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Xlang Serialization Format

Cross-language Serialization Specification

Apache Fory™ xlang serialization enables automatic cross-language object serialization with support for shared references, circular references, and polymorphism. Unlike traditional serialization frameworks that require IDL definitions and schema compilation, Fory serializes objects directly without any intermediate steps.

Key characteristics:

Automatic: No IDL definition, no schema compilation, no manual object-to-protocol conversion
Cross-language: Same binary format works seamlessly across Java, Python, C++, Rust, Go, JavaScript, and more
Reference-aware: Handles shared references and circular references without duplication or infinite recursion
Polymorphic: Supports object polymorphism with runtime type resolution

This specification defines the Fory xlang binary format. The format is dynamic rather than static, which enables flexibility and ease of use at the cost of additional complexity in the wire format.

Type Systems

Data Types

bool: a boolean value (true or false).
int8: a 8-bit signed integer.
int16: a 16-bit signed integer.
int32: a 32-bit signed integer.
var32: a 32-bit signed integer which use fory variable-length encoding.
int64: a 64-bit signed integer.
var64: a 64-bit signed integer which use fory PVL encoding.
hybrid64: a 64-bit signed integer which use fory Hybrid encoding.
uint8: an 8-bit unsigned integer.
uint16: a 16-bit unsigned integer.
uint32: a 32-bit unsigned integer.
varu32: a 32-bit unsigned integer which use fory variable-length encoding.
uint64: a 64-bit unsigned integer.
varu64: a 64-bit unsigned integer which use fory PVL encoding.
hybridu64: a 64-bit unsigned integer which use fory Hybrid encoding.
float16: a 16-bit floating point number.
float32: a 32-bit floating point number.
float64: a 64-bit floating point number including NaN and Infinity.
string: a text string encoded using Latin1/UTF16/UTF-8 encoding.
enum: a data type consisting of a set of named values. Rust enum with non-predefined field values are not supported as an enum.
named_enum: an enum whose value will be serialized as the registered name.
struct: a dynamic(final) type serialized by Fory Struct serializer. i.e. it doesn't have subclasses. Suppose we're deserializing List<SomeClass>, we can save dynamic serializer dispatch since SomeClass is dynamic(final).
compatible_struct: a dynamic(final) type serialized by Fory compatible Struct serializer.
named_struct: a struct whose type mapping will be encoded as a name.
named_compatible_struct: a compatible_struct whose type mapping will be encoded as a name.
ext: a type which will be serialized by a customized serializer.
named_ext: an ext type whose type mapping will be encoded as a name.
list: a sequence of objects.
set: an unordered set of unique elements.
map: a map of key-value pairs. Mutable types such as list/map/set/array are not allowed as key of map.
duration: an absolute length of time, independent of any calendar/timezone, as a count of nanoseconds.
timestamp: a point in time, independent of any calendar/timezone, as a count of nanoseconds. The count is relative to an epoch at UTC midnight on January 1, 1970.
local_date: a naive date without timezone. The count is days relative to an epoch at UTC midnight on Jan 1, 1970.
decimal: exact decimal value represented as an integer value in two's complement.
binary: an variable-length array of bytes.
array: only allow 1d numeric components. Other arrays will be taken as List. The implementation should support the interoperability between array and list.
- bool_array: one dimensional bool array.
- int8_array: one dimensional int8 array.
- int16_array: one dimensional int16 array.
- int32_array: one dimensional int32 array.
- int64_array: one dimensional int64 array.
- float16_array: one dimensional half_float_16 array.
- float32_array: one dimensional float32 array.
- float64_array: one dimensional float64 array.
union: a tagged union type that can hold one of several alternative types. The active alternative is identified by an index.
none: represents an empty/unit value with no data (e.g., for empty union alternatives).

Note:

Unsigned integer types use the same byte sizes as their signed counterparts; the difference is in value interpretation. See Type mapping for language-specific type mappings.

Polymorphisms

For polymorphism, if one non-final class is registered, and only one subclass is registered, then we can take all elements in List/Map have same type, thus reduce runtime check cost.

Collection/Array polymorphism are not fully supported, since some languages such as golang have only one collection type. If users want to get exactly the type he passed, he must pass that type when deserializing or annotate that type to the field of struct.

Type disambiguation

Due to differences between type systems of languages, those types can't be mapped one-to-one between languages. When deserializing, Fory use the target data structure type and the data type in the data jointly to determine how to deserialize and populate the target data structure. For example:

class Foo {
  int[] intArray;
  Object[] objects;
  List<Object> objectList;
}

class Foo2 {
  int[] intArray;
  List<Object> objects;
  List<Object> objectList;
}

intArray has an int32_array type. But both objects and objectList fields in the serialize data have list data type. When deserializing, the implementation will create an Object array for objects, but create a ArrayList for objectList to populate its elements. And the serialized data of Foo can be deserialized into Foo2 too.

Users can also provide meta hints for fields of a type, or the type whole. Here is an example in java which use annotation to provide such information.

@ForyObject(fieldsNullable = false, trackingRef = false)
class Foo {
  @ForyField(trackingRef = false)
  int[] intArray;
  @ForyField(polymorphic = true)
  Object object;
  @ForyField(tagId = 1, nullable = true)
  List<Object> objectList;
}

Such information can be provided in other languages too:

cpp: use macro and template.
golang: use struct tag.
python: use typehint.
rust: use macro.

Type ID

All internal data types are expressed using an ID in range 0~64. Users can use IDs in range 0~8192 for registering their custom types (struct/ext/enum). User type IDs are in a separate namespace and combined with internal type IDs via bit shifting: (user_type_id << 8) | internal_type_id.

