Libraries are similar to contracts, but you can't declare any state variable and you can't send ether.

A library is embedded into the contract if all library functions are internal.

Otherwise the library must be deployed and then linked before the contract is deployed.

库类似于合约，但你不能声明任何状态变量和 你不能发送以太币。

如果所有库函数都是内部的，则将库嵌入到合约中。

否则，必须先部署库，然后在部署合约之前进行链接。

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.13;

library SafeMath {
    function add(uint x, uint y) internal pure returns (uint) {
        uint z = x + y;
        require(z >= x, "uint overflow");

        return z;
    }
}

library Math {
    function sqrt(uint y) internal pure returns (uint z) {
        if (y > 3) {
            z = y;
            uint x = y / 2 + 1;
            while (x < z) {
                z = x;
                x = (y / x + x) / 2;
            }
        } else if (y != 0) {
            z = 1;
        }
        // else z = 0 (default value)
    }
}

contract TestSafeMath {
    using SafeMath for uint;

    uint public MAX_UINT = 2**256 - 1;

    function testAdd(uint x, uint y) public pure returns (uint) {
        return x.add(y);
    }

    function testSquareRoot(uint x) public pure returns (uint) {
        return Math.sqrt(x);
    }
}

// Array function to delete element at index and re-organize the array
// so that their are no gaps between the elements.
library Array {
    function remove(uint[] storage arr, uint index) public {
        // Move the last element into the place to delete
        require(arr.length > 0, "Can't remove from empty array");
        arr[index] = arr[arr.length - 1];
        arr.pop();
    }
}

contract TestArray {
    using Array for uint[];

    uint[] public arr;

    function testArrayRemove() public {
        for (uint i = 0; i < 3; i++) {
            arr.push(i);
        }

        arr.remove(1);

        assert(arr.length == 2);
        assert(arr[0] == 0);
        assert(arr[1] == 2);
    }
}

abi.encode
encodes data into 
bytes
.

abi.decode
decodes 
bytes
back into data.

abi.encode 将数据编码为字节。

 abi.decode 将字节解码回数据。

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.13;

contract AbiDecode {
    struct MyStruct {
        string name;
        uint[2] nums;
    }

    function encode(
        uint x,
        address addr,
        uint[] calldata arr,
        MyStruct calldata myStruct
    ) external pure returns (bytes memory) {
        return abi.encode(x, addr, arr, myStruct);
    }

    function decode(bytes calldata data)
        external
        pure
        returns (
            uint x,
            address addr,
            uint[] memory arr,
            MyStruct memory myStruct
        )
    {
        // (uint x, address addr, uint[] memory arr, MyStruct myStruct) = ...
        (x, addr, arr, myStruct) = abi.decode(data, (uint, address, uint[], MyStruct));
    }
}