328 lines
14 KiB
Markdown
328 lines
14 KiB
Markdown
---
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no_site_title: true
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title: "Preserves: Binary Syntax"
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---
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Tony Garnock-Jones <tonyg@leastfixedpoint.com>
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{{ site.version_date }}. Version {{ site.version }}.
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[LEB128]: https://en.wikipedia.org/wiki/LEB128
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[argdata]: https://github.com/NuxiNL/argdata
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[canonical]: canonical-binary.html
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[google-varint]: https://developers.google.com/protocol-buffers/docs/encoding#varints
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[vlq]: https://en.wikipedia.org/wiki/Variable-length_quantity
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*Preserves* is a data model, with associated serialization formats. This
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document defines one of those formats: a binary syntax for `Value`s from
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the [Preserves data model](preserves.html) that is easy for computer
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software to read and write. An [equivalent human-readable text
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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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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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<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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[^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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the flipped interpretation of the high bit of each byte. It is
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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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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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[^overlong-varint]: **Implementation note.** The spec permits overlong length encodings to
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reduce wasted activity in resource-constrained situations. If an implementation is in
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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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«<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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seq(R_1, ..., R_m) = len(|R_1|) ++ R_1 ++...++ len(|R_m|) ++ R_m
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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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serializing in some other implementation-defined order.
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[^no-sorting-rationale]: In the BitTorrent encoding format,
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[bencoding](http://www.bittorrent.org/beps/bep_0003.html#bencoding),
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dictionary key/value pairs must be sorted by key. This is a
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necessary step for ensuring serialization of `Value`s is
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canonical. We do not require that key/value pairs (or set
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elements) be in sorted order for serialized `Value`s; however, a
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[canonical form][canonical] for `Repr`s does exist where a sorted
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ordering is required.
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[^not-sorted-semantically]: It's important to note that the sort
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ordering for writing out set elements and dictionary key/value
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pairs is *not* the same as the sort ordering implied by the
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semantic ordering of those elements or keys. For example, the
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`Repr` of a negative number very far from zero will start with a
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byte that is *greater* than the byte which starts the `Repr` of
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zero, making it sort lexicographically later by `Repr`, despite
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being semantically *less than* zero.
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**Rationale**. This is for ease-of-implementation reasons: not all
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languages can easily represent sorted sets or sorted dictionaries,
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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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### 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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### 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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[0xBF] ++ len(|r|) ++ r ++ len(|«v_1»|) ++ «v_1» ++...++ len(|«v_m»|) ++ «v_m»
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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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### 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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### SignedIntegers.
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«87112285931760246646623899502532662132736»
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= A3 01 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00
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00 00
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«-257» = A3 FE FF «-3» = A3 FD «128» = A3 00 80
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«-256» = A3 FF 00 «-2» = A3 FE «255» = A3 00 FF
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«-255» = A3 FF 01 «-1» = A3 FF «256» = A3 01 00
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«-254» = A3 FF 02 «0» = A3 «32767» = A3 7F FF
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«-129» = A3 FF 7F «1» = A3 01 «32768» = A3 00 80 00
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«-128» = A3 80 «12» = A3 0C «65535» = A3 00 FF FF
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«-127» = A3 81 «13» = A3 0D «65536» = A3 01 00 00
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«-4» = A3 FC «127» = A3 7F «131072» = A3 02 00 00
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### Annotations.
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The `Repr` corresponding to textual syntax `@a@b[]`, i.e. an empty sequence annotated with two
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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, 0x81, 0xA8, 0x82, 0xA6, 0x61, 0x82, 0xA6, 0x62]
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## Security Considerations
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**Annotations.** In modes where a `Value` is being read while
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annotations are skipped, an endless sequence of annotations may give an
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illusion of progress.
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**Overlong varints.** The binary format allows (but discourages)
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overlong [varint](#varint)s. Because every `Repr` has a bound on its
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length from its surrounding context, this is not a denial-of-service
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vector *per se*; however, implementations may wish to consider optional
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restrictions on the number of redundant leading `0` bytes accepted when
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reading a varint.
