// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

import {Math} from "@openzeppelin/contracts/utils/math/Math.sol";
import {SafeMath} from "@openzeppelin/contracts/utils/math/SafeMath.sol";
import {Ownable} from "@openzeppelin/contracts/access/Ownable.sol";
import {IEpoch} from "../interfaces/IEpoch.sol";

contract Epoch is IEpoch, Ownable {
    using SafeMath for uint256;

    uint256 private period;
    uint256 private startTime;
    uint256 private lastExecutedAt;

    /* ========== CONSTRUCTOR ========== */

    constructor(
        uint256 _period,
        uint256 _startTime,
        uint256 _startEpoch
    ) {
        period = _period;
        startTime = _startTime;
        lastExecutedAt = startTime.add(_startEpoch.mul(period));
    }

    /* ========== Modifier ========== */

    modifier checkStartTime() {
        require(block.timestamp >= startTime, "Epoch: not started yet");
        _;
    }

    modifier checkEpoch() {
        require(block.timestamp > startTime, "Epoch: not started yet");
        require(_callable(), "Epoch: not allowed");
        _;
        lastExecutedAt = block.timestamp;
    }

    function _getLastEpoch() internal view returns (uint256) {
        return lastExecutedAt.sub(startTime).div(period);
    }

    function _getCurrentEpoch() internal view returns (uint256) {
        return Math.max(startTime, block.timestamp).sub(startTime).div(period);
    }

    function callable() external view override returns (bool) {
        return _callable();
    }

    function _callable() internal view returns (bool) {
        return _getCurrentEpoch() >= _getNextEpoch();
    }

    function _getNextEpoch() internal view returns (uint256) {
        if (startTime == lastExecutedAt) {
            return _getLastEpoch();
        }
        return _getLastEpoch().add(1);
    }

    // epoch
    function getLastEpoch() external view override returns (uint256) {
        return _getLastEpoch();
    }

    function getCurrentEpoch() external view override returns (uint256) {
        return Math.max(startTime, block.timestamp).sub(startTime).div(period);
    }

    function getNextEpoch() external view override returns (uint256) {
        return _getNextEpoch();
    }

    function nextEpochPoint() external view override returns (uint256) {
        return startTime.add(_getNextEpoch().mul(period));
    }

    // params
    function getPeriod() external view override returns (uint256) {
        return period;
    }

    function getStartTime() external view override returns (uint256) {
        return startTime;
    }

    /* ========== GOVERNANCE ========== */

    function setPeriod(uint256 _period) external onlyOwner {
        period = _period;
    }
}