mirror of https://github.com/status-im/nim-rlp.git
137 lines
4.8 KiB
Markdown
137 lines
4.8 KiB
Markdown
nim-rlp
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=======
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[](https://travis-ci.org/status-im/nim-rlp)
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A Nim implementation of the Recursive Length Prefix encoding (RLP) as specified
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in the Ethereum's [Yellow Papper](https://ethereum.github.io/yellowpaper/paper.pdf)
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and [Wiki](https://github.com/ethereum/wiki/wiki/RLP).
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## Reading RLP data
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The `Rlp` type provided by this library represents a cursor over a RLP-encoded
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byte stream. Before instantiating such a cursor, you must convert your
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input data to a `BytesRange` object, which represents an immutable and
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thus cheap-to-copy sub-range view over an underlying `seq[byte]` instance:
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``` nim
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proc initBytesRange*(s: var seq[byte], ibegin = 0, iend = -1): BytesRange
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proc rlpFromBytes*(data: BytesRange): Rlp
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```
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### Streaming API
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Once created, the `Rlp` object will offer procs such as `isList`, `isBlob`,
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`getType`, `listLen`, `blobLen` to determine the type of the value under
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the cursor. The contents of blobs can be extracted with procs such as
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`toString`, `toBytes` and `toInt` without advancing the cursor.
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Lists can be traversed with the standard `items` iterator, which will advance
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the cursor to each sub-item position and yield the `Rlp` object at that point.
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As an alternative, `listElem` can return a new `Rlp` object adjusted to a
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particular sub-item position without advancing the original cursor.
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Keep in mind that copying `Rlp` objects is cheap and you can create as many
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cursors pointing to different positions in the RLP stream as necessary.
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`skipElem` will advance the cursor to the next position in the current list.
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`hasData` will indicate that there are no more bytes in the stream that can
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be consumed.
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Another way to extract data from the stream is through the universal `read`
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proc that accepts a type as a parameter. You can pass any supported type
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such as `string`, `int`, `seq[T]`, etc, including composite user-defined
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types (see [Object Serialization](#object-serialization)). The cursor
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will be advanced just past the end of the consumed object.
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The `toXX` and `read` family of procs may raise a `BadCastError` in case
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of type mismatch with the stream contents under the cursor. A corrupted
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RLP stream or an attemp to read past the stream end will be signaled
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with the `MalformedRlpError` exception. If the RLP stream includes data
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that cannot be processed on the current platform (e.g. an integer value
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that is too large), the library will raise an `UnsupportedRlpError` exception.
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### DOM API
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Calling `Rlp.toNodes` at any position within the stream will return a tree
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of `RlpNode` objects representing the collection of values begging at that
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position:
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``` nim
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type
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RlpNodeType* = enum
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rlpBlob
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rlpList
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RlpNode* = object
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case kind*: RlpNodeType
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of rlpBlob:
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bytes*: BytesRange
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of rlpList:
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elems*: seq[RlpNode]
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```
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As a short-cut, you can also call `decode` directly on a byte sequence to
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avoid creating a `Rlp` object when obtaining the nodes.
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For debugging purposes, you can also create a human readable representation
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of the Rlp nodes by calling the `inspect` proc:
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``` nim
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proc inspect*(self: Rlp, indent = 0): string
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```
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## Creating RLP data
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The `RlpWriter` type can be used to encode RLP data. Instances are created
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with the `initRlpWriter` proc. This should be followed by one or more calls
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to `append` which is overloaded to accept arbitrary values. Finally, you can
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call `finish` to obtain the final `BytesRange`.
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If the end result should by a RLP list of particular length, you can replace
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the initial call to `initRlpWriter` with `initRlpList(n)`. Calling `finish`
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before writing a sufficient number of elements will then result in a
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`PrematureFinalizationError`.
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As an alternative short-cut, you can also call `encode` on an arbitrary value
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(including sequences and user-defined types) to execute all of the steps at
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once and directly obtain the final RLP bytes. `encodeList(varargs)` is another
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short-cut for creating RLP lists.
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## Object serialization
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As previously explained, generic procs such as `read`, `append`, `encode` and
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`decode` can be used with arbitrary used-defined object types. By default, the
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library will serialize all of the fields of the object using the `fields`
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iterator, but you can modify the order of serialization or include only a
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subset of the fields by using the `rlpFields` macro:
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``` nim
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macro rlpFields*(T: typedesc, fields: varargs[untyped])
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## example usage:
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type
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Transaction = object
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amount: int
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time: DateTime
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sender: string
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receiver: string
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rlpFields Transaction,
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sender, receiver, amount
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...
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var t1 = rlp.read(Transaction)
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var bytes = encode(t1)
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var t2 = bytes.decode(Transaction)
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```
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## Contributing / Testing
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To test the correctness of any modifications to the library, please execute
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`nimble test` at the root of the repo.
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## License
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This library is licensed under the Apache 2.0 license.
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