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# Machine-oriented binary syntax

The preserves.binary module implements the Preserves machine-oriented binary syntax.

The main entry points are functions encode, canonicalize, decode, and decode_with_annotations.

    >>> encode(Record(Symbol('hi'), []))
    b'\xb4\xb3\x02hi\x84'
    >>> decode(b'\xb4\xb3\x02hi\x84')
    #hi()

## `Decoder(packet=b'', include_annotations=False, decode_embedded=lambda x: x)`

Bases: `BinaryCodec`

Implementation of a decoder for the machine-oriented binary Preserves syntax.

Parameters:

Name | Type | Description | Default
--- | --- | --- | ---
`packet` | `bytes` | initial contents of the input buffer; may subsequently be extended by calling extend. | `b''`
`include_annotations` | `bool` | if `True`, wrap each value and subvalue in an Annotated object. | `False`
`decode_embedded` |  | function accepting a `Value` and returning a possibly-decoded form of that value suitable for placing into an Embedded object. | `lambda x: x`

Normal usage is to supply a buffer, and keep calling next until a ShortPacket exception is raised:

    >>> d = Decoder(b'\xa0{\xb1\x05hello\x85\xb3\x01x\xb5\x84')
    >>> d.next()
    123
    >>> d.next()
    'hello'
    >>> d.next()
    ()
    >>> d.next()
    Traceback (most recent call last):
      ...
    preserves.error.ShortPacket: Short packet

Alternatively, keep calling try_next until it yields `None`, which is not in the domain of Preserves `Value`s:

    >>> d = Decoder(b'\xa0{\xb1\x05hello\x85\xb3\x01x\xb5\x84')
    >>> d.try_next()
    123
    >>> d.try_next()
    'hello'
    >>> d.try_next()
    ()
    >>> d.try_next()

For convenience, Decoder implements the iterator interface, backing it with try_next, so you can simply iterate over all complete values in an input:

    >>> d = Decoder(b'\xa0{\xb1\x05hello\x85\xb3\x01x\xb5\x84')
    >>> list(d)
    [123, 'hello', ()]

    >>> for v in Decoder(b'\xa0{\xb1\x05hello\x85\xb3\x01x\xb5\x84'):
    ...     print(repr(v))
    123
    'hello'
    ()

Supply `include_annotations=True` to read annotations alongside the annotated values:

    >>> d = Decoder(b'\xa0{\xb1\x05hello\x85\xb3\x01x\xb5\x84', include_annotations=True)
    >>> list(d)
    [123, 'hello', @#x ()]

If you are incrementally reading from, say, a socket, you can use extend to add new input as if comes available:

    >>> d = Decoder(b'\xa0{\xb1\x05he')
    >>> d.try_next()
    123
    >>> d.try_next() # returns None because the input is incomplete
    >>> d.extend(b'llo')
    >>> d.try_next()
    'hello'
    >>> d.try_next()

Attributes:

Name | Type | Description
--- | --- | ---
`packet` | `bytes` | buffered input waiting to be processed
`index` | `int` | read position within `packet`

Source code in `preserves/binary.py`

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    def __init__(self, packet=b'', include_annotations=False, decode_embedded=lambda x: x):
        super(Decoder, self).__init__()
        self.packet = packet
        self.index = 0
        self.include_annotations = include_annotations
        self.decode_embedded = decode_embedded

### `extend(data)`

Appends `data` to the remaining bytes in `self.packet`, trimming already-processed bytes from the front of `self.packet` and resetting `self.index` to zero.

Source code in `preserves/binary.py`

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    def extend(self, data):
        """Appends `data` to the remaining bytes in `self.packet`, trimming already-processed
        bytes from the front of `self.packet` and resetting `self.index` to zero."""
        self.packet = self.packet[self.index:] + data
        self.index = 0

### `next()`

Reads the next complete `Value` from the internal buffer, raising ShortPacket if too few bytes are available, or DecodeError if the input is invalid somehow.

