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pragma solidity ^0.8.0;

contract Lottery {
    address public manager;
    address[] public participants;
    address public lastWinner;
    uint public numberOfParticipants;
    uint public poolSize;
    uint public requiredParticipants = 5;
    uint public requiredEth = 0.1 ether;

    constructor() {
        manager = msg.sender;
    }

    function enter() public payable {
        require(msg.value == requiredEth, "You must send 0.1 ether to enter the lottery.");
        require(participants.length < requiredParticipants, "The current lottery pool is full.");

        for (uint i = 0; i < participants.length; i++) {
            require(participants[i] != msg.sender, "You are already a participant.");
        }

        participants.push(msg.sender);

        if (participants.length == requiredParticipants) {
            pickWinner();
            startNewLottery();
        }
    }

    function pickWinner() private {
        require(participants.length == requiredParticipants, "The pool is not full yet.");
        uint index = random() % requiredParticipants;
        lastWinner = participants[index];
        payable(lastWinner).transfer(poolSize);
    }

    function startNewLottery() private {
        delete participants;
        poolSize = 0;
    }

    function random() private view returns (uint) {
        return uint(keccak256(abi.encodePacked(block.difficulty, block.timestamp, participants)));
    }

    function viewParticipants() public view returns (address[] memory, uint) {
        return (participants, participants.length);
    }

    function viewPreviousWinner() public view returns (address) {
        require(lastWinner != address(0), "No lottery has been completed yet.");
        return lastWinner;
    }
}