# Gas Effiecient Smart Contract

- Do not create contracts
- Use fixed sized-type variables
- use bytes32 instead of string or bytes
- use uint256 instead of uint32
- Minimize the frequency of data storage

  // Avoid
  function ttt() {
      a = 1;
      b = 2;
      .... // do something
      c = 3;
      d = 4
  }
  
  // better
  function ttt() {
      a = 1;
      b = 2;
      c = 3;
      d = 4;
      ... do something
  }

  // LOOP
  uint cnt;
  
  // avoid
  function ttt() {
      for(...) {
          cnt++;
      }
  }
  
  // better
  function ttt() {
      uint tmp = cnt;
      for(...) {
          tmp++;
      }
      cnt = tmp;
  }

- Data alignment (256 bits)

  // Avoid
  uint64 a;  --> x1
  uint b     --> x2
  byte c;    -|
  uint64 d;  --> x3
  
  // better
  uint64 a;   -|
  uint64 d;    |
  byte c;     --> x1
  uint b;     --> x2

- Batch execution
- gas cost
- standard tx: 21k
- external call: 1.4k

  // avoid
  function ttt(address user);
  
  // better
  function ttt(address[] users);
  ``
- Short circuit rule

function a() --> cost 5k
function b() --> cost 20k

// should
if (a() || b()) {
...
}

- Use `external` for functions only accessed externally. (not `public`)
  
### Miscs
- data storage (SSTORE)
  - new: 20k (zero --> nonzero)
  - modify: 5k
  - delete: -15k (actual: -10k (-15k+5k) )
- Instanbul hard fork
    - SLOAD: 200 --> 800
    - BALANCE: 400 --> 700
    - input data: 
        - nonzero: 68 --> 16
        - zero: still 4
- Interesting fact

function b() public --> cost n
function a() public --> cost n+22

  [Function Name Optimization](https://emn178.github.io/solidity-optimize-name/)
  

###### tags `note` `solidity`