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https://github.com/karalabe/ssz
Opinionated 0-alloc SSZ codec for Go
https://github.com/karalabe/ssz
encoder ethereum golang ssz
Last synced: 1 day ago
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Opinionated 0-alloc SSZ codec for Go
- Host: GitHub
- URL: https://github.com/karalabe/ssz
- Owner: karalabe
- License: bsd-3-clause
- Created: 2024-06-29T13:41:20.000Z (5 months ago)
- Default Branch: main
- Last Pushed: 2024-09-18T12:13:13.000Z (about 2 months ago)
- Last Synced: 2024-10-29T20:06:26.503Z (18 days ago)
- Topics: encoder, ethereum, golang, ssz
- Language: Go
- Homepage: https://github.com/ethereum/consensus-specs/blob/dev/ssz/simple-serialize.md
- Size: 254 KB
- Stars: 39
- Watchers: 3
- Forks: 7
- Open Issues: 8
-
Metadata Files:
- Readme: README.md
- License: COPYING
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README
# Simple Serialize (SSZ)... v15
[](https://pkg.go.dev/github.com/karalabe/ssz?tab=doc) [](https://github.com/karalabe/ssz/actions/workflows/tests.yml) [](https://codecov.io/github/karalabe/ssz)
Package `ssz` provides a zero-allocation, opinionated toolkit for working with Ethereum's [Simple Serialize (SSZ)](https://github.com/ethereum/consensus-specs/blob/dev/ssz/simple-serialize.md) format through Go. The focus is on code maintainability, only secondarily striving towards raw performance.
***Please note, this repository is a work in progress. The API is unstable and breaking changes will regularly be made. Do not depend on this in publicly available modules.***
*This package heavily inspired from the code generated by- and contained within [`fastssz`](https://github.com/ferranbt/fastssz)!*
## Goals and objectives
- **Elegant API surface:** Binary protocols are low level constructs and writing encoders/decoders entails boilerplate and fumbling with details. Code generators can do a good job in achieving performance, but with a too low level API, the generated code becomes impossible to humanly maintain. That isn't an issue, until you throw something at the generator it cannot understand (e.g. multiplexed types), at which point you'll be in deep pain. By defining an API that is elegant from a dev perspective, we can create maintainable code for the special snowflake types, yet still generate it for the rest of boring types.
- **Reduced redundancies:** The API aims to make the common case easy and the less common case possible. Redundancies in user encoding/decoding code are deliberately avoided to remove subtle bugs (even at a slight hit on performance). If the user's types require some asymmetry, explicit encoding and decoding code paths are still supported.
- **Support existing types:** Serialization libraries often assume the user is going to define a completely new, isolated type-set for all the things they want to encode. That is simply not the case, and to introduce a new encoding library into a pre-existing codebase, it must play nicely with the existing types. That means common Go typing and aliasing patterns should be supported without annotating everything with new methods.
- **Performant, as meaningful:** Encoding/decoding code should be performant, even if we're giving up some of it to cater for the above goals. Language constructs that are known to be slow (e.g. reflection) should be avoided, and code should have performance similar to low level generated ones, including 0 needing allocations. That said, a meaningful application of the library will *do* something with the encoded data, which will almost certainly take more time than generating/parsing a binary blob.
## Expectations
Whilst we aim to be a become the SSZ encoder of `go-ethereum` - and more generally, a go-to encoder for all Go applications requiring to work with Ethereum data blobs - there is no guarantee that this outcome will occur. At the present moment, this package is still in the design and experimentation phase and is not ready for a formal proposal.
There are several possible outcomes from this experiment:
- We determine the effort required to implement all SSZ features are not worth it, abandoning this package.
- All the needed features are shipped, but the package is rejected in favor of some other superior design.
- The API this package gets merged into some existing library and this work gets abandoned in its favor.
- The package turns out simple, safe and performant enough to be added to `go-ethereum` as a test.
- Some other unforeseen outcome of the infinite possibilities.
## Design
### Responsibilities
The `ssz` package splits the responsibility between user code and library code in the way pictured below:
- Users are responsible for creating Go structs, which are mapped one-to-one to the SSZ container type.
- The library is responsible for creating all other SSZ types from the fields of the user-defined structs.
- Some SSZ types require specific types to be used due to robustness and performance reasons.
- SSZ unions are not implemented as they are an unused (and disliked) feature in Ethereum.
### Weird stuff
The [Simple Serialize spec](https://github.com/ethereum/consensus-specs/blob/dev/ssz/simple-serialize.md) has schema definitions for mapping SSZ data to [JSON](https://github.com/ethereum/consensus-specs/blob/dev/ssz/simple-serialize.md#json-mapping). We believe in separation of concerns. This library does not concern itself with encoding/decoding from formats other than SSZ.
## How to use
First up, you need to add the package to your project:
```go
go get github.com/karalabe/ssz
```
### Static types
Some types in Ethereum will only contain a handful of statically sized fields. One example is a `Withdrawal`:
```go
type Address [20]byte
type Withdrawal struct {
Index uint64
Validator uint64
Address Address
Amount uint64
}
```
To encode/decode such an object via SSZ, it needs to implement the `ssz.StaticObject` interface:
```go
type StaticObject interface {
// SizeSSZ returns the total size of an SSZ object.
SizeSSZ() uint32
// DefineSSZ defines how an object would be encoded/decoded.
DefineSSZ(codec *Codec)
}
```
- The `SizeSSZ` seems self-explanatory. It returns the total size of the final SSZ, and for static types such as a `Withdrawal`, you need to calculate this by hand (or by a code generator, more on that later).
- The `DefineSSZ` is more involved. It expects you to define what fields, in what order and with what types are going to be encoded. Essentially, it's the serialization format.
```go
func (w *Withdrawal) SizeSSZ() uint32 { return 44 }
func (w *Withdrawal) DefineSSZ(codec *ssz.Codec) {
ssz.DefineUint64(codec, &w.Index) // Field (0) - Index - 8 bytes
ssz.DefineUint64(codec, &w.Validator) // Field (1) - ValidatorIndex - 8 bytes
ssz.DefineStaticBytes(codec, &w.Address) // Field (2) - Address - 20 bytes
ssz.DefineUint64(codec, &w.Amount) // Field (3) - Amount - 8 bytes
}
```
- The `DefineXYZ` methods should feel self-explanatory. They spill out what fields to encode in what order and into what types. The interesting tidbit is the addressing of the fields. Since this code is used for *both* encoding and decoding, it needs to be able to instantiate any `nil` fields during decoding, so pointers are needed.
To encode the above `Withdrawal` into an SSZ stream, use either `ssz.EncodeToStream` or `ssz.EncodeToBytes`. The former will write into a stream directly, whilst the latter will write into a bytes buffer directly. In both cases you need to supply the output location to avoid GC allocations in the library.
```go
func main() {
out := new(bytes.Buffer)
if err := ssz.EncodeToStream(out, new(Withdrawal)); err != nil {
panic(err)
}
fmt.Printf("ssz: %#x\n", blob)
}
```
To decode an SSZ blob, use `ssz.DecodeFromStream` and `ssz.DecodeFromBytes` with the same disclaimers about allocations. Note, decoding requires knowing the *size* of the SSZ blob in advance. Unfortunately, this is a limitation of the SSZ format.
### Dynamic types
Most data types in Ethereum will contain a cool mix of static and dynamic data fields. Encoding those is much more interesting, yet still proudly simple. One such a data type would be an `ExecutionPayload` as seen below:
```go
type Hash [32]byte
type LogsBLoom [256]byte
type ExecutionPayload struct {
ParentHash Hash
FeeRecipient Address
StateRoot Hash
ReceiptsRoot Hash
LogsBloom LogsBLoom
PrevRandao Hash
BlockNumber uint64
GasLimit uint64
GasUsed uint64
Timestamp uint64
ExtraData []byte
BaseFeePerGas *uint256.Int
BlockHash Hash
Transactions [][]byte
Withdrawals []*Withdrawal
}
```
Do note, we've reused the previously defined `Address` and `Withdrawal` types. You'll need those too to make this part of the code work. The `uint256.Int` type is from the `github.com/holiman/uint256` package.
To encode/decode such an object via SSZ, it needs to implement the `ssz.DynamicObject` interface:
```go
type DynamicObject interface {
// SizeSSZ returns either the static size of the object if fixed == true, or
// the total size otherwise.
