Building an Energy Market on the Blockchain: Evaluation, Wireframing, and Implementation

pragma solidity ^0.5.10;
import "@openzeppelin/contracts/token/ERC20/ERC20.sol";
import "@hq20/contracts/contracts/access/Whitelist.sol";


/**
 * @title Energy Market
 * @notice Implements a simple energy market, using ERC20 and 
 * Whitelist. ERC20 is used to enable payments from the consumers to
 * the distribution network, represented by this contract, and from
 * the distribution network to the producers. Whitelist is used to
 * keep a list of compliant smart meters that communicate the
 * production and consumption of energy.
 */
contract EnergyMarket is ERC20, Whitelist {
  event EnergyProduced(address producer, uint256 time);
  event EnergyConsumed(address consumer, uint256 time);
  // uint128 is used here to facilitate the price formula
  // Casting between uint128 and int256 never overflows
  // int256(uint128) - int256(uint128) never overflows
  mapping(uint256 => uint128) public consumption;
  mapping(uint256 => uint128) public production;
  uint128 public basePrice;

  /**
   * @dev The constructor initializes the underlying currency token
   * and the smart meter whitelist. The constructor also mints the
   * requested amount of the underlying currency token to fund the
   * network load. Also sets the base energy price, used for 
   * calculating prices.
   */
  constructor (uint256 _initialSupply, uint128 _basePrice)
    public
    ERC20()
    Whitelist()
  {
    _mint(address(this), _initialSupply);
    basePrice = _basePrice;
  }

  /**
   * @dev The production price for each time slot.
   */
  function getProductionPrice(uint256 _time)
    public
    view
    returns(uint256)
  {
    return uint256(
      max(
        0,
        int256(basePrice) *
          (3 + safeSub(production[_time], consumption[_time]))
      )
    );
  }

  /**
   * @dev The consumption price for each time slot
   */
  function getConsumptionPrice(uint256 _time)
    public
    view
    returns(uint256)
  {
    return uint256(
      max(
        0,
        int256(basePrice) *
          (3 + safeSub(consumption[_time], production[_time]))
      )
    );
  }

  /**
   * @dev Add one energy unit to the distribution network at the
   * specified time and be paid the production price. Only
   * whitelisted smart meters can call this function.
   */
  function produce(uint256 _time)
    public
  {
    require(isMember(msg.sender), "Unknown meter.");
    this.transfer(
      msg.sender,
      getProductionPrice(_time)
    );
    production[_time] = production[_time] + 1;
    emit EnergyProduced(msg.sender, _time);
  }

  /**
   * @dev Take one energy unit from the distribution network at the
   * specified time by paying the consumption price. Only
   * whitelisted smart meters can call this function.
   */
  function consume(uint256 _time)
    public
  {
    require(isMember(msg.sender), "Unknown meter.");
    this.transferFrom(
      msg.sender,
      address(this),
      getConsumptionPrice(_time)
    );
    consumption[_time] = consumption[_time] + 1;
    emit EnergyConsumed(msg.sender, _time);
  }

  /**
   * @dev Returns the largest of two numbers.
   */
  function max(int256 a, int256 b)
    internal
    pure
    returns(int256)
  {
    return a >= b ? a : b;
  }

  /**
   * @dev Substracts b from a using types safely casting from
   * uint128 to int256.
   */
  function safeSub(uint128 a, uint128 b)
    internal
    pure
    returns(int256)
  {
    return int256(a) - int256(b);
  }
}