. Implementations use the length of
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the containing `Repr`, known from the surrounding context, to decide
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when to stop expecting more contained `Repr`s.
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<a id="varint"></a> Each length is stored as an [argdata][]-compatible
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big-endian base 128 *varint*.[^see-also-leb128] Each byte of a varint
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stores seven bits of the length. All bytes have a clear upper bit,
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except the final byte, which has the upper bit set. We write
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`len(m)` for the varint-encoding of a non-negative integer `m`,
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defined recursively as follows:
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len(m) = e(m, 128)
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where e(v, d) = [v + d] if v < 128
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e(v / 128, 0) ++ [(v % 128) + d] if v ≥ 128
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except the final byte, which has the upper bit set.
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[^see-also-leb128]: Argdata's length representation is very close to
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[Variable-length quantity (VLQ)][VLQ] encoding, differing only in
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@ -56,10 +94,8 @@ defined recursively as follows:
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big-endian, unlike [LEB128][] encoding ([as used by
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Google][google-varint] in protobufs).
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We write `len(|r|)` for the varint-encoding of the length of `Repr` `r`.
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There is no requirement that a varint-encoded `m` in a `Repr` be the
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unique shortest encoding for that `m`.[^overlong-varint] However,
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There is no requirement that a varint-encoded length be the unique
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shortest encoding for the length.[^overlong-varint] However,
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implementations *SHOULD* use the shortest encoding whereever possible
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when writing, and *MAY* reject encodings with more than eight leading
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`0` bytes when reading encoded values.
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@ -69,21 +105,24 @@ when writing, and *MAY* reject encodings with more than eight leading
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anything other than a very low-level language, it is likely to be able to use
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[IOList](./conventions.html#iolists)-style data structures to avoid unnecessary copying.
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### Records, Sequences, Sets and Dictionaries.
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**Records.** A `Record` is encoded as tag `0xA7` followed by the
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length-prefixed encodings of its label and fields.
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«<L F_1...F_m>» = [0xA7] ++ seq(«L», «F_1», ..., «F_m»)
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«[X_1...X_m]» = [0xA8] ++ seq(«X_1», ..., «X_m»)
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«#{E_1...E_m}» = [0xA9] ++ seq(«E_1», ..., «E_m»)
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«{K_1:V_1...K_m:V_m}» = [0xAA] ++ seq(«K_1», «V_1», ..., «K_m», «V_m»)
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**Sequences.** A `Sequence` is encoded as tag `0xA8` followed by the
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length-prefixed encodings of its members.
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seq(R_1, ..., R_m) = len(|R_1|) ++ R_1 ++...++ len(|R_m|) ++ R_m
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**Sets.** A `Set` is encoded like a `Sequence`, but with tag `0xA9`, and
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in some arbitrary order.
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There is *no* ordering requirement on the `E_i` elements or
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`K_i`/`V_i` pairs.[^no-sorting-rationale] They may appear in any
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order. However, the `E_i` and `K_i` *MUST* be pairwise distinct. In
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addition, implementations *SHOULD* default to writing set elements and
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dictionary key/value pairs in order sorted lexicographically by their
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`Repr`s[^not-sorted-semantically], and *MAY* offer the option of
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**Dictionaries.** A `Dictionary` encodes as tag `0xAA` followed by the
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length-prefixed keys and values, in an alternating key/value sequence.
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There is *no* ordering requirement on the elements of sets or the
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key/value pairs of dictionaries.[^no-sorting-rationale] However,
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elements of sets and keys in dictionaries *MUST* be pairwise distinct.
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In addition, implementations *SHOULD* default to writing set elements
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and dictionary key/value pairs in order sorted lexicographically by
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their `Repr`s[^not-sorted-semantically], and *MAY* offer the option of
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serializing in some other implementation-defined order.
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[^no-sorting-rationale]: In the BitTorrent encoding format,
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@ -109,93 +148,33 @@ serializing in some other implementation-defined order.
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but encoding and then sorting byte strings is much more likely to
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be within easy reach.
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No sentinel marks the end of a sequence of length-prefixed `Repr`s.
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During decoding, use the length of the containing `Repr` to decide when
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to stop expecting more contained `Repr`s.
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### Embedded Values.
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### SignedIntegers.
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«x» when x ∈ SignedInteger = [0xA3] ++ intbytes(x)
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The function `intbytes(x)` gives a big-endian two's-complement binary
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representation of `x`, taking at least as many whole bytes as needed to
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unambiguously identify the value and its sign; `intbytes(0)` may be the
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empty byte sequence.[^zero-intbytes] The most-significant bit in the
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first byte in `intbytes(x)` is the sign bit. While every `SignedInteger`
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*SHOULD* be represented with its shortest possible encoding (which will
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often include a necessary leading `0xFF` or `0x00`), redundant leading
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`0xFF` or `0x00` bytes *MAY* be used.[^overlong-signedinteger]
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[^zero-intbytes]: The value 0 needs zero bytes to identify the value,
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so `intbytes(0)` can be the empty byte string. Non-zero values need
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at least one byte.