SizeSSZ(fixed bool) uint32
// DefineSSZ defines how an object would be encoded/decoded.
DefineSSZ(codec *Codec)
}
```
If you look at it more closely, you'll notice that it's almost the same as `ssz.StaticObject`, except the type of `SizeSSZ` is different, here taking an extra boolean argument. The method name/type clash is deliberate: it guarantees compile time that dynamic objects cannot end up in static ssz slots and vice versa.
```go
func (e *ExecutionPayload) SizeSSZ(fixed bool) uint32 {
// Start out with the static size
size := uint32(512)
if fixed {
return size
}
// Append all the dynamic sizes
size += ssz.SizeDynamicBytes(e.ExtraData) // Field (10) - ExtraData - max 32 bytes (not enforced)
size += ssz.SizeSliceOfDynamicBytes(e.Transactions) // Field (13) - Transactions - max 1048576 items, 1073741824 bytes each (not enforced)
size += ssz.SizeSliceOfStaticObjects(e.Withdrawals) // Field (14) - Withdrawals - max 16 items, 44 bytes each (not enforced)
return size
}
```
Opposed to the static `Withdrawal` from the previous section, `ExecutionPayload` has both static and dynamic fields, so we can't just return a pre-computed literal number.
- First up, we will still need to know the static size of the object to avoid costly runtime calculations over and over. Just for reference, that would be the size of all the static fields in the object + 4 bytes for each dynamic field (offset encoding). Feel free to verify the number `512` above.
- If the caller requested only the static size via the `fixed` parameter, return early.
- If the caller, however, requested the total size of the object, we need to iterate over all the dynamic fields and accumulate all their sizes too.
- For all the usual Go suspects like slices and arrays of bytes; 2D sliced and arrays of bytes (i.e. `ExtraData` and `Transactions` above), there are helper methods available in the `ssz` package.
- For types implementing `ssz.StaticObject / ssz.DynamicObject` (e.g. one item of `Withdrawals` above), there are again helper methods available to use them as single objects, static array of objects, of dynamic slice of objects.
The codec itself is very similar to the static example before:
```go
func (e *ExecutionPayload) DefineSSZ(codec *ssz.Codec) {
// Define the static data (fields and dynamic offsets)
ssz.DefineStaticBytes(codec, &e.ParentHash) // Field ( 0) - ParentHash - 32 bytes
ssz.DefineStaticBytes(codec, &e.FeeRecipient) // Field ( 1) - FeeRecipient - 20 bytes
ssz.DefineStaticBytes(codec, &e.StateRoot) // Field ( 2) - StateRoot - 32 bytes
ssz.DefineStaticBytes(codec, &e.ReceiptsRoot) // Field ( 3) - ReceiptsRoot - 32 bytes
ssz.DefineStaticBytes(codec, &e.LogsBloom) // Field ( 4) - LogsBloom - 256 bytes
ssz.DefineStaticBytes(codec, &e.PrevRandao) // Field ( 5) - PrevRandao - 32 bytes
ssz.DefineUint64(codec, &e.BlockNumber) // Field ( 6) - BlockNumber - 8 bytes
ssz.DefineUint64(codec, &e.GasLimit) // Field ( 7) - GasLimit - 8 bytes
ssz.DefineUint64(codec, &e.GasUsed) // Field ( 8) - GasUsed - 8 bytes
ssz.DefineUint64(codec, &e.Timestamp) // Field ( 9) - Timestamp - 8 bytes
ssz.DefineDynamicBytesOffset(codec, &e.ExtraData, 32) // Offset (10) - ExtraData - 4 bytes
ssz.DefineUint256(codec, &e.BaseFeePerGas) // Field (11) - BaseFeePerGas - 32 bytes
ssz.DefineStaticBytes(codec, &e.BlockHash) // Field (12) - BlockHash - 32 bytes
ssz.DefineSliceOfDynamicBytesOffset(codec, &e.Transactions, 1_048_576, 1_073_741_824) // Offset (13) - Transactions - 4 bytes
ssz.DefineSliceOfStaticObjectsOffset(codec, &e.Withdrawals, 16) // Offset (14) - Withdrawals - 4 bytes
// Define the dynamic data (fields)
ssz.DefineDynamicBytesContent(codec, &e.ExtraData, 32) // Field (10) - ExtraData
ssz.DefineSliceOfDynamicBytesContent(codec, &e.Transactions, 1_048_576, 1_073_741_824) // Field (13) - Transactions
ssz.DefineSliceOfStaticObjectsContent(codec, &e.Withdrawals, 16) // Field (14) - Withdrawals
}
```
Most of the `DefineXYZ` methods are similar as before. However, you might spot two distinct sets of method calls, `DefineXYZOffset` and `DefineXYZContent`. You'll need to use these for dynamic fields:
- When SSZ encodes a dynamic object, it encodes it in two steps.
- A 4-byte offset pointing to the dynamic data is written into the static SSZ area.
- The dynamic object's actual encoding are written into the dynamic SSZ area.
- Encoding libraries can take two routes to handle this scenario:
- Explicitly require the user to give one command to write the object offset, followed by another command later to write the object content. This is fast, but leaks out encoding detail into user code.
- Require only one command from the user, under the hood writing the object offset immediately, and stashing the object itself away for later serialization when the dynamic area is reached. This keeps the offset notion hidden from users, but entails a GC hit to the encoder.
- This package was decided to be allocation free, thus the user is needs to be aware that they need to define the dynamic offset first and the dynamic content later. It's a tradeoff to achieve 50-100% speed increase.
- You might also note that dynamic fields also pass in size limits, in two places nonetheless. This is an unfortunate asymmetry in the SSZ spec wrt encoding and hashing data layouts.
- During encoding/decoding, dynamic data is placed at the end of the SSZ blob, so limits need to be passed to the `DefineXYZContent` methods.
- During hashing, dynamic data is merkleized inline, mixed with static data, so limits need to be passed to the `DefineXYZOffset` methods.
- This is a bit unfortunate. Either parameter set could be avoided at the cost of internal tracking, but that would break 0-alloc.
To encode the above `ExecutionPayload` do just as we have done with the static `Withdrawal` object.
### Asymmetric types
For types defined in perfect isolation - dedicated for SSZ - it's easy to define the fields with the perfect types, and perfect sizes, and perfect everything. Generating or writing an elegant encoder for those, is easy.
In reality, often you'll need to encode/decode types which already exist in a codebase, which might not map so cleanly onto the SSZ defined structure spec you want (e.g. you have one union type of `ExecutionPayload` that contains all the Bellatrix, Capella, Deneb, etc fork fields together) and you want to encode/decode them differently based on the context.
Most SSZ libraries will not permit you to do such a thing. Reflection based libraries *cannot* infer the context in which they should switch encoders and can neither can they represent multiple encodings at the same time. Generator based libraries again have no meaningful way to specify optional fields based on different constraints and contexts.
The only way to handle such scenarios is to write the encoders by hand, and furthermore, encoding might be dependent on what's in the struct, whilst decoding might be dependent on what's it contained within. Completely asymmetric, so our unified *codec definition* approach from the previous sections cannot work.
For these scenarios, this package has support for asymmetric encoders/decoders, where the caller can independently implement the two paths with their unique quirks.
To avoid having a real-world example's complexity overshadow the point we're trying to make here, we'll just convert the previously demoed `Withdrawal` encoding/decoding from the unified `codec` version to a separate `encoder` and `decoder` version.
```go
func (w *Withdrawal) DefineSSZ(codec *ssz.Codec) {
codec.DefineEncoder(func(enc *ssz.Encoder) {
ssz.EncodeUint64(enc, w.Index) // Field (0) - Index - 8 bytes
ssz.EncodeUint64(enc, w.Validator) // Field (1) - ValidatorIndex - 8 bytes
ssz.EncodeStaticBytes(enc, &w.Address) // Field (2) - Address - 20 bytes
ssz.EncodeUint64(enc, w.Amount) // Field (3) - Amount - 8 bytes
})
codec.DefineDecoder(func(dec *ssz.Decoder) {
ssz.DecodeUint64(dec, &w.Index) // Field (0) - Index - 8 bytes
ssz.DecodeUint64(dec, &w.Validator) // Field (1) - ValidatorIndex - 8 bytes
ssz.DecodeStaticBytes(dec, &w.Address) // Field (2) - Address - 20 bytes
ssz.DecodeUint64(dec, &w.Amount) // Field (3) - Amount - 8 bytes
})
}
```
- As you can see, we piggie-back on the already existing `ssz.Object`'s `DefineSSZ` method, and do *not* require implementing new functions. This is good because we want to be able to seamlessly use unified or split encoders without having to tell everyone about it.
