Gas Optimizations

[code423n4]

1. Uint variables have default value to 0, so dont have to be initialized

In Cally.sol#L94-L95, uint varibles are initialzied to 0, this can be avoided as they default to zero when they are declared

    // Before
    uint256 public feeRate = 0;
    uint256 public protocolUnclaimedFees = 0;

    // After
    uint256 public feeRate;
    uint256 public protocolUnclaimedFees;

Similar also in src/Cally.sol#L282

      // Before
      uint256 fee = 0;

      // After
      uint256 fee;

2. checking a != 0 is cheaper than a > 0 for uint

in Cally.sol#L170, durationDays > 0 can be replaced with durationDays != 0 since it is uint.

3. Several variables are declared for one time use only

Several variables are declared for using only one times, This can be avoided by using the expression directly to save gas.
eg in src/Cally.sol#L223-L224

    // Before:
        uint256 premium = getPremium(vaultId);
        require(msg.value >= premium, "Incorrect ETH amount sent");

    // After
        require(msg.value >= getPremium(vaultId), "Incorrect ETH amount sent");

Similar issues are also in:

• Cally.sol#L227-L228

        // Before
        uint32 auctionStartTimestamp = vault.currentExpiration;
        require(block.timestamp >= auctionStartTimestamp, "Auction not started");
  
        // After
        require(block.timestamp >= vault.currentExpiration, "Auction not started");

• src/Cally.sol#L278-L279

        // Before
        vault.isExercised = true;
        _vaults[vaultId] = vault;
        
        // After
        _vaults[vaultId].isExercised = vault;

• src/Cally.sol#L333-L334

      // Before
        uint256 optionId = vaultId + 1;
        _burn(optionId);
        _burn(vaultId);
  
      // After
        _burn(vaultId);
        _burn(vaultId + 1);

• src/Cally.sol#L395-L396

  // Before
        Vault memory vault = _vaults[vaultId];
        return premiumOptions[vault.premiumIndex];
  // After
        return premiumOptions[_vaults[vaultId].premiumIndex];

4. prefix increment can be used in loops instead of postfix increment as it is cheaper

In src/CallyNft.sol#L244

  // Before
        for (uint256 i = 0; i < data.length; i++) {
        }
  // After
        for (uint256 i = 0; i < data.length; ++i) {
        }

5. data.length is calculated for each iteration in loop, this can be cached to save gas

In src/CallyNft.sol#L244

  // Before
        for (uint256 i = 0; i < data.length; i++) {
        }
  // After
        uint len = data.length;
        for (uint256 i = 0; i < len; ++i) {
        }

6. Using custom errors to save gase

There are a lots of require statements that use revert strings. They can be replaced with custom Errors introduced in solidity 0.8.4 which are cheaper in terms of gas.