AyCode.Core/AyCode.Core/docs/BINARY_FORMAT.md

AcBinary Wire Format

Complete wire format specification for the AcBinary serializer. Source of truth: Serializers/Binaries/BinaryTypeCode.cs.

For advanced features (compact encoding, string interning, reference tracking, property ordering) see BINARY_FEATURES.md. For configuration options, presets, and option interactions see BINARY_OPTIONS.md.

Stream Layout

[version : 1 byte]  [flags : 1 byte]  [cacheCount : VarUInt?]  [payload...]

• version — FormatVersion = 1 (current).
• flags — See Header Flags.
• cacheCount — Present only when HeaderFlag_HasCacheCount is set. Number of type wrapper slots used by serializer.

Header Flags

The flags byte uses 0x90 (144) as base with bit flags in the lower nibble:

Bit	Mask	Flag	Meaning
0	0x01	Metadata	Property hash metadata included (cross-type deserialization)
1	0x02	RefHandling_OnlyId	Reference tracking for IId objects only
2	0x04	RefHandling_All	Reference tracking for all objects (always combined with bit 1)
3	0x08	HasCacheCount	VarUInt cache count follows the flags byte

Reference handling modes: None = 0x00, OnlyId = 0x02, All = 0x06 (bits 1+2).

Variable-Length Encoding

VarUInt (unsigned)

LEB128: 7 data bits per byte, MSB = continuation flag.

value < 128       → 1 byte   [0xxxxxxx]
value < 16384     → 2 bytes  [1xxxxxxx] [0xxxxxxx]
value < 2097152   → 3 bytes  ...
(max 5 bytes for uint32)

VarInt (signed)

ZigZag encoding maps signed to unsigned, then LEB128:

encode: (value << 1) ^ (value >> 31)
decode: (raw >> 1) ^ -(raw & 1)

Maps: 0 → 0, -1 → 1, 1 → 2, -2 → 3, etc.

VarULong (unsigned 64-bit)

Same LEB128 encoding, max 10 bytes for uint64.

Type Markers

All markers defined in BinaryTypeCode.cs. SlotCount = 64.

FixObj (0–63)

Single-byte object type. The marker byte is the type slot index — no additional type identifier needed.

[FixObj(N)]  [properties...]

Slot allocation: Slots 0–63 are reserved for runtime polymorphic types, assigned dynamically on first encounter during serialization. Source-generated (SGen) types receive slots starting at 64+ via AllocateWrapperSlot() (sequential, Interlocked.Increment). SGen slots are compile-time stable; runtime slots depend on serialization order.

Complex Types (64–71)

Code	Name	Wire format
64	Object	[64] [VarUInt typeIndex] [properties...]
65	ObjectRef	[65] [VarUInt refCacheIndex]
66	Array	[66] [VarUInt count] [elements...]
67	Dictionary	[67] [VarUInt count] [key, value pairs...]
68	ByteArray	[68] [VarUInt length] [raw bytes]
69	ObjectWithMetadata	[69] [VarUInt typeIndex] [VarUInt hashCount] [hashes...] [properties...]
70	ObjectRefFirst	[70] [VarUInt refCacheIndex] [object body...]
71	ObjectWithMetadataRefFirst	[71] [VarUInt refCacheIndex] [metadata + properties...]

Polymorphic Types (72–75)

Used when runtime type differs from declared property type and UseMetadata=false.

Code	Name	Wire format
72	ObjectWithTypeName	[72] [UTF8 typeName] [inner marker] [body...] — prefix, inner Object/Array/Dict follows
73	ObjectWithTypeNameRefFirst	[73] [UTF8 typeName] [VarUInt refCacheIndex] [properties...] — combined, no inner marker
74	ObjectWithTypeIndex	[74] [VarUInt typeIndex] [inner marker] [body...] — prefix
75	ObjectWithTypeIndexRefFirst	[75] [VarUInt typeIndex] [VarUInt refCacheIndex] [properties...] — combined

Second occurrence of a referenced polymorphic object uses plain ObjectRef(65) — no polymorphic prefix needed.

Primitives (76–90)

Code	Name	Wire format
76	Null	[76] — no payload
77	True	[77] — no payload
78	False	[78] — no payload
79	Int8	[79] [1 byte]
80	UInt8	[80] [1 byte]
81	Int16	[81] [VarInt]
82	UInt16	[82] [VarUInt]
83	Int32	[83] [VarInt]
84	UInt32	[84] [VarUInt]
85	Int64	[85] [VarLong]
86	UInt64	[86] [VarULong]
87	Float32	[87] [4 bytes IEEE 754]
88	Float64	[88] [8 bytes IEEE 754]
89	Decimal	[89] [16 bytes]
90	Char	[90] [VarUInt]

Strings (91–94)

Code	Name	Wire format
91	String	[91] [VarUInt byteLength] [UTF-8 bytes]
92	StringInterned	[92] [VarUInt cacheIndex] — 2nd+ occurrence
93	StringEmpty	[93] — no payload
94	StringInternFirst	[94] [VarUInt cacheIndex] [VarUInt byteLength] [UTF-8 bytes] — 1st occurrence

Date/Time (95–98)

Code	Name	Wire format
95	DateTime	[95] [8 bytes ticks]
96	DateTimeOffset	[96] [8 bytes ticks] [VarInt offsetMinutes]
97	TimeSpan	[97] [VarLong ticks]
98	Guid	[98] [16 bytes]

Other Markers

Code	Name	Wire format
99	Enum	[99] [VarInt underlyingValue]
100	MetadataHeader	Legacy: implies RefHandling=true + metadata present
101	NoMetadataHeader	Legacy: implies RefHandling=true, no metadata
102	PropertySkip	[102] — marks skipped property (default/null value)

FixStr (103–134)

Short ASCII strings encoded in a single marker byte + raw bytes (no length prefix):

[FixStrBase + byteLength]  [ASCII bytes]

• Length range: 0–31 bytes (FixStrBase=103, FixStrMax=134)
• Saves 1 byte vs String marker + VarUInt length
• Falls back to String(91) if content is non-ASCII

TinyInt (192–255)

Single-byte integer encoding for small values:

value = marker - 192 - 16    (range: -16 to 47)
marker = value + 16 + 192    (64 values total)

Saves 2+ bytes vs Int32(83) + VarInt for frequently occurring small integers.