perf(shielded-pool): replace O(n) root recomputation with incremental frontier#78
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… frontier insert_leaf formerly called get_all_leaves() and recomputed the full Poseidon Merkle root from scratch on every insert (O(n) reads, O(n log n) hashes). At 10k commitments this meant ~10k storage reads per shield or private_transfer extrinsic. Replace with an incremental frontier algorithm: persist a 20-slot array (MerkleTreeFrontier storage item) that stores the last left-sibling at each tree level. Each insert now takes exactly 20 hashes and 1 storage read/write, regardless of tree size. Complexity is O(depth) = O(20) — constant at any scale. Changes: - lib.rs: add MerkleTreeFrontier<T> StorageValue<_, [[u8;32]; 20]> - storage.rs: add get_frontier / set_frontier to MerkleRepository - merkle.rs: rewrite MerkleTreeService::insert_leaf to use frontier; remove compute_poseidon_merkle_root (now unused); fix old_root in MerkleRootUpdated event (was always [0u8;32], now carries real value) - genesis.rs: initialize MerkleTreeFrontier to all-zeros at genesis Add 5 tests: - incremental_root_matches_batch_root_after_single_insert - incremental_root_matches_batch_root_after_multiple_inserts - incremental_proof_verifies_against_incremental_root - merkle_root_updated_event_carries_correct_old_root - frontier_survives_storage_round_trip_across_separate_calls
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insert_leaf formerly called get_all_leaves() and recomputed the full Poseidon Merkle root from scratch on every insert (O(n) reads, O(n log n) hashes). At 10k commitments this meant ~10k storage reads per shield or private_transfer extrinsic.
Replace with an incremental frontier algorithm: persist a 20-slot array (MerkleTreeFrontier storage item) that stores the last left-sibling at each tree level. Each insert now takes exactly 20 hashes and 1 storage read/write, regardless of tree size. Complexity is O(depth) = O(20) — constant at any scale.
Changes:
Add 5 tests: