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.

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.

Dev: @protolambda on Twitter

MIT, see license file.