Source code in `preserves/binary.py`

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    def next(self):
        """Reads the next complete `Value` from the internal buffer, raising
        [ShortPacket][preserves.error.ShortPacket] if too few bytes are available, or
        [DecodeError][preserves.error.DecodeError] if the input is invalid somehow.

        """
        tag = self.nextbyte()
        if tag == 0x80: return self.wrap(False)
        if tag == 0x81: return self.wrap(True)
        if tag == 0x82: return self.wrap(Float.from_bytes(self.nextbytes(4)))
        if tag == 0x83: return self.wrap(struct.unpack('>d', self.nextbytes(8))[0])
        if tag == 0x84: raise DecodeError('Unexpected end-of-stream marker')
        if tag == 0x85:
            a = self.next()
            v = self.next()
            return self.unshift_annotation(a, v)
        if tag == 0x86:
            if self.decode_embedded is None:
                raise DecodeError('No decode_embedded function supplied')
            return self.wrap(Embedded(self.decode_embedded(self.next())))
        if tag >= 0x90 and tag <= 0x9f: return self.wrap(tag - (0xa0 if tag > 0x9c else 0x90))
        if tag >= 0xa0 and tag <= 0xaf: return self.wrap(self.nextint(tag - 0xa0 + 1))
        if tag == 0xb0: return self.wrap(self.nextint(self.varint()))
        if tag == 0xb1: return self.wrap(self.nextbytes(self.varint()).decode('utf-8'))
        if tag == 0xb2: return self.wrap(self.nextbytes(self.varint()))
        if tag == 0xb3: return self.wrap(Symbol(self.nextbytes(self.varint()).decode('utf-8')))
        if tag == 0xb4:
            vs = self.nextvalues()
            if not vs: raise DecodeError('Too few elements in encoded record')
            return self.wrap(Record(vs[0], vs[1:]))
        if tag == 0xb5: return self.wrap(tuple(self.nextvalues()))
        if tag == 0xb6: return self.wrap(frozenset(self.nextvalues()))
        if tag == 0xb7: return self.wrap(ImmutableDict.from_kvs(self.nextvalues()))
        raise DecodeError('Invalid tag: ' + hex(tag))

### `try_next()`

Like next, but returns `None` instead of raising ShortPacket.

Source code in `preserves/binary.py`

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    def try_next(self):
        """Like [next][preserves.binary.Decoder.next], but returns `None` instead of raising
        [ShortPacket][preserves.error.ShortPacket]."""
        start = self.index
        try:
            return self.next()
        except ShortPacket:
            self.index = start
            return None

## `Encoder(encode_embedded=lambda x: x, canonicalize=False, include_annotations=None)`

Bases: `BinaryCodec`

Implementation of an encoder for the machine-oriented binary Preserves syntax.

    >>> e = Encoder()
    >>> e.append(123)
    >>> e.append('hello')
    >>> e.append(annotate([], Symbol('x')))
    >>> e.contents()
    b'\xa0{\xb1\x05hello\x85\xb3\x01x\xb5\x84'

Parameters:

Name | Type | Description | Default
--- | --- | --- | ---
`encode_embedded` |  | function accepting an Embedded.embeddedValue and returning a `Value` for serialization. | `lambda x: x`
`canonicalize` | `bool` | if `True`, ensures the serialized data are in canonical form. This is slightly more work than producing potentially-non-canonical output. | `False`
`include_annotations` | `bool | None` | if `None`, includes annotations in the output only when `canonicalize` is `False`, because canonical serialization of values demands omission of annotations. If explicitly `True` or `False`, however, annotations will be included resp. excluded no matter the `canonicalize` setting. This can be used to get canonical ordering (`canonicalize=True`) and annotations (`include_annotations=True`). | `None`

Attributes:

Name | Type | Description
--- | --- | ---
`buffer` | `bytearray` | accumulator for the output of the encoder

Source code in `preserves/binary.py`

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    def __init__(self,
                 encode_embedded=lambda x: x,
                 canonicalize=False,
                 include_annotations=None):
        super(Encoder, self).__init__()
        self.buffer = bytearray()
        self._encode_embedded = encode_embedded
        self._canonicalize = canonicalize
        if include_annotations is None:
            self.include_annotations = not self._canonicalize
        else:
            self.include_annotations = include_annotations

### `append(v)`

Extend `self.buffer` with an encoding of `v`.

