go-ethereum/rlp/doc.go

// Copyright 2014 The go-ethereum Authors
// This file is part of the go-ethereum library.
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// the Free Software Foundation, either version 3 of the License, or
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// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU Lesser General Public License for more details.
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/*
Package rlp implements the RLP serialization format.

The purpose of RLP (Recursive Linear Prefix) is to encode arbitrarily nested arrays of
binary data, and RLP is the main encoding method used to serialize objects in Ethereum.
The only purpose of RLP is to encode structure; encoding specific atomic data types (eg.
strings, ints, floats) is left up to higher-order protocols. In Ethereum integers must be
represented in big endian binary form with no leading zeroes (thus making the integer
value zero equivalent to the empty string).

RLP values are distinguished by a type tag. The type tag precedes the value in the input
stream and defines the size and kind of the bytes that follow.


Encoding Rules

Package rlp uses reflection and encodes RLP based on the Go type of the value.

If the type implements the Encoder interface, Encode calls EncodeRLP. It does not
call EncodeRLP on nil pointer values.

To encode a pointer, the value being pointed to is encoded. A nil pointer to a struct
type, slice or array always encodes as an empty RLP list unless the slice or array has
elememt type byte. A nil pointer to any other value encodes as the empty string.

Struct values are encoded as an RLP list of all their encoded public fields. Recursive
struct types are supported.

To encode slices and arrays, the elements are encoded as an RLP list of the value's
elements. Note that arrays and slices with element type uint8 or byte are always encoded
as an RLP string.

A Go string is encoded as an RLP string.

An unsigned integer value is encoded as an RLP string. Zero always encodes as an empty RLP
string. big.Int values are treated as integers. Signed integers (int, int8, int16, ...)
are not supported and will return an error when encoding.

Boolean values are encoded as the unsigned integers zero (false) and one (true).

An interface value encodes as the value contained in the interface.

Floating point numbers, maps, channels and functions are not supported.


Decoding Rules

Decoding uses the following type-dependent rules:

If the type implements the Decoder interface, DecodeRLP is called.

To decode into a pointer, the value will be decoded as the element type of the pointer. If
the pointer is nil, a new value of the pointer's element type is allocated. If the pointer
is non-nil, the existing value will be reused. Note that package rlp never leaves a
pointer-type struct field as nil unless one of the "nil" struct tags is present.

To decode into a struct, decoding expects the input to be an RLP list. The decoded
elements of the list are assigned to each public field in the order given by the struct's
definition. The input list must contain an element for each decoded field. Decoding
returns an error if there are too few or too many elements for the struct.

To decode into a slice, the input must be a list and the resulting slice will contain the
input elements in order. For byte slices, the input must be an RLP string. Array types
decode similarly, with the additional restriction that the number of input elements (or
bytes) must match the array's defined length.

To decode into a Go string, the input must be an RLP string. The input bytes are taken
as-is and will not necessarily be valid UTF-8.

To decode into an unsigned integer type, the input must also be an RLP string. The bytes
are interpreted as a big endian representation of the integer. If the RLP string is larger
than the bit size of the type, decoding will return an error. Decode also supports
*big.Int. There is no size limit for big integers.

To decode into a boolean, the input must contain an unsigned integer of value zero (false)
or one (true).

To decode into an interface value, one of these types is stored in the value:

	  []interface{}, for RLP lists
	  []byte, for RLP strings

Non-empty interface types are not supported when decoding.
Signed integers, floating point numbers, maps, channels and functions cannot be decoded into.


Struct Tags

As with other encoding packages, the "-" tag ignores fields.

    type StructWithIgnoredField struct{
        Ignored uint `rlp:"-"`
        Field   uint
    }

Go struct values encode/decode as RLP lists. There are two ways of influencing the mapping
of fields to list elements. The "tail" tag, which may only be used on the last exported
struct field, allows slurping up any excess list elements into a slice.

    type StructWithTail struct{
        Field   uint
        Tail    []string `rlp:"tail"`
    }

The "optional" tag says that the field may be omitted if it is zero-valued. If this tag is
used on a struct field, all subsequent public fields must also be declared optional.

When encoding a struct with optional fields, the output RLP list contains all values up to
the last non-zero optional field.

When decoding into a struct, optional fields may be omitted from the end of the input
list. For the example below, this means input lists of one, two, or three elements are
accepted.

   type StructWithOptionalFields struct{
        Required  uint
        Optional1 uint `rlp:"optional"`
        Optional2 uint `rlp:"optional"`
   }

The "nil", "nilList" and "nilString" tags apply to pointer-typed fields only, and change
the decoding rules for the field type. For regular pointer fields without the "nil" tag,
input values must always match the required input length exactly and the decoder does not
produce nil values. When the "nil" tag is set, input values of size zero decode as a nil
pointer. This is especially useful for recursive types.

    type StructWithNilField struct {
        Field *[3]byte `rlp:"nil"`
    }

In the example above, Field allows two possible input sizes. For input 0xC180 (a list
containing an empty string) Field is set to nil after decoding. For input 0xC483000000 (a
list containing a 3-byte string), Field is set to a non-nil array pointer.

RLP supports two kinds of empty values: empty lists and empty strings. When using the
"nil" tag, the kind of empty value allowed for a type is chosen automatically. A field
whose Go type is a pointer to an unsigned integer, string, boolean or byte array/slice
expects an empty RLP string. Any other pointer field type encodes/decodes as an empty RLP
list.

The choice of null value can be made explicit with the "nilList" and "nilString" struct
tags. Using these tags encodes/decodes a Go nil pointer value as the empty RLP value kind
defined by the tag.
*/
package rlp