- Whereas previously we had a bunch of `DefineXYZ` method to enumerate the fields for the unified encoding/decoding, here we replaced them with separate definitions for the encoder and decoder via `codec.DefineEncoder` and `codec.DefineDecoder`.
- The implementation of the encoder and decoder follows the exact same pattern and naming conventions as with the `codec` but instead of operating on a `ssz.Codec` object, we're operating on an `ssz.Encoder`/`ssz.Decoder` objects; and instead of calling methods named `ssz.DefineXYZ`, we're calling methods named `ssz.EncodeXYZ` and `ssz.DecodeXYZ`.
- Perhaps note, the `EncodeXYZ` methods do not take pointers to everything anymore, since they do not require the ability to instantiate the field during operation. Still, static bytes are passed by pointer to avoid heavy copy overheads for large arrays.
Encoding the above `Withdrawal` into an SSZ stream, you use the same thing as before. Everything is seamless.
### Checked types
If your types are using strongly typed arrays (e.g. `[32]byte`, and not `[]byte`) for static lists, the above codes work just fine. However, some types might want to use `[]byte` as the field type, but have it still *behave* as if it was `[32]byte`. This poses an issue, because if the decoder only sees `[]byte`, it cannot figure out how much data you want to decode into it. For those scenarios, we have *checked methods*.
The previous `Withdrawal` is a good example. Let's replace the `type Address [20]byte` alias, with a plain `[]byte` slice (not a `[20]byte` array, rather an opaque `[]byte` slice).
```go
type Withdrawal struct {
Index uint64
Validator uint64
Address []byte
Amount uint64
}
```
The code for the `SizeSSZ` remains the same. The code for `DefineSSZ` changes ever so slightly:
```go
func (w *Withdrawal) DefineSSZ(codec *ssz.Codec) {
ssz.DefineUint64(codec, &w.Index) // Field (0) - Index - 8 bytes
ssz.DefineUint64(codec, &w.Validator) // Field (1) - ValidatorIndex - 8 bytes
ssz.DefineCheckedStaticBytes(codec, &w.Address, 20) // Field (2) - Address - 20 bytes
ssz.DefineUint64(codec, &w.Amount) // Field (3) - Amount - 8 bytes
}
```
Notably, the `ssz.DefineStaticBytes` call from our old code (which got given a `[20]byte` array), is replaced with `ssz.DefineCheckedStaticBytes`. The latter method operates on an opaque `[]byte` slice, so if we want it to behave like a static sized list, we need to tell it how large it's needed to be. This will result in a runtime check to ensure that the size is correct before decoding.
Note, *checked methods* entail a runtime cost. When decoding such opaque slices, we can't blindly fill the fields with data, rather we need to ensure that they are allocated and that they are of the correct size. Ideally only use *checked methods* for prototyping or for pre-existing types where you just have to run with whatever you have and can't change the field to an array.
### Monolithic types
We've seen previously, that asymmetric codecs can be used to implement custom serialization logic for types that might encode in a variety of ways depending on their data content.
One verify specific subset of that scenario is the Ethereum consensus typeset. Whenever a new fork is released, a number of types are slightly modified, usually by adding new fields to existing structs. In the beacon specs, this usually results in an explosion of types: a new base type for fork X is created (e.g. `BeaconBlockBodyBellatrix`), but any type including that also needs to be re-created for fork X (e.g. `BeaconBlockBellatrix`), resulting in cascading type creations. Point in case, there are [79 consensus types in Prysm](https://pkg.go.dev/github.com/prysmaticlabs/prysm/[email protected]/proto/eth/v2#pkg-types), most of which are copies of one another with tiny additions.
This design is definitely clean and works well if these containers are used just as data transmission objects or storage objects. However, operating on hundreds of types storing the same thing in a live codebase is unwieldy. In [go-ethereum](https://github.com/ethereum/go-ethereum) we've always used monolithic types that encode just right according to the RLP specs of EL forks and thus this library aims to provide similar support for the SSZ world too.
We define a *monolithic type* as a container that can be encoded/decoded differently, based on what fork the codec runs in. To give an example, let's look at the previous `ExecutionPayload`, but instead of using it to represent a single possible consensus form, let's define *all* possible fields across all possible forks:
```go
type ExecutionPayloadMonolith struct {
ParentHash Hash
FeeRecipient Address
StateRoot Hash
ReceiptsRoot Hash
LogsBloom LogsBLoom
PrevRandao Hash
BlockNumber uint64
GasLimit uint64
GasUsed uint64
Timestamp uint64
ExtraData []byte
BaseFeePerGas *uint256.Int
BlockHash Hash
Transactions [][]byte
Withdrawals []*Withdrawal // Appears in the Shanghai fork
BlobGasUsed *uint64 // Appears in the Cancun fork
ExcessBlobGas *uint64 // Appears in the Cancun fork
}
```
Not much difference versus what we've used previously, but note, the fields that are fork-specific must all be nil-able (`Withdrawal` is a slice that can be `nil` and the blob gas fields are `*uint64`, which again can be `nil`).
Like before, we need to implement the `SizeSSZ` method:
```go
func (e *ExecutionPayloadMonolith) SizeSSZ(sizer *ssz.Sizer, fixed bool) uint32 {
// Start out with the static size
size := uint32(512)
if sizer.Fork() >= ssz.ForkShanghai {
size += 4
}
if sizer.Fork() >= ssz.ForkCancun {
size += 16
}
if fixed {
return size
}
// Append all the dynamic sizes
size += ssz.SizeDynamicBytes(sizer, obj.ExtraData)
size += ssz.SizeSliceOfDynamicBytes(sizer, obj.Transactions)
if sizer.Fork() >= ssz.ForkShanghai {
size += ssz.SizeSliceOfStaticObjects(sizer, obj.Withdrawals)
}
return size
}
```
This time, it was a bit more complex:
- The static size can change depending on which fork we're encoding into. The base Frontier encoding is 512 bytes, but Shanghai adds the dynamic withdrawals (4 bytes static offset) and Cancun adds 2 static uint64s (2x8 bytes). You can retrieve what fork we're encoding into via the `ssz.Sizer` method argument.
- The dynamic size can change just the same, if we're encoding into Shanghai, we need to account for the withdrawals too. The uint64s are not dynamic, so they don't appear in the that section of the size.
