erc20-demurrage-token/solidity/RedistributedDemurrageToken.sol

pragma solidity > 0.6.11;

// SPDX-License-Identifier: GPL-3.0-or-later

contract RedistributedDemurrageToken {

	// Redistribution bit field, with associated shifts and masks
	// (Uses sub-byte boundaries)
	bytes32[] public redistributions; // uint1(isFractional) | uint95(unused) | uint20(demurrageModifier) | uint36(participants) | uint72(value) | uint32(period)
	uint8 constant shiftRedistributionPeriod 	= 0;
	uint256 constant maskRedistributionPeriod 	= 0x00000000000000000000000000000000000000000000000000000000ffffffff; // (1 << 32) - 1
	uint8 constant shiftRedistributionValue 	= 32;
	uint256 constant maskRedistributionValue	= 0x00000000000000000000000000000000000000ffffffffffffffffff00000000; // ((1 << 72) - 1) << 32
	uint8 constant shiftRedistributionParticipants	= 104;
	uint256 constant maskRedistributionParticipants	= 0x00000000000000000000000000000fffffffff00000000000000000000000000; // ((1 << 36) - 1) << 104
	uint8 constant shiftRedistributionDemurrage	= 140;
	uint256 constant maskRedistributionDemurrage	= 0x000000000000000000000000fffff00000000000000000000000000000000000; // ((1 << 20) - 1) << 140
	uint8 constant shiftRedistributionIsFractional	= 255;
	uint256 constant maskRedistributionIsFractional	= 0x8000000000000000000000000000000000000000000000000000000000000000; // 1 << 255

	// Account bit field, with associated shifts and masks
	// Mirrors structure of redistributions for consistency
	mapping (address => bytes32) account; // uint152(unused) | uint32(period) | uint72(value)
	uint8 constant shiftAccountValue		= 0;
	uint256 constant maskAccountValue 		= 0x0000000000000000000000000000000000000000000000ffffffffffffffffff; // (1 << 72) - 1
	uint8 constant shiftAccountPeriod		= 72;
	uint256 constant maskAccountPeriod 		= 0x00000000000000000000000000000000000000ffffffff000000000000000000; // ((1 << 32) - 1) << 72

	// Cached demurrage amount, ppm with 38 digit resolution
	uint128 public demurrageAmount;

	// Cached demurrage period; the period for which demurrageAmount was calculated
	uint128 public demurragePeriod;

	// Implements EIP172
	address public owner;

	// Implements ERC20
	string public name;

	// Implements ERC20
	string public symbol;

	// Implements ERC20
	uint256 public decimals;

	// Implements ERC20
	uint256 public totalSupply;

	// Minimum amount of (demurraged) tokens an account must spend to participate in redistribution for a particular period
	uint256 public minimumParticipantSpend;

	// 128 bit resolution of the demurrage divisor
	// (this constant x 1000000 is contained within 128 bits)
	uint256 constant ppmDivider = 100000000000000000000000000000000;

	// Timestamp of start of periods (time which contract constructor was called)
	uint256 public immutable periodStart;

	// Duration of a single redistribution period in seconds
	uint256 public immutable periodDuration;

	// Demurrage in ppm per minute
	uint256 public immutable taxLevel;

	// Addresses allowed to mint new tokens
	mapping (address => bool) minter;

	// Storage for ERC20 approve/transferFrom methods
	mapping (address => mapping (address => uint256 ) ) allowance; // holder -> spender -> amount (amount is subject to demurrage)

	// Address to send unallocated redistribution tokens
	address sinkAddress; 

	// Implements ERC20
	event Transfer(address indexed _from, address indexed _to, uint256 _value);

	// Implements ERC20
	event Approval(address indexed _owner, address indexed _spender, uint256 _value);

	// New tokens minted
	event Mint(address indexed _minter, address indexed _beneficiary, uint256 _value);

	// New demurrage cache milestone calculated
	event Decayed(uint256 indexed _period, uint256 indexed _periodCount, uint256 indexed _oldAmount, uint256 _newAmount);

	// When a new period threshold has been crossed
	event Period(uint256 _period);

	// Redistribution applied on a single eligible account
	event Redistribution(address indexed _account, uint256 indexed _period, uint256 _value);

	// Temporary event used in development, will be removed on prod
	event Debug(bytes32 _foo);

	constructor(string memory _name, string memory _symbol, uint8 _decimals, uint256 _taxLevelMinute, uint256 _periodMinutes, address _defaultSinkAddress) public {
		// ACL setup
		owner = msg.sender;
		minter[owner] = true;