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**Canonical form for cryptographic hashing and signing.** No canonical
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textual encoding of a `Value` is specified. A
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[canonical form][canonical] exists for binary encoded `Value`s, and
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implementations *SHOULD* produce canonical binary encodings by
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default; however, an implementation *MAY* permit two serializations of
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the same `Value` to yield different binary `Repr`s.
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## Acknowledgements
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The exclusion of lengths from `Repr`s, placing lengths instead ahead of
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contained values in sequences, is inspired by [argdata][], as is the
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inclusion of a `NUL` byte in `String` `Repr`s for C interoperability.
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## Appendix. Autodetection of textual or binary syntax
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Every tag byte in a binary Preserves `Repr` falls within the range
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[`0x80`, `0xBF`]. These bytes, interpreted as UTF-8, are *continuation
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bytes*, and will never occur as the first byte of a UTF-8 encoded code
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point. This means no binary-encoded `Repr` can be misinterpreted as
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valid UTF-8.
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Conversely, a UTF-8 `Document` must start with a valid codepoint,
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meaning in particular that it must not start with a byte in the range
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[`0x80`, `0xBF`]. This means that no UTF-8 encoded textual-syntax
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Preserves `Document` can be misinterpreted as a binary-syntax `Repr`.
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Examination of the top two bits of the first byte of an encoded `Value`
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gives its syntax: if the top two bits are `10`, it should be interpreted
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as a binary-syntax `Repr`; otherwise, it should be interpreted as text.
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**Streaming.** Autodetection is still possible when streaming an
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undetermined number of `Value`s across, say, a TCP/IP connection:
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- If the text syntax is to be used for the connection, simply start
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writing each `Document` one after the other. Documents for `Atom`s
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are in general ambiguous if not separated from their neighbours by
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whitespace; whitespace *SHOULD* be used to separate adjacent
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documents. Specifically, whitespace separating adjacent documents
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*SHOULD* be ASCII newline (10).
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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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is that the entire stream, when complete, is a valid `Sequence`
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`Repr`.
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## Appendix. Table of tag values
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(8x) RESERVED 80-8F
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(9x) RESERVED 90-9F
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A0 - False
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A1 - True
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A2 - Float or Double (length disambiguates)
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A3 - SignedIntegers (0 may be encoded with no bytes at all)
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A4 - String (a trailing NUL is added)
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A5 - ByteString
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A6 - Symbol
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A7 - Record
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A8 - Sequence
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A9 - Set
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AA - Dictionary
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AB - Embedded
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(Ax) RESERVED AC-AF
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(Bx) RESERVED B0-BE
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BF - Annotations. {BF Lval val Lann0 ann0 Lann1 ann1 ...}
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## Appendix. Binary SignedInteger representation
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Languages that provide fixed-width machine word types may find the
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following table useful in encoding and decoding binary `SignedInteger`
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values.
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| Integer range | Bytes required | Encoding (hex) |
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| --- | --- | --- |
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| 0 | 1 | `A3` |
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| -2<sup>7</sup> ≤ n < 2<sup>7</sup> (i8) | 2 | `A3` `XX` |
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| -2<sup>15</sup> ≤ n < 2<sup>15</sup> (i16) | 3 | `A3` `XX` `XX` |
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| -2<sup>23</sup> ≤ n < 2<sup>23</sup> (i24) | 4 | `A3` `XX` `XX` `XX` |
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| -2<sup>31</sup> ≤ n < 2<sup>31</sup> (i32) | 5 | `A3` `XX` `XX` `XX` `XX` |
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| -2<sup>39</sup> ≤ n < 2<sup>39</sup> (i40) | 6 | `A3` `XX` `XX` `XX` `XX` `XX` |
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| -2<sup>47</sup> ≤ n < 2<sup>47</sup> (i48) | 7 | `A3` `XX` `XX` `XX` `XX` `XX` `XX` |
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| -2<sup>55</sup> ≤ n < 2<sup>55</sup> (i56) | 8 | `A3` `XX` `XX` `XX` `XX` `XX` `XX` `XX` |
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| -2<sup>63</sup> ≤ n < 2<sup>63</sup> (i64) | 9 | `A3` `XX` `XX` `XX` `XX` `XX` `XX` `XX` `XX` |
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<!-- Heading to visually offset the footnotes from the main document: -->
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## Notes
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