Similarly to how `SizeSSZ` needs to be fork-enabled, `DefineSSZ` goes through a transformation:
```go
func (obj *ExecutionPayloadMonolith) DefineSSZ(codec *ssz.Codec) {
// Define the static data (fields and dynamic offsets)
ssz.DefineStaticBytes(codec, &obj.ParentHash) // Field ( 0) - ParentHash - 32 bytes
ssz.DefineStaticBytes(codec, &obj.FeeRecipient) // Field ( 1) - FeeRecipient - 20 bytes
ssz.DefineStaticBytes(codec, &obj.StateRoot) // Field ( 2) - StateRoot - 32 bytes
ssz.DefineStaticBytes(codec, &obj.ReceiptsRoot) // Field ( 3) - ReceiptsRoot - 32 bytes
ssz.DefineStaticBytes(codec, &obj.LogsBloom) // Field ( 4) - LogsBloom - 256 bytes
ssz.DefineStaticBytes(codec, &obj.PrevRandao) // Field ( 5) - PrevRandao - 32 bytes
ssz.DefineUint64(codec, &obj.BlockNumber) // Field ( 6) - BlockNumber - 8 bytes
ssz.DefineUint64(codec, &obj.GasLimit) // Field ( 7) - GasLimit - 8 bytes
ssz.DefineUint64(codec, &obj.GasUsed) // Field ( 8) - GasUsed - 8 bytes
ssz.DefineUint64(codec, &obj.Timestamp) // Field ( 9) - Timestamp - 8 bytes
ssz.DefineDynamicBytesOffset(codec, &obj.ExtraData, 32) // Offset (10) - ExtraData - 4 bytes
ssz.DefineUint256(codec, &obj.BaseFeePerGas) // Field (11) - BaseFeePerGas - 32 bytes
ssz.DefineStaticBytes(codec, &obj.BlockHash) // Field (12) - BlockHash - 32 bytes
ssz.DefineSliceOfDynamicBytesOffset(codec, &obj.Transactions, 1048576, 1073741824) // Offset (13) - Transactions - 4 bytes
ssz.DefineSliceOfStaticObjectsOffsetOnFork(codec, &obj.Withdrawals, 16, ssz.ForkFilter{Added: ssz.ForkShanghai}) // Offset (14) - Withdrawals - 4 bytes
ssz.DefineUint64PointerOnFork(codec, &obj.BlobGasUsed, ssz.ForkFilter{Added: ssz.ForkCancun}) // Field (15) - BlobGasUsed - 8 bytes
ssz.DefineUint64PointerOnFork(codec, &obj.ExcessBlobGas, ssz.ForkFilter{Added: ssz.ForkCancun}) // Field (16) - ExcessBlobGas - 8 bytes
// Define the dynamic data (fields)
ssz.DefineDynamicBytesContent(codec, &obj.ExtraData, 32) // Field (10) - ExtraData - ? bytes
ssz.DefineSliceOfDynamicBytesContentOnFork(codec, &obj.Transactions, 1048576, 1073741824) // Field (13) - Transactions - ? bytes
ssz.DefineSliceOfStaticObjectsContentOnFork(codec, &obj.Withdrawals, 16, ssz.ForkFilter{Added: ssz.ForkShanghai}) // Field (14) - Withdrawals - ? bytes
}
```
The above code is eerily similar to our previous codec, yet, weirdly strange. Wherever fork specific fields appear, the methods get suffixed with `OnFork` and get passed the rule as to which fork to apply in (e.g. `ssz.ForkFilter{Added: ssz.ForkCancun}`). There are good reasons for both:
- The `SizeSSZ` method used `if` clauses to check for forks and behaved differently based on which fork we're in. That is clean, however decoding has a quirk: if we decode into a pre-existing object (with fields set to arbitrary junk), the fields not present in a fork needs to be nil-ed out. As such, `if` clauses within the definitions won't work any more, we need to "define" missing fields too to ensure they get nil-ed correctly. Thus `OnFork` suffix for all fields, always.
- Of course, calling an `OnFork` method it kind of pointless without specifying which fork we want a field to be present in. That's the `ssz.ForkFilter` parameter. By making it a slightly more complex filter type, the SSZ library supports both adding new fields in a fork, and also removing old fields (both cases happened in the beacon chain). Other operations will be added as needed.
Lastly, to encode the above `ExecutionPayloadMonolith` into an SSZ stream, we can't use the tried and proven `ssz.EncodeToStream`, since that will not know what fork we'd like to use. Rather, again, we need to call an `OnFork` version:
```go
func main() {
out := new(bytes.Buffer)
if err := ssz.EncodeToStreamOnFork(out, new(ExecutionPayloadMonolith), ssz.ForkCancun); err != nil {
panic(err)
}
fmt.Printf("ssz: %#x\n", blob)
}
```
*As a side emphasis, although the SSZ library has the Ethereum hard-forks included (e.g. `ssz.ForkCancun` and `ssz.ForkDeneb`), there is nothing stopping a user of the library from using their own fork enum (e.g. `mypkg.ForkAlice` and `mypkg.ForkBob`), just type it with `ssz.Fork` and make sure `0` means some variation of `unknown`/`present in all forks`*.
## Generated encoders
More often than not, the Go structs that you'd like to serialize to/from SSZ are simple data containers. Without some particular quirk you'd like to explicitly support, there's little reason to spend precious time counting the bits and digging through a long list of encoder methods to call.
For those scenarios, the library also supports generating the encoding/decoding code via a Go command:
```go
go run github.com/karalabe/ssz/cmd/sszgen --help
```
### Inferred field sizes
Let's go back to our very simple `Withdrawal` type from way back.
```go
type Withdrawal struct {
Index uint64
Validator uint64
Address [20]byte
Amount uint64
}
```
This seems like a fairly simple thing that we should be able to automatically generate a codec for. Let's try:
```
go run github.com/karalabe/ssz/cmd/sszgen --type Withdrawal
```
Calling the generator on this type will produce the following (very nice I might say) code:
```go
// Code generated by github.com/karalabe/ssz. DO NOT EDIT.
package main
import "github.com/karalabe/ssz"
// SizeSSZ returns the total size of the static ssz object.
func (obj *Withdrawal) SizeSSZ() uint32 {
return 8 + 8 + 20 + 8
}
// DefineSSZ defines how an object is encoded/decoded.
func (obj *Withdrawal) DefineSSZ(codec *ssz.Codec) {
ssz.DefineUint64(codec, &obj.Index) // Field (0) - Index - 8 bytes
ssz.DefineUint64(codec, &obj.Validator) // Field (1) - Validator - 8 bytes
ssz.DefineStaticBytes(codec, &obj.Address) // Field (2) - Address - 20 bytes
ssz.DefineUint64(codec, &obj.Amount) // Field (3) - Amount - 8 bytes
}
```
It has everything we would have written ourselves: `SizeSSZ` and `DefineSSZ`... and it also has a lot of useful comments we for sure wouldn't have written outselves. Generator for the win!
Ok, but this was too easy. All the fields of the `Withdrawal` object were primitive types of known lengths, so there's no heavy lifting involved at all. Lets take a look at a juicier example.
### Explicit field sizes
For our complex test, lets pick our dynamic `ExecutionPayload` type from before, but lets make it as hard as it gets and remove all size information from the Go types (e.g. instead of using `[32]byte`, we can make it extra hard by using `[]byte` only).
Now, obviously, if we were to write serialization code by hand, we'd take advantage of our knowledge of what each of these fields is semantically, so we could provide the necessary sizes for a decoder to use. If we want to, however, generate the serialization code, we need to share all that "insider-knowledge" with the code generator somehow.
The standard way in Go world is through struct tags. Specifically in the context of this library, it will be through the `ssz-size` and `ssz-max` tags. These follow the convention set previously by other Go SSZ libraries;
- `ssz-size` can be used to declare a field having a static size
- `ssz-max` can be used to declare a field having a dynamic size with a size cap.
- Both tags support multiple dimensions via comma-separation and omitting via `?`
```go
type ExecutionPayload struct {
ParentHash []byte `ssz-size:"32"`
FeeRecipient []byte `ssz-size:"32"`
StateRoot []byte `ssz-size:"20"`
ReceiptsRoot []byte `ssz-size:"32"`
LogsBloom []byte `ssz-size:"256"`
PrevRandao []byte `ssz-size:"32"`
BlockNumber uint64
GasLimit uint64
GasUsed uint64
Timestamp uint64
ExtraData []byte `ssz-max:"32"`
BaseFeePerGas *uint256.Int
BlockHash []byte `ssz-size:"32"`
Transactions [][]byte `ssz-max:"1048576,1073741824"`
Withdrawals []*Withdrawal `ssz-max:"16"`
}
```
Calling the generator as before, just with the `ExecutionPayload` yields the below, fork-enhanced code:
```go
// Code generated by github.com/karalabe/ssz. DO NOT EDIT.
package main
import "github.com/karalabe/ssz"
// SizeSSZ returns either the static size of the object if fixed == true, or
// the total size otherwise.
func (obj *ExecutionPayload) SizeSSZ(fixed bool) uint32 {
var size = uint32(32 + 32 + 20 + 32 + 256 + 32 + 8 + 8 + 8 + 8 + 4 + 32 + 32 + 4 + 4)
if fixed {
return size
}
size += ssz.SizeDynamicBytes(obj.ExtraData)
size += ssz.SizeSliceOfDynamicBytes(obj.Transactions)
size += ssz.SizeSliceOfStaticObjects(obj.Withdrawals)
return size
}
// DefineSSZ defines how an object is encoded/decoded.