Internal Type ID Table

Type ID	Name	Description
0	UNKNOWN	Unknown type, used for dynamic typing
1	BOOL	Boolean value
2	INT8	8-bit signed integer
3	INT16	16-bit signed integer
4	INT32	32-bit signed integer
5	VARINT32	Variable-length encoded 32-bit signed integer
6	INT64	64-bit signed integer
7	VARINT64	Variable-length encoded 64-bit signed integer
8	TAGGED_INT64	Hybrid encoded 64-bit signed integer
9	UINT8	8-bit unsigned integer
10	UINT16	16-bit unsigned integer
11	UINT32	32-bit unsigned integer
12	VAR_UINT32	Variable-length encoded 32-bit unsigned integer
13	UINT64	64-bit unsigned integer
14	VAR_UINT64	Variable-length encoded 64-bit unsigned integer
15	TAGGED_UINT64	Hybrid encoded 64-bit unsigned integer
16	FLOAT16	16-bit floating point (half precision)
17	FLOAT32	32-bit floating point (single precision)
18	FLOAT64	64-bit floating point (double precision)
19	STRING	UTF-8/UTF-16/Latin1 encoded string
20	LIST	Ordered collection (List, Array, Vector)
21	SET	Unordered collection of unique elements
22	MAP	Key-value mapping
23	ENUM	Enum registered by numeric ID
24	NAMED_ENUM	Enum registered by namespace + type name
25	STRUCT	Struct registered by numeric ID (schema consistent)
26	COMPATIBLE_STRUCT	Struct with schema evolution support (by ID)
27	NAMED_STRUCT	Struct registered by namespace + type name
28	NAMED_COMPATIBLE_STRUCT	Struct with schema evolution (by name)
29	EXT	Extension type registered by numeric ID
30	NAMED_EXT	Extension type registered by namespace + type name
31	UNION	Tagged union type (one of several alternatives)
32	NONE	Empty/unit type (no data)
33	DURATION	Time duration (seconds + nanoseconds)
34	TIMESTAMP	Point in time (nanoseconds since epoch)
35	LOCAL_DATE	Date without timezone (days since epoch)
36	DECIMAL	Arbitrary precision decimal
37	BINARY	Raw binary data
38	ARRAY	Generic array type
39	BOOL_ARRAY	1D boolean array
40	INT8_ARRAY	1D int8 array
41	INT16_ARRAY	1D int16 array
42	INT32_ARRAY	1D int32 array
43	INT64_ARRAY	1D int64 array
44	UINT8_ARRAY	1D uint8 array
45	UINT16_ARRAY	1D uint16 array
46	UINT32_ARRAY	1D uint32 array
47	UINT64_ARRAY	1D uint64 array
48	FLOAT16_ARRAY	1D float16 array
49	FLOAT32_ARRAY	1D float32 array
50	FLOAT64_ARRAY	1D float64 array

Type ID Encoding for User Types

When registering user types (struct/ext/enum), the full type ID combines user ID and internal type ID:

Full Type ID = (user_type_id << 8) | internal_type_id

Examples:

User ID	Type	Internal ID	Full Type ID	Decimal
0	STRUCT	25	`(0 << 8) \| 25`	25
0	ENUM	23	`(0 << 8) \| 23`	23
1	STRUCT	25	`(1 << 8) \| 25`	281
1	COMPATIBLE_STRUCT	26	`(1 << 8) \| 26`	282
2	NAMED_STRUCT	27	`(2 << 8) \| 27`	539

When reading type IDs:

Extract internal type: internal_type_id = full_type_id & 0xFF
Extract user type ID: user_type_id = full_type_id >> 8

Type mapping

See Type mapping

Spec overview

Here is the overall format:

| fory header | object ref meta | object type meta | object value data |

The data are serialized using little endian byte order for all types.

Fory header

Fory header format for xlang serialization:

|        1 byte bitmap           |   1 byte   |          optional 4 bytes          |
+--------------------------------+------------+------------------------------------+
|  4 bits reserved | 4 bits meta |  language  | unsigned int for meta start offset |

Detailed byte layout:

Byte 0:   Bitmap flags
          - Bit 0: null flag (0x01)
          - Bit 1: xlang flag (0x02)
          - Bit 2: oob flag (0x04)
          - Bits 3-7: reserved
Byte 1:   Language ID (only present when xlang flag is set)
Byte 2-5: Meta start offset (only present when meta share mode is enabled)

null flag (bit 0): 1 when object is null, 0 otherwise. If an object is null, only this flag is set.
xlang flag (bit 1): 1 when serialization uses Fory xlang format, 0 when serialization uses Fory language-native format.
oob flag (bit 2): 1 when out-of-band serialization is enabled (BufferCallback is not null), 0 otherwise.

All data is encoded in little-endian format.

language: 1 byte indicating the source language. This allows deserializers to optimize for specific language characteristics.

Language IDs

Language	ID
XLANG	0
JAVA	1
PYTHON	2
CPP	3
GO	4
JAVASCRIPT	5
RUST	6
DART	7

Meta Start Offset

If compatible mode is enabled, an uncompressed unsigned int32 (4 bytes, little endian) is appended to indicate the start offset of metadata. During serialization, this is initially written as a placeholder (e.g., -1 or 0), then updated after all objects are serialized and metadata is collected.

Reference Meta

Reference tracking handles whether the object is null, and whether to track reference for the object by writing corresponding flags and maintaining internal state.

Reference Flags

Flag	Byte Value (int8)	Hex	Description
NULL FLAG	`-3`	`0xFD`	Object is null. No further bytes are written for this object.
REF FLAG	`-2`	`0xFE`	Object was already serialized. Followed by unsigned varint32 reference ID.
NOT_NULL VALUE FLAG	`-1`	`0xFF`	Object is non-null but reference tracking is disabled for this type. Object data follows immediately.
REF VALUE FLAG	`0`	`0x00`	Object is referencable and this is its first occurrence. Object data follows. Assigns next reference ID.

Reference Tracking Algorithm

Writing:

function write_ref_or_null(buffer, obj):
    if obj is null:
        buffer.write_int8(NULL_FLAG)      // -3
        return true  // done, no more data to write

    if reference_tracking_enabled:
        ref_id = lookup_written_objects(obj)
        if ref_id exists:
            buffer.write_int8(REF_FLAG)   // -2
            buffer.write_varuint32(ref_id)
            return true  // done, reference written
        else:
            buffer.write_int8(REF_VALUE_FLAG)  // 0
            add_to_written_objects(obj, next_ref_id++)
            return false  // continue to serialize object data
    else:
        buffer.write_int8(NOT_NULL_VALUE_FLAG)  // -1
        return false  // continue to serialize object data

Reading:

function read_ref_or_null(buffer):
    flag = buffer.read_int8()
    switch flag:
        case NULL_FLAG (-3):
            return (null, true)  // null object, done
        case REF_FLAG (-2):
            ref_id = buffer.read_varuint32()
            obj = get_from_read_objects(ref_id)
            return (obj, true)  // referenced object, done
        case NOT_NULL_VALUE_FLAG (-1):
            return (null, false)  // non-null, continue reading
        case REF_VALUE_FLAG (0):
            reserve_ref_slot()  // will be filled after reading
            return (null, false)  // non-null, continue reading

Reference ID Assignment

Reference IDs are assigned sequentially starting from 0
The ID is assigned when REF_VALUE_FLAG is written (first occurrence)
Objects are stored in a list/map indexed by their reference ID
For reading, a placeholder slot is reserved before deserializing the object, then filled after

When Reference Tracking is Disabled

When reference tracking is disabled globally or for specific types, only the NULL and NOT_NULL VALUE flags will be used for reference meta. This reduces overhead for types that are known not to have references.

Language-Specific Considerations

Languages with nullable and reference types by default (Java, Python, JavaScript):

In xlang mode, for cross-language compatibility:

All fields are treated as not-null by default
Reference tracking is disabled by default
Users can explicitly mark fields as nullable or enable reference tracking via annotations
Optional types (e.g., java.util.Optional, typing.Optional) are treated as nullable

Annotation examples:

// Java: use @ForyField annotation
public class MyClass {
    @ForyField(nullable = true, ref = true)
    private Object refField;

    @ForyField(nullable = false)
    private String requiredField;
}

# Python: use typing with fory field descriptors
from pyfory import Fory, ForyField

class MyClass:
    ref_field: ForyField(SomeType, nullable=True, ref=True)
    required_field: ForyField(str, nullable=False)

Languages with non-nullable types by default:

Language	Null Representation	Reference Tracking Support
Rust	`Option::None`	Via `Rc<T>`, `Arc<T>`, `Weak<T>`
C++	`std::nullopt`, `nullptr`	Via `std::shared_ptr<T>`, `weak_ptr<T>`
Go	`nil` interface/pointer	Via pointer/interface types

Important: For languages like Rust that don't have implicit reference semantics, reference tracking must use explicit smart pointers (Rc, Arc).

