# Tracking Contract Balances

Sometimes the balances a component uses is stored on a contract that is not a dedicated single pool contract. During Tycho VM simulations, token contracts are mocked and any balances checked or used during a swap need to be overwritten for a simulation to succeed. Default behavior is for the component balances reported to be used to overwrite the pool contract balances. This assumes 2 things: there is a one-to-one relationship between contracts and components, and the hex-encoded contract address serves as the component ID.

If a protocol deviates from this assumption, the balances for each appropriate contract needs to be tracked for that contract. All contracts that have their balances checked/accessed during a simulation need to be tracked in this way.

### Implementation Steps:

1. Implement logic/a helper function to extract the absolute balances of the contract. This is protocol specific and might be obtained from an event, or extracted from a storage slot if an appropriate one is identified.
2. Create an `InterimContractChange` for the contract and add the contract balances using `upsert_token_balance`.
3. Add these contract changes to the appropriate `TransactionChangesBuilder` using `add_contract_changes`.

An example for a protocol that uses a single vault contract is as follows:

```rust
use tycho_substreams::models::{InterimContractChange, TransactionChangesBuilder};

// all changes on this block, aggregated by transaction
let mut transaction_changes: HashMap<_, TransactionChanges> = HashMap::new();

// Extract token balances for vault contract
block
    .transaction_traces
    .iter()
    .for_each(|tx| {
        // use helper function to get absolute balances at this transaction
        let vault_balance_change = get_vault_reserves(tx, &components_store, &tokens_store);

        if !vault_balance_change.is_empty() {
            let tycho_tx = Transaction::from(tx);
            let builder = transaction_changes
                .entry(tx.index.into())
                .or_insert_with(|| TransactionChangesBuilder::new(&tycho_tx));

            let mut vault_contract_changes = InterimContractChange::new(VAULT_ADDRESS, false);
            for (token_addr, reserve_value) in vault_balance_change {
                vault_contract_changes.upsert_token_balance(
                    token_addr.as_slice(),
                    reserve_value.value.as_slice(),
                );
            }
            builder.add_contract_changes(&vault_contract_changes);
        }
    });
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://docs.propellerheads.xyz/tycho/for-dexs/protocol-integration/indexing/common-problems-and-patterns/tracking-contract-balances.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.