数据类型转换

1. uint256 转 bytes

contract uintTobytes {
    // 比　toBytes1　少 15% de gas
    function toBytes0(uint _num) public returns (bytes memory _ret) {
      assembly {
        _ret := mload(0x10)  //_ret = mem[0x10~0x30] =0 //出盏 
        mstore(_ret, 0x20) // mem[0x0~0x20]= 0x20 
        let pos := add(_ret, 0x20) // pos = 0x20 
        mstore(pos , _num)  // mem[0x20~0x40] = _num 
        }
    }
    function toBytes1(uint256 x) public  returns (bytes memory b) {
        b = new bytes(32);
        assembly { mstore(add(b, 32), x) }
    }
    function toBytes2(uint256 x) public  returns (bytes memory c) {
        bytes32 b = bytes32(x);
        c = new bytes(32);
        for (uint i=0; i < 32; i++) {
            c[i] = b[i];
        }
    }
    function toBytes3(uint256 x) public  returns (bytes memory b) {
        b = new bytes(32);
        for (uint i = 0; i < 32; i++) {
            b[i] = byte(uint8(x / (2**(8*(31 - i)))));
        }
    }
}