Type Meta

For every type to be serialized, it have a type id to indicate its type.

basic types: the type id
enum:
- Type.ENUM + registered id
- Type.NAMED_ENUM + registered namespace+typename
list: Type.List
set: Type.SET
map: Type.MAP
ext:
- Type.EXT + registered id
- Type.NAMED_EXT + registered namespace+typename
struct:
- Type.STRUCT + struct meta
- Type.NAMED_STRUCT + struct meta

Every type must be registered with an ID or name first. The registration can be used for security check and type identification.

Struct is a special type, depending whether schema compatibility is enabled, Fory will write struct meta differently.

Only ext/enum/struct can be registered using namespaced type.

Struct Schema consistent

If schema consistent mode is enabled globally when creating fory, type meta will be written as a fory unsigned varint of type_id. Schema evolution related meta will be ignored.
If schema evolution mode is enabled globally when creating fory, and current class is configured to use schema consistent mode like struct vs table in flatbuffers:
- Type meta will be add to captured_type_defs: captured_type_defs[type def stub] = map size ahead when registering type.
- Get index of the meta in captured_type_defs, write that index as | unsigned varint: index |.

Struct Schema evolution

If schema evolution mode is enabled globally when creating fory, and enabled for current type, type meta will be written using one of the following mode. Which mode to use is configured when creating fory.

Normal mode(meta share not enabled):
- If type meta hasn't been written before, add type def to captured_type_defs: captured_type_defs[type def] = map size.
- Get index of the meta in captured_type_defs, write that index as | unsigned varint: index |.
- After finished the serialization of the object graph, fory will start to write captured_type_defs:
  - Firstly, set current to meta start offset of fory header
  - Then write captured_type_defs one by one:
    buffer.write_var_uint32(len(writting_type_defs) - len(schema_consistent_type_def_stubs)) for type_meta in writting_type_defs: if not type_meta.is_stub(): type_meta.write_type_def(buffer) writing_type_defs = copy(schema_consistent_type_def_stubs)

Meta share mode: the writing steps are same as the normal mode, but captured_type_defs will be shared across multiple serializations of different objects. For example, suppose we have a batch to serialize:

captured_type_defs = {}
stream = ...
# add `Type1` to `captured_type_defs` and write `Type1`
fory.serialize(stream, [Type1()])
# add `Type2` to `captured_type_defs` and write `Type2`, `Type1` is written before.
fory.serialize(stream, [Type1(), Type2()])
# `Type1` and `Type2` are written before, no need to write meta.
fory.serialize(stream, [Type1(), Type2()])

Streaming mode(streaming mode doesn't support meta share):
- If type meta hasn't been written before, the data will be written as:
```
| unsigned varint: 0b11111111 | type def |
```
- If type meta has been written before, the data will be written as:
```
| unsigned varint: written index << 1 |
```
  written index is the id in captured_type_defs.
- With this mode, meta start offset can be omitted.

The normal mode and meta share mode will forbid streaming writing since it needs to look back for update the start offset after the whole object graph writing and meta collecting is finished. Only in this way we can ensure deserialization failure in meta share mode doesn't lost shared meta.

Type Def

Here we mainly describe the meta layout for schema evolution mode:

|    8 bytes header    |   variable bytes   |  variable bytes   |
+----------------------+--------------------+-------------------+
| global binary header |    meta header     |    fields meta    |

For languages which support inheritance, if parent class and subclass has fields with same name, using field in subclass.

Global binary header

50 bits hash + 1bit compress flag + write fields meta + 12 bits meta size. Right is the lower bits.

lower 12 bits are used to encode meta size. If meta size >= 0b1111_1111_1111, then write meta_ size - 0b1111_1111_1111 next.
13rd bit is used to indicate whether to write fields meta. When this class is schema-consistent or use registered serializer, fields meta will be skipped. Class Meta will be used for share namespace + type name only.
14rd bit is used to indicate whether meta is compressed.
Other 50 bits is used to store the unique hash of flags + all layers class meta.

Meta header

Meta header is a 8 bits number value.

Lowest 5 digits 0b00000~0b11110 are used to record num fields. 0b11111 is preserved to indicate that Fory need to read more bytes for length using Fory unsigned int encoding. Note that num_fields is the number of compatible fields. Users can use tag id to mark some fields as compatible fields in schema consistent context. In such cases, schema consistent fields will be serialized first, then compatible fields will be serialized next. At deserialization, Fory will use fields info of those fields which aren't annotated by tag id for deserializing schema consistent fields, then use fields info in meta for deserializing compatible fields.
The 6th bit: 0 for registered by id, 1 for registered by name.
Remaining 2 bits are reserved for future extension.

Fields meta

Format:

|   field info: variable bytes    | variable bytes  | ... |
+---------------------------------+-----------------+-----+
| header + type info + field name | next field info | ... |

Field Header

Field Header is 8 bits, annotation can be used to provide more specific info. If annotation not exists, fory will infer those info automatically.

The format for field header is:

2 bits field name encoding + 4 bits size + nullability flag + ref tracking flag

Detailed spec:

2 bits field name encoding:
- encoding: UTF8/ALL_TO_LOWER_SPECIAL/LOWER_UPPER_DIGIT_SPECIAL/TAG_ID
- If tag id is used, field name will be written by an unsigned varint tag id, and 2 bits encoding will be 11.
size of field name:
- The 4 bits size: 0~14 will be used to indicate length 1~15, the value 15 indicates to read more bytes, the encoding will encode size - 15 as a varint next.
- If encoding is TAG_ID, then num_bytes of field name will be used to store tag id.
ref tracking: when set to 1, ref tracking will be enabled for this field.
nullability: when set to 1, this field can be null.

Field Type Info

Field type info is written as unsigned int8. Detailed id spec is:

For struct registered by id, it will be Type.STRUCT.
For struct registered by name, it will be Type.NAMED_STRUCT.
For enum registered by id, it will be Type.ENUM.
For enum registered by name, it will be Type.NAMED_ENUM.
For ext type registered by id, it will be Type.EXT.
For ext type registered by name, it will be Type.NAMED_EXT.
For list/set type, it will be written as Type.LIST/SET, then write element type recursively.
For 1D primitive array type, it will be written as Type.XXX_ARRAY.
For multi-dimensional primitive array type with same size on each dim, it will be written as Type.TENSOR.
For other array type, it will be written as Type.LIST, then write element type recursively.
For map type, it will be written as Type.MAP, then write key and value type recursively.
For other types supported by fory directly, it will be fory type id for that type.
For other types not determined at compile time, write Type.UNKNOWN instead. For such types, actual type will be written when serializing such field values.