		// ERC20 setup
		name = _name;
		symbol = _symbol;
		decimals = _decimals;

		// Demurrage setup
		periodStart = block.timestamp;
		periodDuration = _periodMinutes * 60;
		demurrageAmount = uint128(ppmDivider * 1000000); // Represents 38 decimal places
		demurragePeriod = 1;
		taxLevel = _taxLevelMinute; // Represents 38 decimal places
		bytes32 initialRedistribution = toRedistribution(0, 1000000, 0, 1);
		redistributions.push(initialRedistribution);

		// Misc settings
		sinkAddress = _defaultSinkAddress;
		minimumParticipantSpend = 10 ** uint256(_decimals);
	}

	// Given address will be allowed to call the mintTo() function
	function addMinter(address _minter) public returns (bool) {
		require(msg.sender == owner);
		minter[_minter] = true;
		return true;
	}

	// Given address will no longer be allowed to call the mintTo() function
	function removeMinter(address _minter) public returns (bool) {
		require(msg.sender == owner || _minter == msg.sender);
		minter[_minter] = false;
		return true;
	}

	/// Implements ERC20
	function balanceOf(address _account) public view returns (uint256) {
		uint256 baseBalance;
		uint256 currentDemurragedAmount;
		uint256 periodCount;

		baseBalance = baseBalanceOf(_account);

		periodCount = actualPeriod() - demurragePeriod; 

		currentDemurragedAmount = uint128(decayBy(demurrageAmount, periodCount));

		return (baseBalance * currentDemurragedAmount) / (ppmDivider * 1000000);
	}

	/// Balance unmodified by demurrage
	function baseBalanceOf(address _account) public view returns (uint256) {
		return uint256(account[_account]) & maskAccountValue;
	}

	/// Increases base balance for a single account
	function increaseBaseBalance(address _account, uint256 _delta) private returns (bool) {
		uint256 oldBalance;
		uint256 newBalance;
		uint256 workAccount;

		workAccount = uint256(account[_account]);
	
		if (_delta == 0) {
			return false;
		}

		oldBalance = baseBalanceOf(_account);
		newBalance = oldBalance + _delta;
		require(uint160(newBalance) > uint160(oldBalance), 'ERR_WOULDWRAP'); // revert if increase would result in a wrapped value
		workAccount &= (~maskAccountValue); 
		workAccount |= (newBalance & maskAccountValue);
		account[_account] = bytes32(workAccount);
		return true;
	}

	/// Decreases base balance for a single account
	function decreaseBaseBalance(address _account, uint256 _delta) private returns (bool) {
		uint256 oldBalance;
	       	uint256 newBalance;
		uint256 workAccount;

		workAccount = uint256(account[_account]);

		if (_delta == 0) {
			return false;
		}

		oldBalance = baseBalanceOf(_account);	
		require(oldBalance >= _delta, 'ERR_OVERSPEND'); // overspend guard
		newBalance = oldBalance - _delta;
		workAccount &= (~maskAccountValue);
		workAccount |= (newBalance & maskAccountValue);
		account[_account] = bytes32(workAccount);
		return true;
	}

	// Creates new tokens out of thin air, and allocates them to the given address
	// Triggers tax
	function mintTo(address _beneficiary, uint256 _amount) external returns (bool) {
		uint256 baseAmount;

		require(minter[msg.sender]);

		changePeriod();
		baseAmount = _amount;
		totalSupply += _amount;
		increaseBaseBalance(_beneficiary, baseAmount);
		emit Mint(msg.sender, _beneficiary, _amount);
		saveRedistributionSupply();
		return true;
	}

	// Deserializes the redistribution word
	// uint1(isFractional) | uint95(unused) | uint20(demurrageModifier) | uint36(participants) | uint72(value) | uint32(period)
	function toRedistribution(uint256 _participants, uint256 _demurrageModifierPpm, uint256 _value, uint256 _period) private pure returns(bytes32) {
		bytes32 redistribution;

		redistribution |= bytes32((_demurrageModifierPpm << shiftRedistributionDemurrage) & maskRedistributionDemurrage);
		redistribution |= bytes32((_participants << shiftRedistributionParticipants) & maskRedistributionParticipants);
		redistribution |= bytes32((_value << shiftRedistributionValue) & maskRedistributionValue); 
		redistribution |= bytes32(_period & maskRedistributionPeriod);
		return redistribution;
	}