Source code in `preserves/binary.py`

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    def append(self, v):
        """Extend `self.buffer` with an encoding of `v`."""
        v = preserve(v)
        if hasattr(v, '__preserve_write_binary__'):
            v.__preserve_write_binary__(self)
        elif v is False:
            self.buffer.append(0x80)
        elif v is True:
            self.buffer.append(0x81)
        elif isinstance(v, float):
            self.buffer.append(0x83)
            self.buffer.extend(struct.pack('>d', v))
        elif isinstance(v, numbers.Number):
            if v >= -3 and v <= 12:
                self.buffer.append(0x90 + (v if v >= 0 else v + 16))
            else:
                self.encodeint(v)
        elif isinstance(v, bytes):
            self.encodebytes(2, v)
        elif isinstance(v, basestring_):
            self.encodebytes(1, v.encode('utf-8'))
        elif isinstance(v, list):
            self.encodevalues(5, v)
        elif isinstance(v, tuple):
            self.encodevalues(5, v)
        elif isinstance(v, set):
            self.encodeset(v)
        elif isinstance(v, frozenset):
            self.encodeset(v)
        elif isinstance(v, dict):
            self.encodedict(v)
        else:
            try:
                i = iter(v)
            except TypeError:
                i = None
            if i is None:
                self.cannot_encode(v)
            else:
                self.encodevalues(5, i)

### `contents()`

Returns a `bytes` constructed from the contents of `self.buffer`.

Source code in `preserves/binary.py`

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    def contents(self):
        """Returns a `bytes` constructed from the contents of `self.buffer`."""
        return bytes(self.buffer)

### `reset()`

Clears `self.buffer` to a fresh empty `bytearray`.

Source code in `preserves/binary.py`

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    def reset(self):
        """Clears `self.buffer` to a fresh empty `bytearray`."""
        self.buffer = bytearray()

## `canonicalize(v, **kwargs)`

As encode, but sets `canonicalize=True` in the Encoder constructor.

Source code in `preserves/binary.py`

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    def canonicalize(v, **kwargs):
        """As [encode][preserves.binary.encode], but sets `canonicalize=True` in the
        [Encoder][preserves.binary.Encoder] constructor.

        """
        return encode(v, canonicalize=True, **kwargs)

## `decode(bs, **kwargs)`

Yields the first complete encoded value from `bs`, passing `kwargs` through to the Decoder constructor. Raises exceptions as per next.

Parameters:

Name | Type | Description | Default
--- | --- | --- | ---
`bs` | `bytes` | encoded data to decode | required

Source code in `preserves/binary.py`

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    def decode(bs, **kwargs):
        """Yields the first complete encoded value from `bs`, passing `kwargs` through to the
        [Decoder][preserves.binary.Decoder] constructor. Raises exceptions as per
        [next][preserves.binary.Decoder.next].

        Args:
            bs (bytes): encoded data to decode

        """
        return Decoder(packet=bs, **kwargs).next()

## `decode_with_annotations(bs, **kwargs)`

Like decode, but supplying `include_annotations=True` to the Decoder constructor.

Source code in `preserves/binary.py`

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    def decode_with_annotations(bs, **kwargs):
        """Like [decode][preserves.binary.decode], but supplying `include_annotations=True` to the
        [Decoder][preserves.binary.Decoder] constructor."""
        return Decoder(packet=bs, include_annotations=True, **kwargs).next()

## `encode(v, **kwargs)`

Encode a single `Value` `v` to a byte string. Any supplied `kwargs` are passed on to the underlying Encoder constructor.

Source code in `preserves/binary.py`

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    def encode(v, **kwargs):
        """Encode a single `Value` `v` to a byte string. Any supplied `kwargs` are passed on to the
        underlying [Encoder][preserves.binary.Encoder] constructor."""
        e = Encoder(**kwargs)
        e.append(v)
        return e.contents()

Last update: March 16, 2023
Created: March 16, 2023