Polymorphism spec:

struct/named_struct/ext/named_ext are taken as polymorphic, the meta for those types are written separately instead of inlining here to reduce meta space cost if object of this type is serialized in current object graph multiple times, and the field value may be null too.
enum is taken as dynamic, if deserialization doesn't have this field, or the type is not enum, enum value will be skipped.
list/map/set are taken as dynamic, when serializing values of those type, the concrete types won't be written again.
Other types that fory supported are taken as dynamic too.

List/Set/Map nested type spec:

list: | list type id | nested type id << 2 + nullability flag + ref tracking flag | ... multi-layer type info |
set: | set type id | nested type id << 2 + nullability flag + ref tracking flag | ... multi-layer type info |
map: | set type id | key type info | value type info |
- Key type format: | nested type id << 2 + nullability flag + ref tracking flag | ... multi-layer type info |
- Value type format: | nested type id << 2 + nullability flag + ref tracking flag | ... multi-layer type info |

Field Name

If tag id is set, tag id will be used instead. Otherwise meta string of field name will be written instead.

Field order

Field order are left as implementation details, which is not exposed to specification, the deserialization need to resort fields based on Fory fields sort algorithms. In this way, fory can compute statistics for field names or types and using a more compact encoding.

Extended Type Meta with Inheritance support

If one want to support inheritance for struct, one can implement following spec.

Schema consistent

Fields are serialized from parent type to leaf type. Fields are sorted using fory struct fields sort algorithms.

Schema Evolution

Meta layout for schema evolution mode:

|    8 bytes header    | variable bytes | variable bytes |   variable bytes   |   variable bytes   |
+----------------------+----------------+----------------+--------------------+--------------------+
| global binary header |  meta header   |  fields meta   | parent meta header | parent fields meta |

Meta header

Meta header is a 64 bits number value encoded in little endian order.

Lowest 4 digits 0b0000~0b1110 are used to record num classes. 0b1111 is preserved to indicate that Fory need to read more bytes for length using Fory unsigned int encoding. If current type doesn't has parent type, or parent type doesn't have fields to serialize, or we're in a context which serialize fields of current type only, num classes will be 1.
The 5th bit is used to indicate whether this type needs schema evolution.
Other 56 bits are used to store the unique hash of flags + all layers type meta.

Single layer type meta

| unsigned varint | var uint |  field info: variable bytes   | variable bytes  | ... |
+-----------------+----------+-------------------------------+-----------------+-----+
|   num_fields    | type id  | header + type id + field name | next field info | ... |

Other layers type meta

Same encoding algorithm as the previous layer.

Meta String

Meta string is a compressed encoding for metadata strings such as field names, type names, and namespaces. This compression significantly reduces the size of type metadata in serialized data.

Encoding Type IDs

ID	Name	Bits/Char	Character Set
0	UTF8	8	Any UTF-8 character
1	LOWER_SPECIAL	5	`a-z . _ $ \|`
2	LOWER_UPPER_DIGIT_SPECIAL	6	`a-z A-Z 0-9 . _`
3	FIRST_TO_LOWER_SPECIAL	5	First char uppercase, rest `a-z . _`
4	ALL_TO_LOWER_SPECIAL	5	`a-z A-Z . _` (uppercase escaped)

Character Mapping Tables

LOWER_SPECIAL (5 bits per character)

Character	Code (binary)	Code (decimal)
a-z	00000-11001	0-25
.	11010	26
_	11011	27
$	11100	28
\|	11101	29

Note: The | character is used as an escape sequence in ALL_TO_LOWER_SPECIAL encoding.

LOWER_UPPER_DIGIT_SPECIAL (6 bits per character)

Character	Code (binary)	Code (decimal)
a-z	000000-011001	0-25
A-Z	011010-110011	26-51
0-9	110100-111101	52-61
.	111110	62
_	111111	63

Encoding Algorithms

LOWER_SPECIAL Encoding

For strings containing only a-z, ., _, $, |:

function encode_lower_special(str):
    bits = []
    for char in str:
        bits.append(lookup_lower_special[char])  // 5 bits each

    // Pad to byte boundary
    total_bits = len(str) * 5
    padding_bits = (8 - (total_bits % 8)) % 8

    // First bit indicates if last char should be stripped (due to padding)
    strip_last = (padding_bits >= 5)
    if strip_last:
        prepend bit 1
    else:
        prepend bit 0

    return pack_bits_to_bytes(bits)

FIRST_TO_LOWER_SPECIAL Encoding

For strings like MyFieldName where only the first character is uppercase:

function encode_first_to_lower_special(str):
    // Convert first char to lowercase
    modified = str[0].lower() + str[1:]
    // Then use LOWER_SPECIAL encoding
    return encode_lower_special(modified)

ALL_TO_LOWER_SPECIAL Encoding

For strings with multiple uppercase characters like MyTypeName:

function encode_all_to_lower_special(str):
    result = ""
    for char in str:
        if char.is_upper():
            result += "|" + char.lower()  // Escape uppercase with |
        else:
            result += char
    return encode_lower_special(result)

Example: MyType → |my|type → encoded with LOWER_SPECIAL

Encoding Selection Algorithm

function choose_encoding(str):
    if all chars in str are in [a-z . _ $ |]:
        return LOWER_SPECIAL

    if first char is uppercase AND rest are in [a-z . _]:
        return FIRST_TO_LOWER_SPECIAL

    if all chars are in [a-z A-Z . _]:
        lower_special_size = encode_all_to_lower_special(str).size
        luds_size = encode_lower_upper_digit_special(str).size
        if lower_special_size <= luds_size:
            return ALL_TO_LOWER_SPECIAL
        else:
            return LOWER_UPPER_DIGIT_SPECIAL

    if all chars are in [a-z A-Z 0-9 . _]:
        return LOWER_UPPER_DIGIT_SPECIAL

    return UTF8

Meta String Header Format

Meta strings are written with a header that includes the encoding type:

| 3 bits encoding | 5+ bits length | encoded bytes |

Or for larger strings:

| varuint: (length << 3) | encoding | encoded bytes |

Special Character Sets by Context

Different contexts use different special characters:

Context	Special Chars	Notes
Field Name	. _ $ \|	$ for inner classes, \| for escape
Namespace	. _	Package/module separators
Type Name	$ _	$ for inner classes in Java

Deduplication

Meta strings are deduplicated within a serialization session:

First occurrence:  | (length << 1) | [hash if large] | encoding | bytes |
Reference:         | ((id + 1) << 1) | 1 |

Bit 0 of the header indicates: 0 = new string, 1 = reference to previous
Large strings (> 16 bytes) include 64-bit hash for content-based deduplication
Small strings use exact byte comparison

Value Format

Basic types

bool

size: 1 byte
format: 0 for false, 1 for true

int8

size: 1 byte
format: write as pure byte.

int16

size: 2 byte
byte order: raw bytes of little endian order

unsigned int32

size: 4 byte
byte order: raw bytes of little endian order

unsigned varint32

size: 1~5 bytes
Format: The most significant bit (MSB) in every byte indicates whether to have the next byte. If the continuation bit is set (i.e. b & 0x80 == 0x80), then the next byte should be read until a byte with unset continuation bit.

