forked from arti/vt1200
288 lines
13 KiB
Python
288 lines
13 KiB
Python
"""Chunked upload of the 32x32 tile dictionary.
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The dictionary is 256 entries of 128 bytes -- 32 KB -- and it does not fit in
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one datagram. :func:`dict_chunks` splits it into independently framed
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``KIND_DICT_CHUNK`` packets, each carrying at most :data:`MAX_PAYLOAD` payload
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bytes: a 2-byte little-endian **offset** followed by a slice of the dictionary.
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[ 16-byte header ][ offset u16 ][ dictionary bytes ... ]
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The returned packets are frame buffers (header plus payload). Wrapping one in
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an Ethernet frame is the caller's job, exactly as for
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:meth:`harness.udpstream.vidsrc.StripSource.encode`, and the helpers below take
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that frame buffer, not the padded Ethernet envelope.
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The offset is the dictionary position of the first body byte, so a receiver
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can detect a gap, a duplicate or a reordered packet without keeping a stream
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state machine (Task 3 requires ascending offsets with no gaps). A 32 KB
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dictionary needs 23 packets: 22 of 1,448 body bytes (31,856 bytes) and a
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912-byte tail.
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Every packet has its own frame header and therefore its own header CRC; the
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CRC covers header bytes 0..11 only (ruling R14), exactly as for a strip frame.
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There is no body checksum -- the Ethernet FCS of each datagram is the check for
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the payload bytes.
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``rows`` and ``stretch`` are zero because a dictionary chunk has no geometry,
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and ``frame_no`` is zero because an upload is not one of the video's numbered
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frames. The ``frame_no`` field is not what orders the chunks; the offset is.
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The payload's ``frame_parse.h`` mirrors :data:`harness.udpstream.frame.KINDS`,
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which is where ``KIND_DICT_CHUNK`` is defined.
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Delta frames
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------------
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The same module carries Task 5's changed-tile list, because it is the other
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half of the tile wire format and the two layouts share the dictionary:
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KIND_TILES_FULL (2) 1,280 dictionary indices, row-major, one byte each
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KIND_TILES_DELTA (3) a u16 count, then `count` u16 positions and
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`count` one-byte dictionary entries
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A position is a row-major tile index 0..1,279, so it needs **two** bytes while
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one dictionary entry fits in one -- the entry space is 256, the tile grid is
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1,280. :func:`delta_frame` writes the payload, :func:`unpack_delta` reads it
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back with the same rules the C decoder enforces (exact length, positions on the
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grid and strictly ascending), and :func:`keyframe_due` is the cadence rule:
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a ``TILES_FULL`` whenever the host has no previous frame, every
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:data:`KEYFRAME_EVERY` frames, and as soon as the terminal's status frame
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reports a drop.
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Positions travel separately from values (rather than interleaved as pairs) for
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one reason: a receiver can validate the whole position list before it touches a
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pixel, so a frame with one bad position paints **nothing**. Interleaved pairs
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invite a decoder that paints as it reads, which is the failure the harness's
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off-grid negative control exists to catch.
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Pure standard library.
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"""
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import struct
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from harness.udpstream.frame import (HEADER_SIZE, KIND_DICT_CHUNK, MAGIC,
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VERSION, pack_header, unpack_header)
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from harness.udpstream.tilecost import TILES_PER_SCREEN
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__all__ = [
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"DELTA_HDR", "KEYFRAME_EVERY", "MAX_BODY", "MAX_DICT_BYTES", "MAX_PAYLOAD",
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"OFFSET_SIZE", "delta_frame", "dict_chunks", "keyframe_due",
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"payload_body", "payload_offset", "unpack_delta",
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]
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#: Largest chunk payload in one packet, offset included. The same 1450-byte
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#: budget as a strip datagram's chunk (:data:`harness.udpstream.vidsrc.CHUNK`).
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MAX_PAYLOAD = 1450
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#: The offset field: a u16 little-endian dictionary position.
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OFFSET_SIZE = 2
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#: Most dictionary bytes one packet can carry after its offset.
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MAX_BODY = MAX_PAYLOAD - OFFSET_SIZE # 1,448
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#: A u16 offset can name positions 0..65,535, so a dictionary ending at
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#: offset 65,536 is still addressable: its last chunk's offset fits.
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MAX_DICT_BYTES = 0x10000
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#: A delta payload opens with the u16 entry count (see frame_parse.h).
