> ## Documentation Index
> Fetch the complete documentation index at: https://docs.base.org/llms.txt
> Use this file to discover all available pages before exploring further.

# Predeploys

> Predeploy changes in the Fjord upgrade, adding FastLZ fee computation constants to the GasPriceOracle contract.

## GasPriceOracle

Following the Fjord upgrade, three additional values used for L1 fee computation are:

* costIntercept
* costFastlzCoef
* minTransactionSize

These values are hard-coded constants in the `GasPriceOracle` contract. The
calculation follows the same formula outlined in the
[Fjord L1-Cost fee changes (FastLZ estimator)](./exec-engine#fjord-l1-cost-fee-changes-fastlz-estimator)
section.

A new method is introduced: `getL1FeeUpperBound(uint256)`. This method returns an upper bound for the L1 fee
for a given transaction size. It is provided for callers who wish to estimate L1 transaction costs in the
write path, and is much more gas efficient than `getL1Fee`.

The upper limit overhead is assumed to be `original/255+16`, borrowed from LZ4. According to historical data, this
approach can encompass more than 99.99% of transactions.

This is implemented as follows:

```solidity theme={null}
function getL1FeeUpperBound(uint256 unsignedTxSize) external view returns (uint256) {
    // Add 68 to account for unsigned tx
    uint256 txSize = unsignedTxSize + 68;
    // txSize / 255 + 16 is the practical fastlz upper-bound covers 99.99% txs.
    uint256 flzUpperBound = txSize + txSize / 255 + 16;

    int256 estimatedSize = costIntercept + costFastlzCoef * flzUpperBound;
    if (estimatedSize < minTransactionSize) {
        estimatedSize = minTransactionSize;
    }
  
    uint256 l1FeeScaled = baseFeeScalar() * l1BaseFee() * 16 + blobBaseFeeScalar() * blobBaseFee();
    return uint256(estimatedSize) * l1FeeScaled / (10 ** (DECIMALS * 2));
}
```

### L1 Gas Usage Estimation

The `getL1GasUsed` method is updated to take into account the improved [compression estimation](./exec-engine#fees)
accuracy as part of the Fjord upgrade.

```solidity theme={null}
function getL1GasUsed(bytes memory _data) public view returns (uint256) {
    if (isFjord) {
      // Add 68 to the size to account for the unsigned tx
      int256 flzSize = LibZip.flzCompress(_data).length + 68;

      int256 estimatedSize = costIntercept + costFastlzCoef * flzSize;
      if (estimatedSize < minTransactionSize) {
        estimatedSize = minTransactionSize;
      }

      // Assume the compressed data is mostly non-zero, and would pay 16 gas per calldata byte
      return estimatedSize * 16;
    }
    // ...
}
```

The `getL1GasUsed` method is deprecated as of Fjord because it does not capture that there are
two kinds of gas being consumed due to the introduction of blobs. This function will revert when
called in a future upgrade.

Users can continue to use the `getL1Fee` method to estimate the L1 fee for a given transaction, or the
new `getL1FeeUpperBound` method introduced by Fjord as a lower gas alternative.