Add tricks to save gas

Solidity-Expert/Tricks-to-save-gas/README.md

@@ -2,3 +2,83 @@
 
 
 <br>
+
+- *In DeFi, any gas saving is an edge. Below are some notes on tricks we use.*
+- Note that in Solidity, the maximum size of a contract is restricted to 24 KB by [EIP 170](https://github.com/ethereum/EIPs/blob/master/EIPS/eip-170.md).
+
+---
+
+## Function names
+
+- In Ninja we brute force hashes of function names to find those that starts `0000`, so this can save around 50 gas
+- TBA: show the code
+
+---
+
+## Pack variables
+
+The below code is an example of poor code and will consume 3 storage slot:
+
+```
+uint8 numberOne;
+uint256 bigNumber;
+uint8 numberTwo;
+```
+
+A much more efficient way to do this in solidity will be:
+
+```
+uint8 numberOne;
+uint8 numberTwo;
+uint256 bigNumber;
+```
+
+---
+
+### Mappings are cheaper than Arrays
+
+- An array is not stored sequentially in memory but as a mapping.
+- You can pack Arrays but not Mappings.
+- It’s cheaper to use arrays if you are using smaller elements like `uint8` which can be packed together.
+- You can’t get the length of a mapping or parse through all its elements, so depending on your use case, you might be forced to use an Array even though it might cost you more gas.
+
+---
+
+## **Use bytes32 rather than string/bytes**
+
+- If you can fit your data in 32 bytes, then you should use bytes32 datatype rather than bytes or strings as it is much cheaper in solidity.
+- Any fixed size variable in solidity is cheaper than variable size.
+
+---
+
+## Use external function modifier
+
+- For all the public functions, the input parameters are copied to memory automatically, and it costs gas.
+- If your function is only called externally, then you should explicitly mark it as external.
+- External function’s parameters are not copied into memory but are read from `calldata` directly.
+- By the way, calling internal functions is cheaper.
+
+---
+
+## Delete variables that you don’t need
+
+- If you don’t need a variable anymore, you should delete it using the delete keyword provided by solidity or by setting it to its default value.
+
+---
+
+## **No need to initialize variables with default values**
+
+- If a variable is not set/initialized, it is assumed to have the default value (0, false, 0x0 etc depending on the data type). If you explicitly initialize it with its default value, you are just wasting gas.
+
+```
+uint256 hello = 0; //bad, expensive
+uint256 world; //good, cheap
+```
+
+---
+
+## **Make use of single line swaps**
+
+```
+(hello, world) = (world, hello)
+```