func (obj *ExecutionPayload) DefineSSZ(codec *ssz.Codec) {
// Define the static data (fields and dynamic offsets)
ssz.DefineCheckedStaticBytes(codec, &obj.ParentHash, 32) // Field ( 0) - ParentHash - 32 bytes
ssz.DefineCheckedStaticBytes(codec, &obj.FeeRecipient, 32) // Field ( 1) - FeeRecipient - 32 bytes
ssz.DefineCheckedStaticBytes(codec, &obj.StateRoot, 20) // Field ( 2) - StateRoot - 20 bytes
ssz.DefineCheckedStaticBytes(codec, &obj.ReceiptsRoot, 32) // Field ( 3) - ReceiptsRoot - 32 bytes
ssz.DefineCheckedStaticBytes(codec, &obj.LogsBloom, 256) // Field ( 4) - LogsBloom - 256 bytes
ssz.DefineCheckedStaticBytes(codec, &obj.PrevRandao, 32) // Field ( 5) - PrevRandao - 32 bytes
ssz.DefineUint64(codec, &obj.BlockNumber) // Field ( 6) - BlockNumber - 8 bytes
ssz.DefineUint64(codec, &obj.GasLimit) // Field ( 7) - GasLimit - 8 bytes
ssz.DefineUint64(codec, &obj.GasUsed) // Field ( 8) - GasUsed - 8 bytes
ssz.DefineUint64(codec, &obj.Timestamp) // Field ( 9) - Timestamp - 8 bytes
ssz.DefineDynamicBytesOffset(codec, &obj.ExtraData, 32) // Offset (10) - ExtraData - 4 bytes
ssz.DefineUint256(codec, &obj.BaseFeePerGas) // Field (11) - BaseFeePerGas - 32 bytes
ssz.DefineCheckedStaticBytes(codec, &obj.BlockHash, 32) // Field (12) - BlockHash - 32 bytes
ssz.DefineSliceOfDynamicBytesOffset(codec, &obj.Transactions, 1048576, 1073741824) // Offset (13) - Transactions - 4 bytes
ssz.DefineSliceOfStaticObjectsOffset(codec, &obj.Withdrawals, 16) // Offset (14) - Withdrawals - 4 bytes
// Define the dynamic data (fields)
ssz.DefineDynamicBytesContent(codec, &obj.ExtraData, 32) // Field (10) - ExtraData - ? bytes
ssz.DefineSliceOfDynamicBytesContent(codec, &obj.Transactions, 1048576, 1073741824) // Field (13) - Transactions - ? bytes
ssz.DefineSliceOfStaticObjectsContent(codec, &obj.Withdrawals, 16) // Field (14) - Withdrawals - ? bytes
}
```
Points of interests to note:
- The generator realized that this type contains dynamic fields (either through `ssz-max` tags or via embedded dynamic objects), so it generated an implementation for `ssz.DynamicObject` (vs. `ssz.StaticObject` in the previous section).
- The generator took into consideration all the size `ssz-size` and `ssz-max` fields to generate serialization calls with different based types and runtime size checks.
- *Note, it is less performant to have runtime size checks like this, so if you know the size of a field, arrays are always preferable vs dynamic lists.*
### Cross-validated field sizes
We've seen that the size of a field can either be deduced automatically, or it can be provided to the generator explicitly. But what happens if we provide an ssz struct tag for a field of known size?
```go
type Withdrawal struct {
Index uint64 `ssz-size:"8"`
Validator uint64 `ssz-size:"8"`
Address [20]byte `ssz-size:"32"` // Deliberately wrong tag size
Amount uint64 `ssz-size:"8"`
}
```
```go
go run github.com/karalabe/ssz/cmd/sszgen --type Withdrawal
failed to validate field Withdrawal.Address: array of byte basic type tag conflict: field is 20 bytes, tag wants [32] bytes
```
The code generator will take into consideration the information in both the field's Go type and the struct tag, and will cross validate them against each other. If there's a size conflict, it will abort the code generation.
This functionality can be very helpful in detecting refactor issues, where the user changes the type of a field, which would result in a different encoding. By having the field tagged with an `ssz-size`, such an error would be detected.
As such, we'd recommend *always* tagging all SSZ encoded fields with their sizes. It results in both safer code and self-documenting code.
### Monolithic types
This library supports monolithic types that encode differently based on what fork the codec is operating in. Naturally, that is a perfect example of something that would be useful to be able to generate, and indeed, can do.
- Monolithic type fields can be tagged with a `ssz-fork:"name"` Go struct tag, which will be picked up and mapped by the code generator from their textual form to pre-declared fork identifiers.
- The fork names follow the Go build constraint rules:
- A field can be declared introduced in fork `X` via `ssz-fork:"x"`.
- A field can be declared removed in fork `X` via `ssz-fork:"!x"`.
```go
type ExecutionPayloadMonolith struct {
ParentHash Hash
FeeRecipient Address
StateRoot Hash
ReceiptsRoot Hash
LogsBloom LogsBloom
PrevRandao Hash
BlockNumber uint64
GasLimit uint64
GasUsed uint64
Timestamp uint64
ExtraData []byte `ssz-max:"32"`
BaseFeePerGas *uint256.Int
BlockHash Hash
Transactions [][]byte `ssz-max:"1048576,1073741824"`
Withdrawals []*Withdrawal `ssz-max:"16" ssz-fork:"shanghai"`
BlobGasUsed *uint64 ` ssz-fork:"cancun"`
ExcessBlobGas *uint64 ` ssz-fork:"cancun"`
}
```
Calling the generator as before, just with the `ExecutionPayloadMonolith` yields the below, much more interesting code:
```go
// Code generated by github.com/karalabe/ssz. DO NOT EDIT.
package main
import "github.com/karalabe/ssz"
// SizeSSZ returns either the static size of the object if fixed == true, or
// the total size otherwise.
func (obj *ExecutionPayloadMonolith) SizeSSZ(sizer *ssz.Sizer, fixed bool) (size uint32) {
size = 32 + 20 + 32 + 32 + 256 + 32 + 8 + 8 + 8 + 8 + 4 + 32 + 32 + 4
if sizer.Fork() >= ssz.ForkShanghai {
size += 4
}
if sizer.Fork() >= ssz.ForkCancun {
size += 8 + 8
}
if fixed {
return size
}
size += ssz.SizeDynamicBytes(sizer, obj.ExtraData)
size += ssz.SizeSliceOfDynamicBytes(sizer, obj.Transactions)
if sizer.Fork() >= ssz.ForkShanghai {
size += ssz.SizeSliceOfStaticObjects(sizer, obj.Withdrawals)
}
return size
}
// DefineSSZ defines how an object is encoded/decoded.
func (obj *ExecutionPayloadMonolith) DefineSSZ(codec *ssz.Codec) {
// Define the static data (fields and dynamic offsets)
ssz.DefineStaticBytes(codec, &obj.ParentHash) // Field ( 0) - ParentHash - 32 bytes
ssz.DefineStaticBytes(codec, &obj.FeeRecipient) // Field ( 1) - FeeRecipient - 20 bytes
ssz.DefineStaticBytes(codec, &obj.StateRoot) // Field ( 2) - StateRoot - 32 bytes
ssz.DefineStaticBytes(codec, &obj.ReceiptsRoot) // Field ( 3) - ReceiptsRoot - 32 bytes
ssz.DefineStaticBytes(codec, &obj.LogsBloom) // Field ( 4) - LogsBloom - 256 bytes
ssz.DefineStaticBytes(codec, &obj.PrevRandao) // Field ( 5) - PrevRandao - 32 bytes
ssz.DefineUint64(codec, &obj.BlockNumber) // Field ( 6) - BlockNumber - 8 bytes
ssz.DefineUint64(codec, &obj.GasLimit) // Field ( 7) - GasLimit - 8 bytes
ssz.DefineUint64(codec, &obj.GasUsed) // Field ( 8) - GasUsed - 8 bytes
ssz.DefineUint64(codec, &obj.Timestamp) // Field ( 9) - Timestamp - 8 bytes
ssz.DefineDynamicBytesOffset(codec, &obj.ExtraData, 32) // Offset (10) - ExtraData - 4 bytes
ssz.DefineUint256(codec, &obj.BaseFeePerGas) // Field (11) - BaseFeePerGas - 32 bytes
ssz.DefineStaticBytes(codec, &obj.BlockHash) // Field (12) - BlockHash - 32 bytes
ssz.DefineSliceOfDynamicBytesOffset(codec, &obj.Transactions, 1048576, 1073741824) // Offset (13) - Transactions - 4 bytes
ssz.DefineSliceOfStaticObjectsOffsetOnFork(codec, &obj.Withdrawals, 16, ssz.ForkFilter{Added: ssz.ForkShanghai}) // Offset (14) - Withdrawals - 4 bytes
ssz.DefineUint64PointerOnFork(codec, &obj.BlobGasUsed, ssz.ForkFilter{Added: ssz.ForkCancun}) // Field (15) - BlobGasUsed - 8 bytes
ssz.DefineUint64PointerOnFork(codec, &obj.ExcessBlobGas, ssz.ForkFilter{Added: ssz.ForkCancun}) // Field (16) - ExcessBlobGas - 8 bytes
// Define the dynamic data (fields)
ssz.DefineDynamicBytesContent(codec, &obj.ExtraData, 32) // Field (10) - ExtraData - ? bytes
ssz.DefineSliceOfDynamicBytesContent(codec, &obj.Transactions, 1048576, 1073741824) // Field (13) - Transactions - ? bytes
ssz.DefineSliceOfStaticObjectsContentOnFork(codec, &obj.Withdrawals, 16, ssz.ForkFilter{Added: ssz.ForkShanghai}) // Field (14) - Withdrawals - ? bytes
}
```
To explicitly highlight, the `ssz-fork` tags have been extracted from the struct definition and mapped into both an updated `SizeSSZ` method as well as a new definition style in `DefineSSZ`.
