Orderbook template contract for Blockchain for Fintect course

Orderbook.sol

pragma solidity ^0.5.2;

import "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/v2.5.0/contracts/token/ERC20/ERC20.sol";


contract Orderbook {
    using SafeMath for uint256;

    struct Order {
        uint256 Amount;
        uint256 Price;
        uint256 TimeStamp;
        address Trader;
        bytes2 Status; // ('A'—Available, 'T'—Traded).
    }

    Order[] Buys;
    Order[] Sells;
    ERC20 public ERC20Base;
    ERC20 public ERC20Counter;

    constructor(address Base, address Counter) public {
        ERC20Base = ERC20(Base);
        ERC20Counter = ERC20(Counter);
    }

    event BuyAdded(
        uint256 indexed Order_No,
        uint256 Amt,
        uint256 Price,
        address trader
    );

    event SellAdded(
        uint256 indexed Order_No,
        uint256 Amt,
        uint256 Price,
        address trader
    );

    event TradeAdd(
        uint256 indexed Order_No,
        uint256 Amt,
        uint256 Price,
        address maker, // The trader who placed the order that is being traded
        address taker // The trader who matches against the order.
    );

    function addBuy(uint256 Amt, uint256 BuyPrice) public returns (uint256) {
        // Transfer from senders address to this contract Amt*BuyPrice base tokens; Hint: use transferFrom ERC20 interface function
        // Create a new order;
        // Add the order to the Buys Array;
        // emit a BuyAdded event with the appropriate parameters
    }

    function addSell(uint256 Amt, uint256 SellPrice) public returns (uint256) {
        // Transfer from senders address to this contract Amt counter tokens; Hint: use transferFrom ERC20 interface function
        // Create a new order;
        // Add the order to the Sells array
        // emit a SellAdded event with the appropriate parameters
    }

    function viewLengthBuy() public view returns (uint256) {
        // return length of buys array;
    }

    function viewLengthSell() public view returns (uint256) {
        // return length of sells array;
    }

    function viewBuy(uint256 OrderNo)
        public
        view
        returns (
            uint256,
            uint256,
            uint256,
            address
        )
    {
        // return the fields of the specified sell order
    }

    function viewSell(uint256 OrderNo)
        public
        view
        returns (
            uint256,
            uint256,
            uint256,
            address
        )
    {
        // return the fields of the specified buy order
    }

    function trade(
        uint256 OrderNo,
        uint256 Amt,
        uint256 TradePrice,
        uint256 trade_type
    )
        public
        returns (
            uint256,
            uint256,
            address
        )
    {
        // tradeType: 1 is Buy trade , 2 is Sell Trade

        // =================================================================
        // Full Sell Order Fulfillment
        // =================================================================
        // If is buy trade and the given sell order matches the amount:
            // REQUIRE that trade price is larger or equal to sell order price, otherwise return "Invalid Price" error message
            // Transfer from the buyer to the seller Amt*SellOrder.price base tokens
            // Set the sell's order amount to 0 
            // Set the sell's order status to T
            // Transfer trade's amount counter tokens to buyer. Remember: The contract holds the counter tokens so it can directly send it to buyer
            // emit a TradeAdd event with the appropriate parameters
            // return Sell Order number that was fullfilled, amount and the buyer's address

        // =================================================================
        // Partial Sell Order Fulfillment
        // =================================================================
        // If is buy trade and the given sell order's amount is larger than the trade amount
            // Transfer from the buyer to the seller Amt*SellOrder.price base tokens
            // REQUIRE that trade price is larger or equal to sell order price, otherwise return "Invalid Price" error message
            // Subtract the trade's amount from sell's order amount and set this as the new sell order amount
            // Sell sell order's status to A
            // Transfer trade's amount counter tokens to buyer. Remember: The contract holds the counter tokens so it can directly send it to buyer
            // emit a TradeAdd event with the appropriate parameters
            // return Sell Order number that was fullfilled, amount and the buyer's address
        
        // =================================================================
        // Full Buy Order Fulfillment
        // =================================================================
        // If is sell trade and the given buy order's amount is equal to the trade amount 
            // Transfer from seller to buyer counter tokens that equal the trade amount
            // Set the buy order's  amount to 0
            // Set the buy order's status to 'T'
            // Transfer the trade's amount base tokens to seller 
            // emit a TradeAdd event with the appropriate parameters
            // return Buy Order number that was fullfilled, amount and the seller's address


        // =================================================================
        // Partial Buy Order Fulfillment
        // =================================================================
        // If is sell trade and the given buy order's amount is larger than the trade amount
            // Transfer from the seller to the buyer counter tokens that equal the trade amount
            // Subtract the trade's amount from buy order's amount and set this as the new buy order amount
            // Sell buy order's status to A
            // Transfer trade's amount base tokens to seller. 
            // emit a TradeAdd event with the appropriate parameters
            // return buy Order number that was fullfilled, amount and the seller's address

        // For any other case REVERT the transaction and return "Invalid trade parameters" error message. Hint: use the "revert" operation
    }
}