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The encoding package converts arbitrary objects into byte slices, and vice versa. Objects are encoded as binary data, without type information. The decoder will attempt to decode its input bytes into whatever type it is passed. For example:

Marshal(int64(3))  ==  []byte{3, 0, 0, 0, 0, 0, 0, 0}

var x int64
Unmarshal([]byte{3, 0, 0, 0, 0, 0, 0, 0}, &x)
// x == 3

Note that this leads to some ambiguity. Since an int64 and a uint64 are both 8 bytes long, it is possible to encode an int64 and successfully decode it as a uint64. As a result, it is imperative that the decoder knows exactly what it is decoding. Developers must rely on context to determine what type to decode into.

The specific rules for encoding Go's builtin types are as follows:

Integers are little-endian, and are always encoded as 8 bytes, i.e. their int64 or uint64 equivalent.

Booleans are encoded as one byte, either zero (false) or one (true). No other values may be used.

Nil pointers are equivalent to "false," i.e. a single zero byte. Valid pointers are represented by a "true" byte (0x01) followed by the encoding of the dereferenced value.

Variable-length types, such as strings and slices, are represented by an 8-byte unsigned length prefix followed by the encoded value. Strings are encoded as their literal UTF-8 bytes. Slices are encoded as the concatenation of their encoded elements. For example:

//                                  slice len: 1     string len: 3   string data
Marshal([]string{"foo"}) == []byte{1,0,0,0,0,0,0,0, 3,0,0,0,0,0,0,0, 'f','o','o'}

Maps are not supported; attempting to encode a map will cause Marshal to panic. This is because their elements are not ordered in a consistent way, and it is imperative that this encoding scheme be deterministic. To encode a map, either convert it to a slice of structs, or define a MarshalSia method (see below).

Arrays and structs are simply the concatenation of their encoded elements. Byte slices are not subject to the 8-byte integer rule; they are encoded as their literal representation, one byte per byte.

All struct fields must be exported. (For some types this is a bit awkward, so this rule is subject to change.) The ordering of struct fields is determined by their type definition. For example:

type foo struct {
    S string
    I int

Marshal(foo{"bar", 3}) == append(Marshal("bar"), Marshal(3)...)

Finally, if a type implements the SiaMarshaler interface, its MarshalSia method will be used to encode the type. Similarly, if a type implements the SiaUnmarshal interface, its UnmarshalSia method will be used to decode the type. Note that unless a type implements both interfaces, it must conform to the spec above. Otherwise, it may encode and decode itself however desired. This may be an attractive option where speed is critical, since it allows for more compact representations, and bypasses the use of reflection.