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DELTA_HDR = 2
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#: Send a full frame (``TILES_FULL``) this often, whether or not anything was
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#: lost. Ruling R2 of the plan: 60 frames is ~2 s at the sender's 30 fps, so a
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#: desynchronised screen can never stay wrong for longer than that.
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KEYFRAME_EVERY = 60
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_OFFSET = struct.Struct("<H")
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_COUNT = struct.Struct("<H")
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def _dict_payload(packet: bytes) -> bytes:
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"""Return a dictionary chunk's payload, or raise ValueError.
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Validates what the helpers rely on: a whole ``VS`` frame of the right
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version, the dictionary-chunk kind, a length that matches the packet, and
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room for the offset. The header CRC is *not* rejected here -- a caller
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that wants to count bad headers can read ``unpack_header`` itself -- but a
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truncated or foreign packet is an error rather than a silent misread.
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"""
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try:
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header = unpack_header(packet)
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except ValueError as exc:
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raise ValueError("not a dictionary chunk: %s" % exc)
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if header["magic"] != MAGIC or header["version"] != VERSION:
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raise ValueError("not a VS frame (magic %r, version %d)"
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% (header["magic"], header["version"]))
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if header["kind"] != KIND_DICT_CHUNK:
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raise ValueError("kind %d is not a dictionary chunk (%d)"
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% (header["kind"], KIND_DICT_CHUNK))
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if len(packet) != HEADER_SIZE + header["payload_len"]:
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raise ValueError("packet is %d bytes but the header describes %d"
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% (len(packet), HEADER_SIZE + header["payload_len"]))
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if header["payload_len"] < OFFSET_SIZE:
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raise ValueError("a dictionary chunk payload is at least %d bytes, "
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"got %d" % (OFFSET_SIZE, header["payload_len"]))
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return packet[HEADER_SIZE:HEADER_SIZE + header["payload_len"]]
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def dict_chunks(dictionary: bytes, offset_base: int = 0) -> list[bytes]:
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"""Split ``dictionary`` into framed ``KIND_DICT_CHUNK`` packets.
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``offset_base`` is added to every offset, for placing the upload at a
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non-zero position in the receiver's buffer. The dictionary must be
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non-empty and must end within the u16 offset space; those are errors here
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rather than a wrap on the wire.
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"""
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if not isinstance(dictionary, (bytes, bytearray, memoryview)):
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raise TypeError("a dictionary is bytes-like, got %s"
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% type(dictionary).__name__)
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data = bytes(dictionary)
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if not data:
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raise ValueError("a dictionary upload needs at least one byte")
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if (not isinstance(offset_base, int) or isinstance(offset_base, bool)
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or not 0 <= offset_base <= 0xFFFF):
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raise ValueError("offset_base is 0..65535, got %r" % (offset_base,))
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end = offset_base + len(data)
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if end > MAX_DICT_BYTES:
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raise ValueError("a %d-byte dictionary at offset %d ends at %d, past "
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"the u16 offset space (%d)"
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% (len(data), offset_base, end, MAX_DICT_BYTES))
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packets = []
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for start in range(0, len(data), MAX_BODY):
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body = data[start:start + MAX_BODY]
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payload = _OFFSET.pack(offset_base + start) + body
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packets.append(pack_header(KIND_DICT_CHUNK, 0, len(payload), 0, 0,
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payload=payload) + payload)
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return packets
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def payload_offset(packet: bytes) -> int:
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"""Dictionary position of the first body byte in ``packet``."""
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return _OFFSET.unpack_from(_dict_payload(packet), 0)[0]
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def payload_body(packet: bytes) -> bytes:
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"""The dictionary bytes ``packet`` carries, offset excluded."""
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return _dict_payload(packet)[OFFSET_SIZE:]
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# --- Task 5: changed-tile lists -------------------------------------------
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def _indices(name: str, frame) -> bytes:
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"""Range-check one full-screen index frame and return it as ``bytes``."""
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if not isinstance(frame, (bytes, bytearray, memoryview)):
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raise TypeError("%s is bytes-like, got %s"
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% (name, type(frame).__name__))
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data = bytes(frame)
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if len(data) != TILES_PER_SCREEN:
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raise ValueError("%s is one index byte per tile: %d bytes, got %d"
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% (name, TILES_PER_SCREEN, len(data)))
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return data
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def delta_frame(prev: bytes, cur: bytes) -> bytes | None:
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"""Return the ``KIND_TILES_DELTA`` payload from ``prev`` to ``cur``.
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``None`` when the frames are identical: there is nothing to put on the
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wire, and an empty list is not a frame the receiver should have to accept.
