Crate vote_commitment_tree 

Append-only Poseidon Merkle tree for the Vote Commitment Tree (Gov Steps V1).

This tree holds both Vote Authority Notes (VANs) and Vote Commitments (VCs) as leaves. Domain separation (DOMAIN_VAN / DOMAIN_VC) is applied when constructing leaf values (in circuits / chain); this crate stores and hashes already-committed field elements.

Insertion order (per cosmos-sdk-messages-spec):

• MsgDelegateVote → append 1 leaf (VAN)
• MsgCastVote → append 2 leaves (new VAN, then VC)

Architecture

The crate is split into server and client layers with a sync API boundary:

• Shared types (MerkleHashVote, Anchor, MerklePath) — used by both sides.
• TreeServer — authoritative full tree: append, checkpoint, serve data via TreeSyncApi.
• TreeClient — sparse tree: sync from server, mark positions, generate witnesses.
• TreeSyncApi — trait defining the communication boundary (in-process for POC, maps to Cosmos SDK endpoints in production).

Built on incrementalmerkletree / shardtree (same crates that back Orchard’s note commitment tree), with two substitutions:

• Hash: Poseidon (no layer tagging) instead of Sinsemilla
• Empty leaf: poseidon_hash(0, 0) instead of Fp::from(2)

Re-exports

pub use client::SyncError;

pub use client::TreeClient;

pub use server::AppendFromKvError;

pub use server::MemoryTreeServer;

pub use server::SyncableServer;

pub use server::TreeServer;

pub use sync_api::TreeSyncApi;

Modules

client

Client-side vote commitment tree.

kv_shard_store

KvShardStore — a ShardStore implementation backed by Go’s Cosmos KV store via C function pointer callbacks.

memory_server

SyncableServer — augments [GenericTreeServer] with per-block leaf tracking and implements TreeSyncApi for in-process sync.

serde

Binary serialization for shard trees and checkpoints.

server

Generic and production vote commitment tree servers.

sync_api

Communication boundary between server and client.

Structs

Anchor

A committed vote commitment tree root at a specific block height.

MerkleHashVote

Leaf and internal node digest for the vote commitment tree.

MerklePath

Merkle authentication path from a leaf to the tree root.

Constants

DOMAIN_VC

Domain tag for Vote Commitments (matches orchard::vote_proof::circuit::DOMAIN_VC).

MERKLE_PATH_BYTES

Serialized size: 4 bytes (position u32 LE) + 32 * TREE_DEPTH bytes (auth path).

TREE_DEPTH

Fixed depth of the Vote Commitment Tree (2^24 ≈ 16.7M leaf capacity).

Functions

poseidon_hash

Poseidon hash of two field elements (delegates to imt-tree for circuit consistency).

poseidon_hash_6

Poseidon hash of six field elements (ConstantLength<6>, width 3, rate 2).

vote_commitment_hash

Compute the vote commitment leaf hash (arity-5 Poseidon).