Encoding Algorithm:

function write_varuint32(value):
    while value >= 0x80:
        buffer.write_byte((value & 0x7F) | 0x80)  // 7 bits of data + continuation bit
        value = value >> 7
    buffer.write_byte(value)  // final byte without continuation bit

Decoding Algorithm:

function read_varuint32():
    result = 0
    shift = 0
    while true:
        byte = buffer.read_byte()
        result = result | ((byte & 0x7F) << shift)
        if (byte & 0x80) == 0:
            break
        shift = shift + 7
    return result

Byte sizes by value range:

Value Range	Bytes
0 ~ 127	1
128 ~ 16383	2
16384 ~ 2097151	3
2097152 ~ 268435455	4
268435456 ~ 4294967295	5

signed int32

size: 4 bytes
byte order: raw bytes of little endian order

signed varint32

size: 1~5 bytes
Format: First convert the number into positive unsigned int using ZigZag encoding, then encode as unsigned varint.

ZigZag Encoding:

// Encode: convert signed to unsigned
zigzag_value = (value << 1) ^ (value >> 31)

// Decode: convert unsigned back to signed
original = (zigzag_value >> 1) ^ (-(zigzag_value & 1))
// Or equivalently:
original = (zigzag_value >> 1) ^ (~(zigzag_value & 1) + 1)

ZigZag encoding maps signed integers to unsigned integers so that small absolute values (positive or negative) have small encoded values:

Original	ZigZag Encoded
0	0
-1	1
1	2
-2	3
2	4
...	...

unsigned int64

size: 8 bytes
byte order: raw bytes of little endian order

unsigned varint64

size: 1~9 bytes

Uses PVL (Progressive Variable-Length) encoding:

function write_varuint64(value):
    while value >= 0x80:
        buffer.write_byte((value & 0x7F) | 0x80)
        value = value >> 7
    buffer.write_byte(value)

Value Range	Bytes
0 ~ 127	1
128 ~ 16383	2
...	...
2^56 ~ 2^63-1	9

unsigned hybrid int64 (TAGGED_UINT64)

size: 4 or 9 bytes

Optimized for unsigned values that fit in 31 bits (common case for IDs, sizes, counts, etc.):

if value in [0, 2147483647]:  // fits in 31 bits (2^31 - 1), full unsigned range
    write 4 bytes: ((int32) value) << 1  // bit 0 is 0, indicating 4-byte encoding
else:
    write 1 byte:  0x01                  // bit 0 is 1, indicating 9-byte encoding
    write 8 bytes: value as little-endian uint64

Reading:

first_int32 = read_int32_le()
if (first_int32 & 1) == 0:
    return (uint64)(first_int32 >> 1)  // 4-byte encoding, unsigned
else:
    return read_uint64_le()            // read remaining 8 bytes

Note: TAGGED_UINT64 uses the full 31 bits for positive values [0, 2^31-1], compared to TAGGED_INT64 which splits the range for signed values [-2^30, 2^30-1].

VarUint36Small

A specialized encoding used for string headers that combines size (up to 36 bits) with encoding flags:

// Write: encodes (size << 2) | encoding_flags
function write_varuint36_small(value):
    if value < 0x80:
        buffer.write_byte(value)
    else:
        // Standard varint encoding for values >= 128
        write_varuint64(value)

This encoding is optimized for the common case where string length fits in 7 bits (strings < 32 characters).

signed int64

size: 8 bytes
byte order: raw bytes of little endian order

signed varint64

size: 1~9 bytes

Uses ZigZag encoding first, then PVL varint:

// Encode
zigzag_value = (value << 1) ^ (value >> 63)
write_varuint64(zigzag_value)

// Decode
zigzag_value = read_varuint64()
value = (zigzag_value >> 1) ^ (-(zigzag_value & 1))

signed hybrid int64 (TAGGED_INT64)

size: 4 or 9 bytes

Optimized for small signed values:

if value in [-1073741824, 1073741823]:  // fits in 30 bits + sign ([-2^30, 2^30-1])
    write 4 bytes: ((int32) value) << 1  // bit 0 is 0, indicating 4-byte encoding
else:
    write 1 byte:  0x01                  // bit 0 is 1, indicating 9-byte encoding
    write 8 bytes: value as little-endian int64

Reading:

first_int32 = read_int32_le()
if (first_int32 & 1) == 0:
    return (int64)(first_int32 >> 1)  // 4-byte encoding, sign-extended
else:
    return read_int64_le()            // read remaining 8 bytes

Note: TAGGED_INT64 uses 30 bits + sign for values [-2^30, 2^30-1], while TAGGED_UINT64 uses full 31 bits for unsigned values [0, 2^31-1].

float32

size: 4 byte
format: encode the specified floating-point value according to the IEEE 754 floating-point "single format" bit layout, preserving Not-a-Number (NaN) values, then write as binary by little endian order.

float64

size: 8 byte
format: encode the specified floating-point value according to the IEEE 754 floating-point "double format" bit layout, preserving Not-a-Number (NaN) values. then write as binary by little endian order.

string

Format:

| varuint36_small: (size << 2) | encoding | binary data |

String Header

The header is encoded using varuint36_small format, which combines the byte length and encoding type:

header = (byte_length << 2) | encoding_type

Encoding Type	Value	Description
LATIN1	0	ISO-8859-1 single-byte encoding
UTF16	1	UTF-16 encoding (2 bytes per code unit)
UTF8	2	UTF-8 variable-length encoding
Reserved	3	Reserved for future use

Encoding Algorithm

Writing:

function write_string(str):
    bytes = encode_to_bytes(str, chosen_encoding)
    header = (bytes.length << 2) | encoding_type
    buffer.write_varuint36_small(header)
    buffer.write_bytes(bytes)

Reading:

function read_string():
    header = buffer.read_varuint36_small()
    encoding = header & 0x03
    byte_length = header >> 2
    bytes = buffer.read_bytes(byte_length)
    return decode_bytes(bytes, encoding)

Encoding Selection by Language

Writing:

Language	Encoding Strategy
Java (JDK8)	Detect at runtime: LATIN1 if all chars < 256, else UTF16
Java (JDK9+)	Use String's internal coder: LATIN1 or UTF16
Python	Can write LATIN1, UTF16, or UTF8 based on string content
C++	UTF8 (`std::string`) or UTF16 (`std::u16string`)
Rust	UTF8 (`String`)
Go	UTF8 (`string`)
JavaScript	UTF8

Reading: All languages support decoding all three encodings (LATIN1, UTF16, UTF8).

Recommendation: Select encoding based on maximum performance - use the encoding that matches the language's native string representation to avoid conversion overhead.

Empty String

Empty strings are encoded with header 0 (length 0, any encoding) followed by no data bytes.

duration

Duration is an absolute length of time, independent of any calendar/timezone, as a count of seconds and nanoseconds.

