# Metapool Factory: Oracles¶

Factory contracts include Time-Weighted Average Price oracles. To understand these a bit better, you need to understand how Curve calculates price.

A curve pool is an array of balances of the tokens it holds. To provide a price, it calculates how much of x you can receive given amount y.

## Time-Weighted Average Price oracles¶

MetaPool.get_price_cumulative_last() → uint256[N_COINS]:

Returns the current time-weighted average price (TWAP). This will represent the underlying balances of the pool.

The value returned is the cumulative pool shifting balances over time

MetaPool.block_timestamp_last() → uint256:

Returns the last timestamp that a TWAP reading was taken in unix time.

MetaPool.get_twap_balances(_first_balances: uint256[N_COINS], _last_balances: uint256[N_COINS], _time_elapsed: uint256) → uint256[N_COINS]:

Calculate the current effective TWAP balances given two snapshots over time, and the time elapsed between the two snapshots.

• _first_balances: First price_cumulative_last array that was snapshot via get_price_cumulative_last

• _last_balances: Second price_cumulative_last array that was snapshot via get_price_cumulative_last

• _time_elapsed: The elapsed time in seconds between _first_balances and _last_balances

Returns the balances of the TWAP value.

MetaPool.get_dy(i: int128, j: int128, dx: uint256, _balances: uint256[N_COINS] = [0, 0]) → uint256:

Calculate the price for exchanging a token with index i to token with index j and amount dx given the _balances provided.

• i: The index of the coin being sent to the pool, as it related to the metapool

• j: The index of the coin being received from the pool, as it relates to the metapool

• dx: The amount of i being sent to the pool

• _balances: The array of balances to be used for purposes of calculating the output amount / exchange rate, this is the value returned in get_twap_balances

Returns the quote / price as dy given dx.

## Security¶

The Curve TWAP is greatly inspired by Uniswap TWAP architecture, in that the price is a cumulative value over time, which reduces balance shifts due to flash loans, but also records the balances based on the previous block, to avoid recording flashloan data.