Libraries

Libraries are similar to Contracts but are mainly intended for reuse. A Library contains functions which other contracts can call. Solidity have certain restrictions on use of a Library. Following are the key characteristics of a Solidity Library.

Library functions can be called directly if they do not modify the state. That means pure or view functions only can be called from outside the library.
Library can not be destroyed as it is assumed to be stateless.
A Library cannot have state variables.
A Library cannot inherit any element.
A Library cannot be inherited.

Example: -1

// SPDX-License-Identifier: GPL-3.0

pragma solidity >=0.8.0 <0.9.0;

library Search {
    //mark self keyword here
    function indexOf(uint256[] storage self, uint256 value)
        public
        view
        returns (uint256)
    {
        for (uint256 i = 0; i < self.length; i++) {
            if (self[i] == value) return i;
        }
        return 0;
    }
}

contract SearchLibrary {
    uint256[] data;

    constructor() {
        for (uint256 i = 1; i < 6; i++) {
            data.push(i);
        }
    }
    
    function isValuePresent(uint256 val) external view returns (uint256) {
        return Search.indexOf(data, val); // mark how library function is called here
    }
}

Example:- 2

// SPDX-License-Identifier: GPL-3.0

pragma solidity >=0.8.0 <0.9.0;

library Search {
    function indexOf(uint256[] storage self, uint256 val)
        public
        view
        returns (uint256)
    {
        for (uint256 i = 0; i < self.length; i++) {
            if (self[i] == val) return i;
        }
        return 0;
    }
}

contract SearchFrom {
    // using A for B library pattern assignment
    using Search for uint256[];
    uint256[] data;

    constructor() {
        data.push(1);
        data.push(2);
        data.push(3);
        data.push(4);
        data.push(5);
    }

    function getIndex() external view returns (uint256) {
        uint256 val = 4;
        return data.indexOf(val); // insanly smart. essenctially we are creating a custom function for data here!!!
    }
}

Example:- 3

// SPDX-License-Identifier: GPL-3.0

pragma solidity >=0.8.0 <0.9.0;

library Arithmatic {
    function add(uint256 _a, uint256 _b) external pure returns (uint256) {
        uint256 sum = _a + _b;
        return sum;
    }

    function substract(uint256 _a, uint256 _b) external pure returns (uint256) {
        uint256 sum;
        if (_a > _b) sum = _a - _b;
        else sum = _b - _a;
        return sum;
    }

    function multiply(uint256 _a, uint256 _b) external pure returns (uint256) {
        uint256 sum = _a * _b;
        return sum;
    }

    modifier checkZero(uint256 _b) {
        require(_b > 0, "Denominator cannot be zero");
        _;
    }

    function divide(uint256 _a, uint256 _b)
        external
        pure
        checkZero(_b)
        returns (uint256)
    {
        uint256 sum;
        sum = _a / _b;
        return sum;
    }
}

contract PerformArithmatic {
    function add(uint256 _a, uint256 _b) external pure returns (uint256) {
        return Arithmatic.add(_a, _b);
    }

    function subtract(uint256 _a, uint256 _b) external pure returns (uint256) {
        return Arithmatic.substract(_a, _b);
    }

    function multiply(uint256 _a, uint256 _b) external pure returns (uint256) {
        return Arithmatic.multiply(_a, _b);
    }

    function divide(uint256 _a, uint256 _b) external pure returns (uint256) {
        return Arithmatic.divide(_a, _b);
    }
}

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Libraries