Format:

| signed varint64: seconds | signed int32: nanoseconds |

seconds: Number of seconds in the duration, encoded as a signed varint64. Can be positive or negative.
nanoseconds: Nanosecond adjustment to the duration, encoded as a signed int32. Value range is [0, 999,999,999] for positive durations, and [-999,999,999, 0] for negative durations.

Notes:

The duration is stored as two separate fields to maintain precision and avoid overflow issues.
Seconds are encoded using varint64 for compact representation of common duration values.
Nanoseconds are stored as a fixed int32 since the range is limited.
The sign of the duration is determined by the seconds field. When seconds is 0, the sign is determined by nanoseconds.

collection/list

Format:

| varuint32: length | 1 byte elements header | [optional type info] | elements data |

Elements Header

The elements header is a single byte that encodes metadata about the collection elements to optimize serialization:

| bit 7-4 (reserved) |    bit 3    |      bit 2       |   bit 1  |   bit 0   |
+--------------------+-------------+------------------+----------+-----------+
|      reserved      | is_same_type| is_decl_elem_type| has_null | track_ref |

Bit	Name	Value	Meaning when SET (1)	Meaning when UNSET (0)
0	track_ref	0x01	Track references for elements	Don't track element references
1	has_null	0x02	Collection may contain null elements	No null elements (skip null checks)
2	is_decl_elem_type	0x04	Elements are the declared generic type	Element types differ from declared type
3	is_same_type	0x08	All elements have the same runtime type	Elements have different runtime types

Common header values:

Header	Hex	Meaning
0x0C	12	Declared type + same type, non-null, no ref tracking (optimal)
0x0D	13	Declared type + same type, non-null, with ref tracking
0x0E	14	Declared type + same type, may have nulls, no ref tracking
0x08	8	Same type but not declared type (type info written once)
0x00	0	Different types, non-null, no ref tracking (type per element)

Type Info After Header

When is_decl_elem_type (bit 2) is NOT set, the element type info is written once after the header if is_same_type (bit 3) is set:

| header (0x08) | type_id (varuint32) | elements... |

When both is_decl_elem_type and is_same_type are NOT set, type info is written per element.

Element Serialization Based on Header

The header determines how each element is serialized:

elements data

Based on the elements header, the serialization of elements data may skip ref flag/null flag/element type info.

fory = ...
buffer = ...
elems = ...
if element_type_is_same:
    if not is_declared_type:
        fory.write_type(buffer, elem_type)
    elem_serializer = get_serializer(...)
    if track_ref:
        for elem in elems:
            if not ref_resolver.write_ref_or_null(buffer, elem):
                elem_serializer.write(buffer, elem)
    elif has_null:
        for elem in elems:
            if elem is None:
                buffer.write_byte(null_flag)
            else:
                buffer.write_byte(not_null_flag)
                elem_serializer.write(buffer, elem)
    else:
        for elem in elems:
            elem_serializer.write(buffer, elem)
else:
    if track_ref:
        for elem in elems:
            fory.write_ref(buffer, elem)
    elif has_null:
        for elem in elems:
            fory.write_nullable(buffer, elem)
    else:
        for elem in elems:
            fory.write_value(buffer, elem)

CollectionSerializer#writeElements can be taken as an example.

array

primitive array

Primitive array are taken as a binary buffer, serialization will just write the length of array size as an unsigned int, then copy the whole buffer into the stream.

Such serialization won't compress the array. If users want to compress primitive array, users need to register custom serializers for such types or mark it as list type.

Tensor

Tensor is a special primitive multi-dimensional array which all dimensions have same size and type. The serialization format is:

| num_dims(unsigned varint) | shape[0](unsigned varint) | shape[...] | shape[N] | element type | data |

The data is continuous to reduce copy and may zero-copy in some cases.

object array

Object array is serialized using the list format. Object component type will be taken as list element generic type.

map

Map uses a chunk-based format to handle heterogeneous key-value pairs efficiently:

| varuint32: total_size | chunk_1 | chunk_2 | ... | chunk_n |

Map Chunk Format

Each chunk contains up to 255 key-value pairs with the same metadata characteristics:

|    1 byte    |     1 byte     |        variable bytes        |
+--------------+----------------+------------------------------+
|  KV header   |  chunk size N  |  N key-value pairs (N*2 obj) |

KV Header Bits

The KV header is a single byte encoding metadata for both keys and values:

|  bit 7-6   |     bit 5     |     bit 4    |     bit 3     |     bit 2     |     bit 1    |     bit 0     |
+------------+---------------+--------------+---------------+---------------+--------------+---------------+
|  reserved  | val_decl_type | val_has_null | val_track_ref | key_decl_type | key_has_null | key_track_ref |

Bit	Name	Value	Meaning when SET (1)
0	key_track_ref	0x01	Track references for keys
1	key_has_null	0x02	Keys may be null (rare, usually invalid)
2	key_decl_type	0x04	Key is the declared generic type
3	val_track_ref	0x08	Track references for values
4	val_has_null	0x10	Values may be null
5	val_decl_type	0x20	Value is the declared generic type

Common KV header values:

Header	Hex	Meaning
0x24	36	Key + value are declared types, non-null, no ref tracking (optimal)
0x2C	44	Key + value declared types, value tracks refs
0x34	52	Key + value declared types, value may be null
0x00	0	Key + value not declared types, non-null, no ref tracking

Chunk Size

Maximum chunk size: 255 pairs (fits in 1 byte)
When key or value is null, that entry is serialized as a separate chunk with implicit size 1 (chunk size byte is skipped)
Reader tracks accumulated count against total map size to know when to stop reading chunks

Why Chunk-Based Format?

Map iteration is expensive. Computing a single header for all pairs would require two passes. The chunk-based approach allows:

Optimistic prediction: Use first key-value pair to predict header
Adaptive chunking: Start new chunk if prediction fails for a pair
Efficient reading: Most maps fit in single chunk (< 255 pairs)
Memory efficiency: Minimal overhead for common homogeneous maps

Why serialize chunk by chunk?

When fory will use first key-value pair to predict header optimistically, it can't know how many pairs have same meta(tracking kef ref, key has null and so on). If we don't write chunk by chunk with max chunk size, we must write at least X bytes to take up a place for later to update the number which has same elements, X is the num_bytes for encoding varint encoding of map size.

And most map size are smaller than 255, if all pairs have same data, the chunk will be 1. This is common in golang/rust, which object are not reference by default.

Also, if only one or two keys have different meta, we can make it into a different chunk, so that most pairs can share meta.

The implementation can accumulate read count with map size to decide whether to read more chunks.

enum

Enums are serialized as an unsigned var int. If the order of enum values change, the deserialized enum value may not be the value users expect. In such cases, users must register enum serializer by make it write enum value as an enumerated string with unique hash disabled.

decimal

Not supported for now.

struct

Struct means object of class/pojo/struct/bean/record type. Struct will be serialized by writing its fields data in fory order.