Do note, this type (or anything embedding it) will require the `OnFork` versions of `ssz.Encode`, `ssz.Decode`, `ssz.Hash` and `ssz.Size` to be called, since naturally it relies on a correct fork being set in the codec's context.
*Lastly, whilst the library itself supports custom fork enums, there is no support yet for these in the code generator. This will probably be added eventually via a `--forks=mypkg` or similar CLI flag, but it's a TODO for now.*
### Go generate
Perhaps just a mention, anyone using the code generator should call it from a `go:generate` compile instruction. It is much simpler and once added to the code, it can always be called via running `go generate`.
### Multi-type ordering
When generating code for multiple types at once (with one call or many), there's one ordering issue you need to be aware of.
When the code generator finds a field that is a struct of some sort, it needs to decide if it's a static or a dynamic type. To do that, it relies on checking if the type implements the `ssz.StaticObject` or `ssz.DynamicObject` interface. If if doesn't implement either, the generator will error.
This means, however, that if you have a type that's embedded in another type (e.g. in our examples above, `Withdrawal` was embedded inside `ExecutionPayload` in a slice), you need to generate the code for the inner type first, and then the outer type. This ensures that when the outer type is resolving the interface of the inner one, that is already generated and available.
## Merkleization
Half the SSZ spec is about encoding/decoding data into a binary format, the other half is about proving the data via [Merkle Proofs](https://github.com/ethereum/consensus-specs/blob/dev/ssz/merkle-proofs.md).
### Symmetric API
The same way that encoding/decoding has a "symmetric" and "asymmetric" API, so does merkleization. What's more, the symmetric API is actually exactly the same as for encoding/decoding, with no code changes necessary!
Taking our very simple `Withdrawal` type and it's codec code:
```go
type Address [20]byte
type Withdrawal struct {
Index uint64
Validator uint64
Address Address
Amount uint64
}
func (w *Withdrawal) SizeSSZ() uint32 { return 44 }
func (w *Withdrawal) DefineSSZ(codec *ssz.Codec) {
ssz.DefineUint64(codec, &w.Index) // Field (0) - Index - 8 bytes
ssz.DefineUint64(codec, &w.Validator) // Field (1) - ValidatorIndex - 8 bytes
ssz.DefineStaticBytes(codec, &w.Address) // Field (2) - Address - 20 bytes
ssz.DefineUint64(codec, &w.Amount) // Field (3) - Amount - 8 bytes
}
```
Hashing this works out of the box. To merkleize the above `Withdrawal` and calculate it's merkel trie root, use either `ssz.HashSequential` or `ssz.HashConcurrent`. The former will run on a single thread and use 0 allocations, whereas the latter might run on multiple threads concurrently (if large enough fields are present) and use O(1) memory.
```go
func main() {
hash := ssz.HashSequential(new(Withdrawal))
fmt.Printf("hash: %#x\n", hash)
}
```
### Asymmetric API
If for some reason you have a type that requires custom encoders/decoders, high chance, that it will also require a custom hasher. For those cases, this library provides an API surface very similar to how the asymmetric encoding/decoding worked:
```go
func (w *Withdrawal) DefineSSZ(codec *ssz.Codec) {
codec.DefineEncoder(func(enc *ssz.Encoder) {
ssz.EncodeUint64(enc, w.Index) // Field (0) - Index - 8 bytes
ssz.EncodeUint64(enc, w.Validator) // Field (1) - ValidatorIndex - 8 bytes
ssz.EncodeStaticBytes(enc, &w.Address) // Field (2) - Address - 20 bytes
ssz.EncodeUint64(enc, w.Amount) // Field (3) - Amount - 8 bytes
})
codec.DefineDecoder(func(dec *ssz.Decoder) {
ssz.DecodeUint64(dec, &w.Index) // Field (0) - Index - 8 bytes
ssz.DecodeUint64(dec, &w.Validator) // Field (1) - ValidatorIndex - 8 bytes
ssz.DecodeStaticBytes(dec, &w.Address) // Field (2) - Address - 20 bytes
ssz.DecodeUint64(dec, &w.Amount) // Field (3) - Amount - 8 bytes
})
codec.DefineHasher(func(has *ssz.Hasher) {
ssz.HashUint64(has, w.Index) // Field (0) - Index - 8 bytes
ssz.HashUint64(has, w.Validator) // Field (1) - ValidatorIndex - 8 bytes
ssz.HashStaticBytes(has, &w.Address) // Field (2) - Address - 20 bytes
ssz.HashUint64(has, w.Amount) // Field (3) - Amount - 8 bytes
})
}
```
Hashing the above `Withdrawal` into a Merkle trie root, you use the same thing as before. Everything is seamless.
## Quick reference
The table below is a summary of the methods available for `SizeSSZ` and `DefineSSZ`:
- The *Size API* is to be used to implement the `SizeSSZ` method's dynamic parts.
- The *Symmetric API* is to be used if the encoding/decoding/hashing doesn't require specialised logic.
- The *Asymmetric API* is to be used if encoding or decoding or hashing requires special casing.
