ZTYP

Another approach to SSZ, focused on typing around merkle-tree-representations of state.

ZTYP mirrors most features of my Python implementation remerkleable,

In addition to tree structures and views, ZTYP also provides encoding/decoding utils for flat native Go structures, in the codec package.

ZRNT uses both the ZTYP tree structures (state) and flat utils (messages) to implement the Eth2 API spec.

Design

Construction:

• ZTYP describes types as interfaces to a binary merkle tree: views over a backing tree.
• Every node in the backing tree is referred to as a Node, build with:
  • Commit: combines two subtrees
  • Root: a bytes32. Can be a summary root of an unexpanded subtree, or a just a leaf node.
  • The backing is immutable: on modification the tree is forked, with both forks sharing the common parts
  • The full tree is cached automatically! Since child nodes never change their identity, the hash-tree-root result is cached in every merkle-tree node. Without the need to even know the type
• a TypeDef and BasicTypeDef are implemented to describe the raw type information.
  • Create default trees from the type-definition. No views required.
  • Convert an untyped backing into a typed view by attaching the type definition.
  • Convert a Root and sub-index into a typed sub-view by attaching a basic type definition.
• a View is used to interact with a subtree. A general view only tracks its backing.
  • Typed views allow you to interact with a backing in typed ways:
    • Basic types: Uint256Type, Uint64Type, Uint32Type, Uint16Type, Uint8Type, BoolType (uint128 is not supported)
    • Composite types: Container, ComplexList, ComplexVector
    • Union type: UnionType
    • Basic composite types (to enable packing of consecutive elements): BasicList, BasicVector
    • Bitfields: BitVector, BitList
    • Optimized small byte vectors: SmallByteVecMeta: to derive any BytesN (N <= 32) from.
    • RootView for an efficient 32 byte (single node) immutable view.
  • Semi-typed views are useful to build your own types: SubtreeView
• ReadProp/WriteProp functions can be used to describe reusable Getter/Setter -> View -> *my-type* pipelines.
  • Take a typed view that has getters/setters, create closure to fix it to a property index
  • Type the property function and add a receiver func to return the typed view instead.
  • Type ReadPropFn and WritePropFn with your own function types to not repeat the boilerplate.
• BackingHooks enable you to create smaller views attached to their parent views. Program like you are mutating references, and have the backing-hook propagate up the changes.
• The backing tree can be partial, and summarised/expanded dynamically. The type-definition will safely handle a tree, and return an error when the expected data for an operation is inconsistent or missing.
• The hash-function for hash-tree-root is:
  • passed by reference, to reuse a single state during hashing.
  • pluggable. Just define a H(a [32]byte, b [32]byte) [32]byte and plug it into Hash and InitZeroHashes.

Contact

Dev: @protolambda on Twitter

License

MIT, see license file.