Depending on schema compatibility, structs will have different formats.

field order

Field will be ordered as following, every group of fields will have its own order:

primitive fields:
- larger size type first, smaller later, variable size type last.
- when same size, sort by type id
- when same size and type id, sort by snake case field name
- types: bool/int8/int16/int32/var32/int64/var64/h64/float16/float32/float64
nullable primitive fields: same order as primitive fields
other internal type fields: sort by type id then snake case field name
list fields: sort by snake case field name
set fields: sort by snake case field name
map fields: sort by snake case field name
other fields: sort by snake case field name

If two fields have same type, then sort by snake_case styled field name.

schema consistent

Object will be written as:

|    4 byte     |  variable bytes  |
+---------------+------------------+
|   type hash   |   field values   |

Type hash is used to check the type schema consistency across languages. Type hash will be the first 32 bits of 56 bits value of the type meta.

Object fields will be serialized one by one using following format:

not null primitive field value:
|   var bytes    |
+----------------+
|   value data   |
+----------------+
nullable primitive field value:
| one byte  |   var bytes   |
+-----------+---------------+
| null flag |  field value  |
+-----------+---------------+
other interal types supported by fory
| var bytes | var objects |
+-----------+-------------+
| null flag | value data  |
+-----------+-------------+
list field type:
| one byte  | var objects |
+-----------+-------------+
| ref meta  | value data  |
set field type:
| one byte  | var objects |
+-----------+-------------+
| ref meta  | value data  |
map field type:
| one byte  | var objects |
+-----------+-------------+
| ref meta  | value data  |
+-----------+-------------+-------------+
other types such as enum/struct/ext
| one byte  | var bytes | var objects |
+-----------+------------+------------+
| ref  flag | type meta | value data |
+-----------+------------+------------+

Type hash algorithm:

Sort fields by fields sort algorithm
Start with string ""
Iterate every field, append string by:
- snow_case(field_name),. For camelcase name, convert it to snow_case first.
- $type_id,, for other fields, use type id TypeId::UNKNOWN instead.
- $nullable;, 1 if nullable, 0 otherwise.
Then convert string to utf8 bytes
Compute murmurhash3_x64_128, and use first 32 bits

Schema evolution

Schema evolution have similar format as schema consistent mode for object except:

For the object type, schema consistent mode will write type by id only, but schema evolution mode will write type consisting of field names, types and other meta too, see Type meta.
Type meta of final custom type needs to be written too, because peers may not have this type defined.

Type

Type will be serialized using type meta format.

Implementation guidelines

How to reduce memory read/write code

Try to merge multiple bytes into an int/long write before writing to reduce memory IO and bound check cost.
Read multiple bytes as an int/long, then split into multiple bytes to reduce memory IO and bound check cost.
Try to use one varint/long to write flags and length together to save one byte cost and reduce memory io.
Condition branches are less expensive compared to memory IO cost unless there are too many branches.

Fast deserialization for static languages without runtime codegen support

For type evolution, the serializer will encode the type meta into the serialized data. The deserializer will compare this meta with class meta in the current process, and use the diff to determine how to deserialize the data.

For java/javascript/python, we can use the diff to generate serializer code at runtime and load it as class/function for deserialization. In this way, the type evolution will be as fast as type consist mode.

For C++/Rust, we can't generate the serializer code at runtime. So we need to generate the code at compile-time using meta programming. But at that time, we don't know the type schema in other processes, so we can't generate the serializer code for such inconsistent types. We may need to generate the code which has a loop and compare field name one by one to decide whether to deserialize and assign the field or skip the field value.

One fast way is that we can optimize the string comparison into jump instructions:

Assume the current type has n fields, and the peer type has n1 fields.
Generate an auto growing field id from 0 for every sorted field in the current type at the compile time.
Compare the received type meta with current type, generate same id if the field name is same, otherwise generate an auto growing id starting from n, cache this meta at runtime.
Iterate the fields of received type meta, use a switch to compare the field id to deserialize data and assign/skip field value. Continuous field id will be optimized into jump in switch block, so it will very fast.

Here is an example, suppose process A has a class Foo with version 1 defined as Foo1, process B has a class Foo with version 2 defined as Foo2:

// class Foo with version 1
class Foo1 {
  int32_t v1; // id 0
  std::string v2; // id 1
};
// class Foo with version 2
class Foo2 {
  // id 0, but will have id 2 in process A
  bool v0;
  // id 1, but will have id 0 in process A
  int32_t v1;
  // id 2, but will have id 3 in process A
  int64_t long_value;
  // id 3, but will have id 1 in process A
  std::string v2;
  // id 4, but will have id 4 in process A
  std::vector<std::string> list;
};

When process A received serialized Foo2 from process B, here is how it deserialize the data:

Foo1 foo1 = ...;
const std::vector<fory::FieldInfo> &field_infos = type_meta.field_infos;
for (const auto &field_info : field_infos) {
  switch (field_info.field_id) {
    case 0:
      foo1.v1 = buffer.read_varint32();
      break;
    case 1:
      foo1.v2 = fory.read_string();
      break;
    default:
      fory.skip_data(field_info);
  }
}

Implementation Checklist for New Languages

This section provides a step-by-step guide for implementing Fory xlang serialization in a new language.

Phase 1: Core Infrastructure

Buffer Implementation
- Create a byte buffer with read/write cursor tracking
- Implement little-endian byte order for all multi-byte writes
- Implement write_int8, write_int16, write_int32, write_int64
- Implement write_float32, write_float64
- Implement read_* counterparts for all write methods
- Implement buffer growth strategy (e.g., doubling)
Varint Encoding
- Implement write_varuint32 / read_varuint32
- Implement write_varint32 / read_varint32 (with ZigZag)
- Implement write_varuint64 / read_varuint64
- Implement write_varint64 / read_varint64 (with ZigZag)
- Implement write_varuint36_small / read_varuint36_small (for strings)
- Optionally implement Hybrid encoding (TAGGED_INT64/TAGGED_UINT64) for int64
Header Handling
- Write/read bitmap flags (null, endian, xlang, oob)
- Write/read language ID
- Handle meta start offset placeholder (for schema evolution)

Phase 2: Basic Type Serializers

Primitive Types
- bool (1 byte: 0 or 1)
- int8, int16, int32, int64 (little endian)
- float32, float64 (IEEE 754, little endian)
String Serialization
- Implement string header: (byte_length << 2) | encoding
- Support UTF-8 encoding (required for xlang)
- Optionally support LATIN1 and UTF-16
Temporal Types
- Duration (seconds + nanoseconds)
- Timestamp (nanoseconds since epoch)
- LocalDate (days since epoch)
Reference Tracking
- Implement write-side object tracking (object → ref_id map)
- Implement read-side object tracking (ref_id → object list)
- Handle all four reference flags: NULL(-3), REF(-2), NOT_NULL(-1), REF_VALUE(0)
- Support disabling reference tracking per-type or globally

Phase 3: Collection Types

List/Array Serialization
- Write length as varuint32
- Write elements header byte
- Handle homogeneous vs heterogeneous elements
- Handle null elements
Map Serialization
- Write total size as varuint32
- Implement chunk-based format (max 255 pairs per chunk)
- Write KV header byte per chunk
- Handle key and value type variations
Set Serialization
- Same format as List (reuse implementation)

Phase 4: Meta String Encoding

Meta strings are required for enum and struct serialization (encoding field names, type names, namespaces).

