Auction.userClaimableArbTokens amountOut calculations can be simplified

[code423n4]

Handle

hyh

Vulnerability details

Impact

Gas is overspent on access and operations.

Proof of Concept

amountOut is calculated in 3 steps, which can be made simpler.
https://github.com/code-423n4/2021-11-malt/blob/main/src/contracts/Auction.sol#L302-305

Recommended Mitigation Steps

Now:

uint256 amountTokens = commitment.commitment.mul(auction.pegPrice).div(price);
uint256 redeemedTokens = commitment.redeemed.mul(auction.pegPrice).div(price);
uint256 amountOut = amountTokens.mul(claimablePerc).div(auction.pegPrice).sub(redeemedTokens);

To be (amountTokens and redeemedTokens aren't used elsewhere):

 /*
* uint256 amountTokens = commitment.commitment.mul(auction.pegPrice).div(price);
* uint256 redeemedTokens = commitment.redeemed.mul(auction.pegPrice).div(price);
* uint256 amountOut = amountTokens.mul(claimablePerc).div(auction.pegPrice).sub(redeemedTokens);
*/
uint256 redeemed = commitment.redeemed.mul(auction.pegPrice);
uint256 amountOut = commitment.commitment.mul(claimablePerc).sub(redeemed).div(price);

[GalloDaSballo]

Through the symbolic substitutions

AT = (Cc * A) / P
RT = Cr * A / P
AO = (AT * CP) / A - RT

AO = ((Cc * A) / P * (CP))/ A - (Cr * A) / P
AO = (Cc * CP - (Cr * A)) / P

R = Cr * A
AO = (Cc * CP - R) / P
AO = (Cc * CP - Cr * A) / P

I agree that the warden has shown a way to save gas while keeping the math correct