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Otherwise the payload is
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u16 count | count * u16 position | count * u8 dictionary entry
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with positions in ascending order (a scan of the row-major grid produces
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them that way) and each entry the *new* value for that tile. Everything is
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recomputed from the two frames -- nothing is carried over from a previous
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delta -- so a delta lost in transit cannot corrupt the next one; it is the
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periodic keyframe that repairs the screen.
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"""
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before = _indices("prev", prev)
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after = _indices("cur", cur)
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positions = [i for i in range(TILES_PER_SCREEN) if before[i] != after[i]]
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if not positions:
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return None
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values = bytes(after[i] for i in positions)
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return (_COUNT.pack(len(positions))
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+ b"".join(_COUNT.pack(i) for i in positions)
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+ values)
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def unpack_delta(payload: bytes) -> list[tuple[int, int]]:
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"""Decode a ``KIND_TILES_DELTA`` payload, or raise ``ValueError``.
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The rules are the C decoder's (``paint_delta`` in ``vidstream.c``), and
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they are the reason a delta cannot corrupt the screen:
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* the payload holds exactly ``2 + 3 * count`` bytes, so the list cannot run
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past the frame the header described;
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* ``count`` is at least 1 and at most :data:`TILES_PER_SCREEN`;
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* every position is on the 40 x 32 grid, so no blit is ever aimed past the
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last tile;
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* positions are strictly ascending, so no tile is named twice and a
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reordered list is a refusal rather than a different picture.
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Each value is a byte and therefore already a dictionary entry, so the
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"entry index < 256" rule needs no check on this side.
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"""
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if not isinstance(payload, (bytes, bytearray, memoryview)):
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raise TypeError("a delta payload is bytes-like, got %s"
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% type(payload).__name__)
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body = bytes(payload)
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if len(body) < DELTA_HDR:
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raise ValueError("a delta payload opens with a %d-byte count, got %d"
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% (DELTA_HDR, len(body)))
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count = _COUNT.unpack_from(body, 0)[0]
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if not 1 <= count <= TILES_PER_SCREEN:
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raise ValueError("a delta lists 1..%d tiles, got %d"
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% (TILES_PER_SCREEN, count))
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if len(body) != DELTA_HDR + 3 * count:
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raise ValueError("a %d-entry delta is %d bytes, got %d"
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% (count, DELTA_HDR + 3 * count, len(body)))
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positions = []
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previous = -1
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for k in range(count):
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position = _COUNT.unpack_from(body, DELTA_HDR + 2 * k)[0]
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if position >= TILES_PER_SCREEN:
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raise ValueError("tile position %d is off the %d-tile grid"
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% (position, TILES_PER_SCREEN))
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if position <= previous:
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raise ValueError("tile positions must be strictly ascending: "
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"%d follows %d" % (position, previous))
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previous = position
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positions.append(position)
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values = body[DELTA_HDR + 2 * count:]
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return list(zip(positions, values))
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def keyframe_due(frame_no: int, drops: int = 0, seen_drops: int = 0,
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every: int = KEYFRAME_EVERY) -> bool:
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"""True when the host must send a ``TILES_FULL`` instead of a delta.
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Two triggers, and both are "the terminal may not have what the host thinks
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it has":
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* ``frame_no`` is a multiple of ``every`` -- a fixed cadence that bounds
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how long a desynchronised screen can stay wrong (ruling R2). Frame 0
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is in this set because there is no previous frame to delta against;
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* ``drops`` rose since ``seen_drops``: the status frame's counter is
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cumulative, so any increase means at least one frame never reached the
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screen and the host's idea of the screen is stale.
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``crc_err`` is deliberately not a trigger. A refused delta *does* leave
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the terminal's screen behind the host's -- the host has moved to the next
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frame and the terminal never painted that one -- so the reason is not that
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nothing was desynchronised. It is that a fixed 60-frame cadence already
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bounds how long that desync lasts, and a refused frame carries no screen
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state a keyframe could recover sooner than the schedule does. ``drops`` is
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different: it counts frames the terminal *accepted* and then lost, which is
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the counter the terminal raises for the same repair.
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"""
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if (not isinstance(frame_no, int) or isinstance(frame_no, bool)
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or frame_no < 0):
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raise ValueError("frame_no is a non-negative int, got %r"
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% (frame_no,))
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if not isinstance(every, int) or isinstance(every, bool) or every < 1:
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raise ValueError("every is a positive int, got %r" % (every,))
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return frame_no % every == 0 or drops > seen_drops
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