*If some type you need is missing, please open an issue, so it can be added.*
| Type | Size API | Symmetric API | Asymmetric Encoding | Asymmetric Decoding | Asymmetric Hashing |
|:---------------------------:|:---------------------------------------------------------------------------------------------------:|:-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------:|:-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------:|:-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------:|:-----------------------------------------------------------------------------------------------------------:|
| `bool` | `1 byte` | [`DefineBool`](https://pkg.go.dev/github.com/karalabe/ssz#DefineBool) | [`EncodeBool`](https://pkg.go.dev/github.com/karalabe/ssz#EncodeBool) | [`DecodeBool`](https://pkg.go.dev/github.com/karalabe/ssz#DecodeBool) | [`HashBool`](https://pkg.go.dev/github.com/karalabe/ssz#HashBool) |
| `uint8` | `1 bytes` | [`DefineUint8`](https://pkg.go.dev/github.com/karalabe/ssz#DefineUint8) | [`EncodeUint8`](https://pkg.go.dev/github.com/karalabe/ssz#EncodeUint8) | [`DecodeUint8`](https://pkg.go.dev/github.com/karalabe/ssz#DecodeUint8) | [`HashUint8`](https://pkg.go.dev/github.com/karalabe/ssz#HashUint8) |
| `uint16` | `2 bytes` | [`DefineUint16`](https://pkg.go.dev/github.com/karalabe/ssz#DefineUint16) | [`EncodeUint16`](https://pkg.go.dev/github.com/karalabe/ssz#EncodeUint16) | [`DecodeUint16`](https://pkg.go.dev/github.com/karalabe/ssz#DecodeUint16) | [`HashUint16`](https://pkg.go.dev/github.com/karalabe/ssz#HashUint16) |
| `uint32` | `4 bytes` | [`DefineUint32`](https://pkg.go.dev/github.com/karalabe/ssz#DefineUint32) | [`EncodeUint32`](https://pkg.go.dev/github.com/karalabe/ssz#EncodeUint32) | [`DecodeUint32`](https://pkg.go.dev/github.com/karalabe/ssz#DecodeUint32) | [`HashUint32`](https://pkg.go.dev/github.com/karalabe/ssz#HashUint32) |
| `uint64` | `8 bytes` | [`DefineUint64`](https://pkg.go.dev/github.com/karalabe/ssz#DefineUint64) | [`EncodeUint64`](https://pkg.go.dev/github.com/karalabe/ssz#EncodeUint64) | [`DecodeUint64`](https://pkg.go.dev/github.com/karalabe/ssz#DecodeUint64) | [`HashUint64`](https://pkg.go.dev/github.com/karalabe/ssz#HashUint64) |
| `[N]byte` as `bitvector[N]` | `N bytes` | [`DefineArrayOfBits`](https://pkg.go.dev/github.com/karalabe/ssz#DefineArrayOfBits) | [`EncodeArrayOfBits`](https://pkg.go.dev/github.com/karalabe/ssz#EncodeArrayOfBits) | [`DecodeArrayOfBits`](https://pkg.go.dev/github.com/karalabe/ssz#DecodeArrayOfBits) | [`HashArrayOfBits`](https://pkg.go.dev/github.com/karalabe/ssz#HashArrayOfBits) |
| `bitfield.Bitlist`² | [`SizeSliceOfBits`](https://pkg.go.dev/github.com/karalabe/ssz#SizeSliceOfBits) | [`DefineSliceOfBitsOffset`](https://pkg.go.dev/github.com/karalabe/ssz#DefineSliceOfBitsOffset) [`DefineSliceOfBitsContent`](https://pkg.go.dev/github.com/karalabe/ssz#DefineSliceOfBitsContent) | [`EncodeSliceOfBitsOffset`](https://pkg.go.dev/github.com/karalabe/ssz#EncodeSliceOfBitsOffset) [`EncodeSliceOfBitsContent`](https://pkg.go.dev/github.com/karalabe/ssz#EncodeSliceOfBitsContent) | [`DecodeSliceOfBitsOffset`](https://pkg.go.dev/github.com/karalabe/ssz#DecodeSliceOfBitsOffset) [`DecodeSliceOfBitsContent`](https://pkg.go.dev/github.com/karalabe/ssz#DecodeSliceOfBitsContent) | [`HashSliceOfBits`](https://pkg.go.dev/github.com/karalabe/ssz#HashSliceOfBits) |
| `[N]uint64` | `N * 8 bytes` | [`DefineArrayOfUint64s`](https://pkg.go.dev/github.com/karalabe/ssz#DefineArrayOfUint64s) | [`EncodeArrayOfUint64s`](https://pkg.go.dev/github.com/karalabe/ssz#EncodeArrayOfUint64s) | [`DecodeArrayOfUint64s`](https://pkg.go.dev/github.com/karalabe/ssz#DecodeArrayOfUint64s) | [`HashArrayOfUint64s`](https://pkg.go.dev/github.com/karalabe/ssz#HashArrayOfUint64s) |
| `[]uint64` | [`SizeSliceOfUint64s`](https://pkg.go.dev/github.com/karalabe/ssz#SizeSliceOfUint64s) | [`DefineSliceOfUint64sOffset`](https://pkg.go.dev/github.com/karalabe/ssz#DefineSliceOfUint64sOffset) [`DefineSliceOfUint64sContent`](https://pkg.go.dev/github.com/karalabe/ssz#DefineSliceOfUint64sContent) | [`EncodeSliceOfUint64sOffset`](https://pkg.go.dev/github.com/karalabe/ssz#EncodeSliceOfUint64sOffset) [`EncodeSliceOfUint64sContent`](https://pkg.go.dev/github.com/karalabe/ssz#EncodeSliceOfUint64sContent) | [`DecodeSliceOfUint64sOffset`](https://pkg.go.dev/github.com/karalabe/ssz#DecodeSliceOfUint64sOffset) [`DecodeSliceOfUint64sContent`](https://pkg.go.dev/github.com/karalabe/ssz#DecodeSliceOfUint64sContent) | [`HashSliceOfUint64s`](https://pkg.go.dev/github.com/karalabe/ssz#HashSliceOfUint64s) |
| `*uint256.Int`¹ | `32 bytes` | [`DefineUint256`](https://pkg.go.dev/github.com/karalabe/ssz#DefineUint256) | [`EncodeUint256`](https://pkg.go.dev/github.com/karalabe/ssz#EncodeUint256) | [`DecodeUint256`](https://pkg.go.dev/github.com/karalabe/ssz#DecodeUint256) | [`HashUint256`](https://pkg.go.dev/github.com/karalabe/ssz#HashUint256) |
| `*big.Int` as `uint256` | `32 bytes` | [`DefineUint256BigInt`](https://pkg.go.dev/github.com/karalabe/ssz#DefineUint256BigInt) | [`EncodeUint256BigInt`](https://pkg.go.dev/github.com/karalabe/ssz#EncodeUint256BigInt) | [`DecodeUint256BigInt`](https://pkg.go.dev/github.com/karalabe/ssz#DecodeUint256BigInt) | [`HashUint256BigInt`](https://pkg.go.dev/github.com/karalabe/ssz#HashUint256BigInt) |
| `[N]byte` | `N bytes` | [`DefineStaticBytes`](https://pkg.go.dev/github.com/karalabe/ssz#DefineStaticBytes) | [`EncodeStaticBytes`](https://pkg.go.dev/github.com/karalabe/ssz#EncodeStaticBytes) | [`DecodeStaticBytes`](https://pkg.go.dev/github.com/karalabe/ssz#DecodeStaticBytes) | [`HashStaticBytes`](https://pkg.go.dev/github.com/karalabe/ssz#HashStaticBytes) |
| `[N]byte` in `[]byte` | `N bytes` | [`DefineCheckedStaticBytes`](https://pkg.go.dev/github.com/karalabe/ssz#DefineCheckedStaticBytes) | [`EncodeCheckedStaticBytes`](https://pkg.go.dev/github.com/karalabe/ssz#EncodeCheckedStaticBytes) | [`DecodeCheckedStaticBytes`](https://pkg.go.dev/github.com/karalabe/ssz#DecodeCheckedStaticBytes) | [`HashCheckedStaticBytes`](https://pkg.go.dev/github.com/karalabe/ssz#HashCheckedStaticBytes) |
| `[]byte` | [`SizeDynamicBytes`](https://pkg.go.dev/github.com/karalabe/ssz#SizeDynamicBytes) | [`DefineDynamicBytesOffset`](https://pkg.go.dev/github.com/karalabe/ssz#DefineDynamicBytesOffset) [`DefineDynamicBytesContent`](https://pkg.go.dev/github.com/karalabe/ssz#DefineDynamicBytesContent) | [`EncodeDynamicBytesOffset`](https://pkg.go.dev/github.com/karalabe/ssz#EncodeDynamicBytesOffset) [`EncodeDynamicBytesContent`](https://pkg.go.dev/github.com/karalabe/ssz#EncodeDynamicBytesContent) | [`DecodeDynamicBytesOffset`](https://pkg.go.dev/github.com/karalabe/ssz#DecodeDynamicBytesOffset) [`DecodeDynamicBytesContent`](https://pkg.go.dev/github.com/karalabe/ssz#DecodeDynamicBytesContent) | [`HashDynamicBytes`](https://pkg.go.dev/github.com/karalabe/ssz#HashDynamicBytes) |
