MerkleTreeAccount

The main account structure that stores the Merkle tree state for tracking private commitments

Overview

The MerkleTreeAccount is a zero-copy account that stores the sparse Merkle tree state used to track all private commitments in the Privacy Cash protocol. Each tree can handle either native SOL or a specific SPL token.

Account Structure

authority · Pubkey · required

The public key of the authority that can update this tree’s configuration

next_index · u64 · required

The index where the next leaf will be inserted in the Merkle tree

subtrees · [[u8; 32]; 26] · required

Array of intermediate node hashes representing the state of each level in the Merkle tree (height 26)

root · [u8; 32] · required

The current root hash of the Merkle tree

root_history · [[u8; 32]; 100] · required

Circular buffer storing the last 100 root hashes to allow proofs against recent tree states

root_index · u64 · required

The current position in the root_history circular buffer

max_deposit_amount · u64 · required

Maximum amount (in lamports for SOL, or token units for SPL tokens) that can be deposited in a single transaction

height · u8 · required

The height of the Merkle tree (fixed at 26)

root_history_size · u8 · required

The size of the root history buffer (fixed at 100)

bump · u8 · required

The PDA bump seed used for account derivation

_padding · [u8; 5] · required

Padding bytes required for zero-copy account alignment

PDA Seeds

For SOL (Native)

[b"merkle_tree"]

For SPL Tokens

[b"merkle_tree", mint.key().as_ref()]

Notes

• This account uses the #[account(zero_copy)] attribute for efficient memory access without deserialization
• The tree height of 26 allows for 2^26 (67,108,864) unique commitments
• The root history enables users to create proofs using recent tree states even while new deposits are being processed
• Each SPL token has its own separate Merkle tree account