Meta String Compression
- Implement LOWER_SPECIAL encoding (5 bits/char)
- Implement LOWER_UPPER_DIGIT_SPECIAL encoding (6 bits/char)
- Implement FIRST_TO_LOWER_SPECIAL encoding
- Implement ALL_TO_LOWER_SPECIAL encoding
- Implement encoding selection algorithm
- Implement meta string deduplication

Phase 5: Enum Serialization

Enum Serialization
- Write ordinal as varuint32
- Support named enum (namespace + type name)

Phase 6: Struct Serialization

Type Registration
- Support registration by numeric ID
- Support registration by namespace + type name
- Maintain type → serializer mapping
- Generate type IDs: (user_id << 8) | internal_type_id
Field Ordering
- Implement Fory field ordering algorithm
- Sort primitives by size (larger first), then type ID, then name
- Handle nullable vs non-nullable fields
- Convert field names to snake_case for sorting
Schema Consistent Mode
- Compute type hash (MurmurHash3 of field info string)
- Write 4-byte type hash before fields
- Serialize fields in Fory order
Schema Evolution Mode (Optional)
- Implement type meta writing
- Support field addition/removal
- Handle unknown fields (skip during read)

Phase 7: Other types

Binary/Array Types
- Primitive arrays (direct buffer copy)
- Tensor (multi-dimensional arrays)

Testing Strategy

Cross-Language Compatibility Tests
- Serialize in new language, deserialize in Java/Python
- Serialize in Java/Python, deserialize in new language
- Test all primitive types
- Test strings with various encodings
- Test collections (empty, single, multiple elements)
- Test maps with various key/value types
- Test nested structs
- Test circular references (if supported)

Language-Specific Implementation Notes

Java

Uses runtime code generation (JIT) for maximum performance
Supports all reference tracking modes
Uses internal String coder for encoding selection
Thread-safe via ThreadSafeFory wrapper

Python

Two modes: Pure Python (debugging) and Cython (performance)
Uses id(obj) for reference tracking
Latin1/UTF-16/UTF-8 encoding for all strings in xlang mode
dataclass support via code generation

C++

Compile-time reflection via macros (FORY_STRUCT, FORY_FIELD_INFO)
Template meta programming for type dispatch and serializer selection
Uses std::shared_ptr for reference tracking
Compile-time field ordering
No runtime code generation

Rust

Derive macros for automatic serialization (#[derive(ForyObject)])
Uses Rc<T> / Arc<T> for reference tracking
Thread-local context caching for performance
Compile-time field ordering

Go

Reflection-based and codegen-based modes
Struct tags for field annotations
Interface types for polymorphism

Common Pitfalls

Byte Order: Always use little-endian for multi-byte values
Varint Sign Extension: Ensure proper handling of signed vs unsigned varints
Reference ID Ordering: IDs must be assigned in serialization order
Field Order Consistency: Must match exactly across languages (schema consistent mode only; in evolution mode, deserialization follows serialization field order from type meta)
String Encoding: Use best encoding for current language
Null Handling: Different languages represent null differently
Empty Collections: Still write length (0) and header byte
Type Hash Calculation: Must use exact same algorithm across languages

Cross-language Serialization Specification​

Type Systems​

Data Types​

Polymorphisms​

Type disambiguation​

Type ID​

Internal Type ID Table​

Type ID Encoding for User Types​

Type mapping​

Spec overview​

Fory header​

Language IDs​

Meta Start Offset​

Reference Meta​

Reference Flags​

Reference Tracking Algorithm​

Reference ID Assignment​

When Reference Tracking is Disabled​

Language-Specific Considerations​

Type Meta​

Struct Schema consistent​

Struct Schema evolution​

Type Def​

Global binary header​

Meta header​

Fields meta​

Field Header​

Field Type Info​

Field Name​

Field order​

Extended Type Meta with Inheritance support​

Schema consistent​

Schema Evolution​

Meta header​

Single layer type meta​

Other layers type meta​

Meta String​

Encoding Type IDs​

Character Mapping Tables​

LOWER_SPECIAL (5 bits per character)​

LOWER_UPPER_DIGIT_SPECIAL (6 bits per character)​

Encoding Algorithms​

LOWER_SPECIAL Encoding​

FIRST_TO_LOWER_SPECIAL Encoding​

ALL_TO_LOWER_SPECIAL Encoding​

Encoding Selection Algorithm​

Meta String Header Format​

Special Character Sets by Context​

Deduplication​

Value Format​

Basic types​

bool​

int8​

int16​

unsigned int32​

unsigned varint32​

signed int32​

signed varint32​

unsigned int64​

unsigned varint64​

unsigned hybrid int64 (TAGGED_UINT64)​

VarUint36Small​

signed int64​

signed varint64​

signed hybrid int64 (TAGGED_INT64)​

float32​

float64​

string​

String Header​

Encoding Algorithm​

Encoding Selection by Language​

Empty String​

duration​

collection/list​

Elements Header​

Type Info After Header​

Element Serialization Based on Header​

elements data​

array​

primitive array​

Cross-language Serialization Specification

Type Systems

Data Types

Polymorphisms

Type disambiguation

Type ID

Internal Type ID Table

Type ID Encoding for User Types

Type mapping

Spec overview

Fory header

Language IDs

Meta Start Offset

Reference Meta

Reference Flags

Reference Tracking Algorithm

Reference ID Assignment

When Reference Tracking is Disabled

Language-Specific Considerations

Type Meta

Struct Schema consistent

Struct Schema evolution

Type Def

Global binary header

Meta header

Fields meta

Field Header

Field Type Info

Field Name

Field order

Extended Type Meta with Inheritance support

Schema consistent

Schema Evolution

Meta header

Single layer type meta

Other layers type meta

Meta String

Encoding Type IDs

Character Mapping Tables

LOWER_SPECIAL (5 bits per character)

LOWER_UPPER_DIGIT_SPECIAL (6 bits per character)

Encoding Algorithms

LOWER_SPECIAL Encoding

FIRST_TO_LOWER_SPECIAL Encoding

ALL_TO_LOWER_SPECIAL Encoding

Encoding Selection Algorithm

Meta String Header Format

Special Character Sets by Context

Deduplication

Value Format

Basic types

bool

int8

int16

unsigned int32

unsigned varint32

signed int32

signed varint32

unsigned int64

unsigned varint64

unsigned hybrid int64 (TAGGED_UINT64)

VarUint36Small

signed int64

signed varint64

signed hybrid int64 (TAGGED_INT64)

float32

float64

string

String Header

Encoding Algorithm

Encoding Selection by Language

Empty String

duration

collection/list

Elements Header

Type Info After Header

Element Serialization Based on Header

elements data

array

primitive array