| `[M][N]byte` | `M * N bytes` | [`DefineArrayOfStaticBytes`](https://pkg.go.dev/github.com/karalabe/ssz#DefineArrayOfStaticBytes) | [`EncodeArrayOfStaticBytes`](https://pkg.go.dev/github.com/karalabe/ssz#EncodeArrayOfStaticBytes) | [`DecodeArrayOfStaticBytes`](https://pkg.go.dev/github.com/karalabe/ssz#DecodeArrayOfStaticBytes) | [`HashArrayOfStaticBytes`](https://pkg.go.dev/github.com/karalabe/ssz#HashArrayOfStaticBytes) |
| `[M][N]byte` in `[][N]byte` | `M * N bytes` | [`DefineCheckedArrayOfStaticBytes`](https://pkg.go.dev/github.com/karalabe/ssz#DefineCheckedArrayOfStaticBytes) | [`EncodeCheckedArrayOfStaticBytes`](https://pkg.go.dev/github.com/karalabe/ssz#EncodeCheckedArrayOfStaticBytes) | [`DecodeCheckedArrayOfStaticBytes`](https://pkg.go.dev/github.com/karalabe/ssz#DecodeCheckedArrayOfStaticBytes) | [`HashCheckedArrayOfStaticBytes`](https://pkg.go.dev/github.com/karalabe/ssz#HashCheckedArrayOfStaticBytes) |
| `[][N]byte` | [`SizeSliceOfStaticBytes`](https://pkg.go.dev/github.com/karalabe/ssz#SizeSliceOfStaticBytes) | [`DefineSliceOfStaticBytesOffset`](https://pkg.go.dev/github.com/karalabe/ssz#DefineSliceOfStaticBytesOffset) [`DefineSliceOfStaticBytesContent`](https://pkg.go.dev/github.com/karalabe/ssz#DefineSliceOfStaticBytesContent) | [`EncodeSliceOfStaticBytesOffset`](https://pkg.go.dev/github.com/karalabe/ssz#EncodeSliceOfStaticBytesOffset) [`EncodeSliceOfStaticBytesContent`](https://pkg.go.dev/github.com/karalabe/ssz#EncodeSliceOfStaticBytesContent) | [`DecodeSliceOfStaticBytesOffset`](https://pkg.go.dev/github.com/karalabe/ssz#DecodeSliceOfStaticBytesOffset) [`DecodeSliceOfStaticBytesContent`](https://pkg.go.dev/github.com/karalabe/ssz#DecodeSliceOfStaticBytesContent) | [`HashSliceOfStaticBytes`](https://pkg.go.dev/github.com/karalabe/ssz#DecodeHashSliceOfStaticBytes) |
| `[][]byte` | [`SizeSliceOfDynamicBytes`](https://pkg.go.dev/github.com/karalabe/ssz#SizeSliceOfDynamicBytes) | [`DefineSliceOfDynamicBytesOffset`](https://pkg.go.dev/github.com/karalabe/ssz#DefineSliceOfDynamicBytesOffset) [`DefineSliceOfDynamicBytesContent`](https://pkg.go.dev/github.com/karalabe/ssz#DefineSliceOfDynamicBytesContent) | [`EncodeSliceOfDynamicBytesOffset`](https://pkg.go.dev/github.com/karalabe/ssz#EncodeSliceOfDynamicBytesOffset) [`EncodeSliceOfDynamicBytesContent`](https://pkg.go.dev/github.com/karalabe/ssz#EncodeSliceOfDynamicBytesContent) | [`DecodeSliceOfDynamicBytesOffset`](https://pkg.go.dev/github.com/karalabe/ssz#DecodeSliceOfDynamicBytesOffset) [`DecodeSliceOfDynamicBytesContent`](https://pkg.go.dev/github.com/karalabe/ssz#DecodeSliceOfDynamicBytesContent) | [`HashSliceOfDynamicBytes`](https://pkg.go.dev/github.com/karalabe/ssz#DecodeHashSliceOfDynamicBytes) |
| `ssz.StaticObject` | `Object(nil).SizeSSZ()` | [`DefineStaticObject`](https://pkg.go.dev/github.com/karalabe/ssz#DefineStaticObject) | [`EncodeStaticObject`](https://pkg.go.dev/github.com/karalabe/ssz#EncodeStaticObject) | [`DecodeStaticObject`](https://pkg.go.dev/github.com/karalabe/ssz#DecodeStaticObject) | [`HashStaticObject`](https://pkg.go.dev/github.com/karalabe/ssz#DecodeHashStaticObject) |
| `[]ssz.StaticObject` | [`SizeSliceOfStaticObjects`](https://pkg.go.dev/github.com/karalabe/ssz#SizeSliceOfStaticObjects) | [`DefineSliceOfStaticObjectsOffset`](https://pkg.go.dev/github.com/karalabe/ssz#DefineSliceOfStaticObjectsOffset) [`DefineSliceOfStaticObjectsContent`](https://pkg.go.dev/github.com/karalabe/ssz#DefineSliceOfStaticObjectsContent) | [`EncodeSliceOfStaticObjectsOffset`](https://pkg.go.dev/github.com/karalabe/ssz#EncodeSliceOfStaticObjectsOffset) [`EncodeSliceOfStaticObjectsContent`](https://pkg.go.dev/github.com/karalabe/ssz#EncodeSliceOfStaticObjectsContent) | [`DecodeSliceOfStaticObjectsOffset`](https://pkg.go.dev/github.com/karalabe/ssz#DecodeSliceOfStaticObjectsOffset) [`DecodeSliceOfStaticObjectsContent`](https://pkg.go.dev/github.com/karalabe/ssz#DecodeSliceOfStaticObjectsContent) | [`HashSliceOfStaticObjects`](https://pkg.go.dev/github.com/karalabe/ssz#DecodeHashSliceOfStaticObjects) |
| `ssz.DynamicObject` | [`SizeDynamicObject`](https://pkg.go.dev/github.com/karalabe/ssz#SizeDynamicObject) | [`DefineDynamicBytesOffset`](https://pkg.go.dev/github.com/karalabe/ssz#DefineDynamicBytesOffset) [`DefineDynamicBytesContent`](https://pkg.go.dev/github.com/karalabe/ssz#DefineDynamicBytesContent) | [`EncodeDynamicBytesOffset`](https://pkg.go.dev/github.com/karalabe/ssz#EncodeDynamicBytesOffset) [`EncodeDynamicBytesContent`](https://pkg.go.dev/github.com/karalabe/ssz#EncodeDynamicBytesContent) | [`DecodeDynamicBytesOffset`](https://pkg.go.dev/github.com/karalabe/ssz#DecodeDynamicBytesOffset) [`DecodeDynamicBytesContent`](https://pkg.go.dev/github.com/karalabe/ssz#DecodeDynamicBytesContent) | [`HashDynamicBytes`](https://pkg.go.dev/github.com/karalabe/ssz#DecodeHashDynamicBytes) |
| `[]ssz.DynamicObject` | [`SizeSliceOfDynamicObjects`](https://pkg.go.dev/github.com/karalabe/ssz#SizeSliceOfDynamicObjects) | [`DefineSliceOfDynamicObjectsOffset`](https://pkg.go.dev/github.com/karalabe/ssz#DefineSliceOfDynamicObjectsOffset) [`DefineSliceOfDynamicObjectsContent`](https://pkg.go.dev/github.com/karalabe/ssz#DefineSliceOfDynamicObjectsContent) | [`EncodeSliceOfDynamicObjectsOffset`](https://pkg.go.dev/github.com/karalabe/ssz#EncodeSliceOfDynamicObjectsOffset) [`EncodeSliceOfDynamicObjectsContent`](https://pkg.go.dev/github.com/karalabe/ssz#EncodeSliceOfDynamicObjectsContent) | [`DecodeSliceOfDynamicObjectsOffset`](https://pkg.go.dev/github.com/karalabe/ssz#DecodeSliceOfDynamicObjectsOffset) [`DecodeSliceOfDynamicObjectsContent`](https://pkg.go.dev/github.com/karalabe/ssz#DecodeSliceOfDynamicObjectsContent) | [`HashSliceOfDynamicObjects`](https://pkg.go.dev/github.com/karalabe/ssz#DecodeHashSliceOfDynamicObjects) |
*¹Type is from `github.com/holiman/uint256`.* \
*²Type is from `github.com/prysmaticlabs/go-bitfield`*.
## Performance
The goal of this package is to be close in performance to low level generated encoders, without sacrificing maintainability. It should, however, be significantly faster than runtime reflection encoders.
The package includes a set of benchmarks for handling the beacon spec types and test datasets. You can run them with `go test ./tests --bench=.`. These can be interesting for some baseline numbers, but they are unrealistic with regard to live beacon state data.
If you want to see the performance on a more realistic piece of data, you'll need to provide a beacon state SSZ object and place it into the project root named `state.ssz`. You can then run `go test --bench=Mainnet ./tests/manual_test.go` to explicitly run this one benchmark. A sample output running against a 208MB state export from around June 11, 2024, on a MacBook Pro M2 Max:
```
go test --bench=Mainnet ./tests/manual_test.go
BenchmarkMainnetState/beacon-state/208757379-bytes/encode-12 26 45164494 ns/op 4622.16 MB/s 74 B/op 0 allocs/op
BenchmarkMainnetState/beacon-state/208757379-bytes/decode-12 27 40984980 ns/op 5093.51 MB/s 8456490 B/op 54910 allocs/op
BenchmarkMainnetState/beacon-state/208757379-bytes/merkleize-sequential-12 2 659472250 ns/op 316.55 MB/s 904 B/op 1 allocs/op
BenchmarkMainnetState/beacon-state/208757379-bytes/merkleize-concurrent-12 9 113414449 ns/op 1840.66 MB/s 16416 B/op 108 allocs/op
```