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[^overlong-signedinteger]: **Implementation note.** The spec permits
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overlong `SignedInteger` encodings to allow e.g. construction of
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`Repr`s by filling in partially-completed templates, which can be
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useful in resource-constrained situations.
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### Strings, ByteStrings and Symbols.
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«S» = [0xA4] ++ utf8(S) ++ [0] if S ∈ String
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[0xA5] ++ S if S ∈ ByteString
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[0xA6] ++ utf8(S) if S ∈ Symbol
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For `String` and `Symbol`, the data following the tag is a UTF-8
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encoding of the `Value`'s code points, while for `ByteString` it is the
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raw data contained within the `Value` unmodified.
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Each `String` has a trailing zero byte appended. This extra byte *MUST
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NOT* be treated as part of the `Value`: it exists to permit zero-copy C
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interoperability.[^zero-copy-c-string-interop]
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[^zero-copy-c-string-interop]: Some care must still be taken when
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passing `String` `Repr`s directly to a C-style ABI, since `String`s
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may contain the zero Unicode code point, which C library routines
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will usually misinterpret as an end-of-string marker.
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### Booleans.
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«#f» = [0xA0]
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«#t» = [0xA1]
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### Floats and Doubles.
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«F» when F ∈ Float = [0xA2] ++ binary32(F)
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«D» when D ∈ Double = [0xA2] ++ binary64(D)
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The functions `binary32(F)` and `binary64(D)` yield big-endian 4- and
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8-byte IEEE 754 binary representations of `F` and `D`, respectively.
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### Embeddeds.
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The `Repr` of an `Embedded` is the `Repr` of a `Value` chosen to
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represent the denoted object, prefixed with `[0xAB]`.
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«#!V» = [0xAB] ++ «V»
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Embedded values are encoded as tag `0xAB` followed by the encoding of
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some `Value` chosen to represent the denoted embedded object.
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### Annotations.
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To annotate a `Repr` `r` with some sequence of `Value`s `[v_1, ...,
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v_m]`, surround `r` as follows:
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The encoding of a sequence of annotations for a `Repr` uses tag `0xBF`,
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followed by the length-prefixed `Repr`, followed by the length-prefixed
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encoded annotations, in order. The `Repr` *MUST NOT* already have
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annotations (must not begin with `0xBF`), and there *MUST* be at least
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one `Value` in the sequence following the `Repr`.
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[0xBF] ++ len(|r|) ++ r ++ len(|«v_1»|) ++ «v_1» ++...++ len(|«v_m»|) ++ «v_m»
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## Examples (normative)
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The `Repr` `r` *MUST NOT* already have annotations; that is, it must not
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begin with `0xBF`. The sequence `[v_1, ..., v_m]` *MUST* contain at
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least one `Value`.
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## Examples
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We write `«v»` for the `Repr` of some `Value` `v`, and `varint(|«v»|)` for
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the varint-encoded length of the `Repr` of `v`.
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### Varints (length representations).
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The following table illustrates varint-encoding.
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| Number, `m` | `m` in binary, grouped into 7-bit chunks | `len(m)` bytes |
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|-------------|-------------------------------------------|-----------------|
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| 15 | `0001111` | 143 |
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| 300 | `0000010 0101100` | 2 172 |
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| 1000000000 | `0000011 1011100 1101011 0010100 0000000` | 3 92 107 20 128 |
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| Number, `m` | `m` in binary, grouped into 7-bit chunks | `varint(m)` bytes |
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|-------------|-------------------------------------------|-------------------|
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| 15 | `0001111` | 143 |
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| 300 | `0000010 0101100` | 2 172 |
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| 1000000000 | `0000011 1011100 1101011 0010100 0000000` | 3 92 107 20 128 |
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### Atoms.
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@ -288,7 +267,9 @@ The `Repr` corresponding to textual syntax `@a@b[]`, i.e. an empty sequence anno
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symbols, `a` and `b`, is
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«@a @b []»
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= [0xBF] ++ len(|«[]»|) ++ «[]» ++ len(|«a»|) ++ «a» ++ len(|«b»|) ++ «b»
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= [0xBF] ++ varint(|«[]»|) ++ «[]»
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++ varint(|«a»|) ++ «a»
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++ varint(|«b»|) ++ «b»
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= [0xBF, 0x81, 0xA8, 0x82, 0xA6, 0x61, 0x82, 0xA6, 0x62]
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## Security Considerations
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@ -346,7 +327,7 @@ undetermined number of `Value`s across, say, a TCP/IP connection:
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- If the binary syntax is to be used for the connection, start the
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connection with byte `0xA8` (sequence). After the initial byte, send
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each value `v` as `len(|«v»|) ++ «v»`. A side effect of this approach
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each value `v` as `varint(|«v»|) ++ «v»`. A side effect of this approach
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is that the entire stream, when complete, is a valid `Sequence`
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`Repr`.
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