decrease approval condition fix (greater than and equal to)

[zaryabbkh]

🚀 Description

The decreaseApproval function in StandardToken.sol has a condition that if the subtracted amount is greater than the allowed amount, simply make the allowance 0, otherwise, it invokes a subtraction function for reducing the approval amount:

 if (_subtractedValue > oldValue) {
      allowed[msg.sender][_spender] = 0;
    } else {
      allowed[msg.sender][_spender] = oldValue.sub(_subtractedValue);
    }

If the subtracted amount is exactly equal to the allowed tokens, this will execute both the first condition, and will then default to the else where the subtract function is called which unnecessarily increases the gas amount of such a transaction. If we modify the condition to be (_subtractedValue >= oldValue), the allowance will be reset to 0. This will only increase 3 units of gas per transaction but it will save 88 units of gas when subtractedValue is equal to the oldValue.

Before update in statement the transaction cost is 16278 to decrease the exactly same allowed amount

After the update the transaction cost is 16190

• 📘 I've reviewed the OpenZeppelin Contributor Guidelines
• ✅ I've added tests where applicable to test my new functionality.
• 📖 I've made sure that my contracts are well-documented.
• 🎨 I've run the JS/Solidity linters and fixed any issues (npm run lint:all:fix).

[nventuro]

Cool, thanks @zaryab19aug! I modified the test cases a bit to make them more consistent with the rest of the suite, hopefully a more comprehensive guide will be added soon.