Ports the fragment layer of the AC UDP protocol. A UDP packet's body is
zero or more message fragments back-to-back; a logical GameMessage that
doesn't fit in ~448 bytes gets split across multiple fragments sharing
the same Id with differing Index values. The assembler handles
reassembly across arbitrary arrival ordering and duplicate fragments.
Added (all reimplemented from ACE's AGPL reference, see NOTICE.md):
- Packets/MessageFragmentHeader.cs: 16-byte fragment header struct
with Pack/Unpack, constants for MaxFragmentSize (464) and
MaxFragmentDataSize (448). Bit-layout doc comment documents what
each field is for.
- Packets/MessageFragment.cs: readonly record struct bundling a
header with its payload bytes; TryParse(source) parses one fragment
from the start of a buffer and returns (fragment, consumed) for
incremental parsing of multi-fragment packets. Refuses to parse
fragments with impossible TotalSize (too small for header, too
large for the 464-byte max, or larger than the source buffer).
- Packets/FragmentAssembler.cs: buffers partial messages keyed by
fragment Id. Ingest(frag, out queue) returns the assembled byte[]
when the last fragment arrives, null while still waiting. Key
correctness properties, all tested:
* Single-fragment (Count=1) shortcut releases with no buffering
* Out-of-order arrival (e.g. 2, 0, 1) releases on last arrival
and assembles in INDEX order, not arrival order
* Duplicate-fragment idempotence (re-sending same index is a no-op)
* Missing fragments stay buffered; DropAll() forcibly clears them
* Two independent messages can be assembled in parallel without
interfering
* messageQueue captured from first-arriving fragment (it's a
property of the logical message, not individual fragments)
Tests (17 new, 37 total in net project, 114 across both test projects):
- MessageFragmentHeader (4): pack/unpack round-trip, little-endian
wire format, constants, size-check throw
- MessageFragment (6): complete parse, insufficient header, oversized
TotalSize, undersized TotalSize, incomplete body, two-back-to-back
incremental parse
- FragmentAssembler (7): single-fragment, in-order 3-fragment,
out-of-order 3-fragment (tests index-order assembly), duplicate
idempotence, missing-fragment buffered, two parallel messages,
DropAll
Phase 4.4 (GameMessage reader + opcode handlers) next.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Adds the 20-byte AC UDP packet header struct + pack/unpack + its
checksum helper, and the Hash32 primitive the checksum uses.
Hash32 (Cryptography/Hash32.cs):
- Seeds accumulator with length << 16
- Sums input as little-endian uint32s word-aligned
- Folds any trailing 1-3 bytes via descending shift (24 → 16 → 8)
- Hand-computed golden values for 4-byte, 5-byte, and each 1/2/3
tail-byte case — no oracle needed, algorithm is simple enough to
verify by tracing
PacketHeader (Packets/PacketHeader.cs):
- Pack/Unpack: Sequence, Flags, Checksum, Id, Time, DataSize, Iteration
(20 bytes, little-endian on the wire)
- CalculateHeaderHash32: substitutes the 0xBADD70DD sentinel for the
Checksum field before hashing (matches AC retail + ACE convention —
without it the checksum would chicken-and-egg on itself). Uses a
local struct copy so the real Checksum isn't mutated on the caller.
- HasFlag for bitmask queries
PacketHeaderFlags (Packets/PacketHeaderFlags.cs):
- Full flag enum from ACE reference: Retransmission, EncryptedChecksum,
BlobFragments, ServerSwitch, ConnectRequest/Response, LoginRequest,
AckSequence, TimeSync, Disconnect, NetError, EchoRequest/Response,
Flow, and friends
Tests (15 new, 20 total in net project, 97 across both projects):
Hash32 (7):
- Empty returns 0
- 4-byte known value (hand-computed from bit layout)
- 5-byte value with one tail byte
- 1/2/3 tail-byte boundary cases (verifies 24/16/8 shift ordering)
- Determinism
PacketHeader (8):
- Pack/Unpack round-trip preserving all 7 fields
- Pack writes little-endian wire format in byte order
- HasFlag single and multi-bit
- CalculateHeaderHash32 invariance under Checksum field changes
(the critical property — verifies the BADD sentinel substitution)
- CalculateHeaderHash32 doesn't mutate
- CalculateHeaderHash32 determinism
- Unpack/Pack size-check throw
User confirmed an ACE server is running on localhost for the future
Phase 4.6 live integration step. Credentials will be read from env
vars at runtime, never committed.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
