Introduction

We will create a referral smart contract which is quite famous in multi level marketing applications.

Smart Contract

First need to add solidity compiler version

// SPDX-License-Identifier: MIT

pragma solidity  >=0.4.22 <0.9.0;

Then creating Refer contract ,

contract refer{
      ...
}

Now let's create variables and objects.

Declaring Variables

We wil create a user struct which will have that user's referrer and a flag if the user is referral or not.

This referral smart contract will be of four levels and we need to assign a struct for those levels for his referral levels and we will map those structs with addresses.

struct User {
    bool referred;
    address referred_by;
}

struct Referal_levels {
    uint256 level_1;
    uint256 level_2;
    uint256 level_3;
    uint256 level_4;
}

mapping(address => Referal_levels) public refer_info;
mapping(address => User) public user_info;

Add Referee Function

Function will take an address as an argument and will have a public visibility. We will increment each level of reference with the help of referred_by  variable from User struct whernever this function is called until user has no referral.

function referee(address ref_add) public {
        require(user_info[msg.sender].referred == false, " Already referred ");
        require(ref_add != msg.sender, " You cannot refer yourself ");

        user_info[msg.sender].referred_by = ref_add;
        user_info[msg.sender].referred = true;

        address level1 = user_info[msg.sender].referred_by;
        address level2 = user_info[level1].referred_by;
        address level3 = user_info[level2].referred_by;
        address level4 = user_info[level3].referred_by;

        if ((level1 != msg.sender) && (level1 != address(0))) {
            refer_info[level1].level_1 += 1;
        }
        if ((level2 != msg.sender) && (level2 != address(0))) {
            refer_info[level2].level_2 += 1;
        }
        if ((level3 != msg.sender) && (level3 != address(0))) {
            refer_info[level3].level_3 += 1;
        }
        if ((level4 != msg.sender) && (level4 != address(0))) {
            refer_info[level4].level_4 += 1;
        }
}

Complete Code

// SPDX-License-Identifier: MIT

pragma solidity  >=0.4.22 <0.9.0;

contract refer{
    
struct User {
    bool referred;
    address referred_by;
}

struct Referal_levels {
    uint256 level_1;
    uint256 level_2;
    uint256 level_3;
    uint256 level_4;
}

mapping(address => Referal_levels) public refer_info;
mapping(address => User) public user_info;

function referee(address ref_add) public {
        require(user_info[msg.sender].referred == false, " Already referred ");
        require(ref_add != msg.sender, " You cannot refer yourself ");

        user_info[msg.sender].referred_by = ref_add;
        user_info[msg.sender].referred = true;

        address level1 = user_info[msg.sender].referred_by;
        address level2 = user_info[level1].referred_by;
        address level3 = user_info[level2].referred_by;
        address level4 = user_info[level3].referred_by;

        if ((level1 != msg.sender) && (level1 != address(0))) {
            refer_info[level1].level_1 += 1;
        }
        if ((level2 != msg.sender) && (level2 != address(0))) {
            refer_info[level2].level_2 += 1;
        }
        if ((level3 != msg.sender) && (level3 != address(0))) {
            refer_info[level3].level_3 += 1;
        }
        if ((level4 != msg.sender) && (level4 != address(0))) {
            refer_info[level4].level_4 += 1;
        }
}

}