	// Serializes the demurrage period part of the redistribution word
	function toRedistributionPeriod(bytes32 redistribution) public pure returns (uint256) {
		return uint256(redistribution) & maskRedistributionPeriod;
	}

	// Serializes the supply part of the redistribution word
	function toRedistributionSupply(bytes32 redistribution) public pure returns (uint256) {
		return (uint256(redistribution) & maskRedistributionValue) >> shiftRedistributionValue;
	}

	// Serializes the number of participants part of the redistribution word
	function toRedistributionParticipants(bytes32 redistribution) public pure returns (uint256) {
		return (uint256(redistribution) & maskRedistributionParticipants) >> shiftRedistributionParticipants;
	}

	// Serializes the number of participants part of the redistribution word
	function toRedistributionDemurrageModifier(bytes32 redistribution) public pure returns (uint256) {
		return (uint256(redistribution) & maskRedistributionDemurrage) >> shiftRedistributionDemurrage;
	}

	// Client accessor to the redistributions array length
	function redistributionCount() public view returns (uint256) {
		return redistributions.length;
	}

	// Add number of participants for the current redistribution period by one
	function incrementRedistributionParticipants() private returns (bool) {
		bytes32 currentRedistribution;
		uint256 tmpRedistribution;
		uint256 participants;

		currentRedistribution = redistributions[redistributions.length-1];
		participants = toRedistributionParticipants(currentRedistribution) + 1;
		tmpRedistribution = uint256(currentRedistribution);
		tmpRedistribution &= (~maskRedistributionParticipants);
		tmpRedistribution |= ((participants << shiftRedistributionParticipants) & maskRedistributionParticipants);

		redistributions[redistributions.length-1] = bytes32(tmpRedistribution);

		return true;
	}

	// Save the current total supply amount to the current redistribution period
	function saveRedistributionSupply() private returns (bool) {
		uint256 currentRedistribution;

		currentRedistribution = uint256(redistributions[redistributions.length-1]);
		currentRedistribution &= (~maskRedistributionValue);
		currentRedistribution |= (totalSupply << shiftRedistributionValue);

		redistributions[redistributions.length-1] = bytes32(currentRedistribution);
		return true;
	}

	// Get the demurrage period of the current block number
	function actualPeriod() public view returns (uint128) {
		return uint128((block.timestamp - periodStart) / periodDuration + 1);
	}

	// Add an entered demurrage period to the redistribution array
	function checkPeriod() private view returns (bytes32) {
		bytes32 lastRedistribution;
		uint256 currentPeriod;

		lastRedistribution =  redistributions[redistributions.length-1];
		currentPeriod = this.actualPeriod();
		if (currentPeriod <= toRedistributionPeriod(lastRedistribution)) {
			return bytes32(0x00);
		}
		return lastRedistribution;
	}

	// Deserialize the pemurrage period for the given account is participating in
	function accountPeriod(address _account) public view returns (uint256) {
		return (uint256(account[_account]) & maskAccountPeriod) >> shiftAccountPeriod;
	}

	// Save the given demurrage period as the currently participation period for the given address
	function registerAccountPeriod(address _account, uint256 _period) private returns (bool) {
		account[_account] &= bytes32(~maskAccountPeriod);
		account[_account] |= bytes32((_period << shiftAccountPeriod) & maskAccountPeriod);
		incrementRedistributionParticipants();
		return true;
	}

	// Determine whether the unit number is rounded down, rounded up or evenly divides.
	// Returns 0 if evenly distributed, or the remainder as a positive number
	// A _numParts value 0 will be interpreted as the value 1
	function remainder(uint256 _numParts, uint256 _sumWhole) public pure returns (uint256) {
		uint256 unit;
		uint256 truncatedResult;

		if (_numParts == 0) { // no division by zero please
			revert('ERR_NUMPARTS_ZERO');
		}
		require(_numParts < _sumWhole); // At least you are never LESS than the sum of your parts. Think about that.

		unit = _sumWhole / _numParts;
		truncatedResult = unit * _numParts;
		return _sumWhole - truncatedResult;
	}

	// Called in the edge case where participant number is 0. It will override the participant count to 1.
	// Returns the remainder sent to the sink address
	function applyDefaultRedistribution(bytes32 _redistribution) private returns (uint256) {
		uint256 redistributionSupply;
		uint256 redistributionPeriod;
		uint256 unit;
		uint256 truncatedResult;

		redistributionSupply = toRedistributionSupply(_redistribution);

		unit = (redistributionSupply * taxLevel) / 1000000;
		truncatedResult = (unit * 1000000) / taxLevel;

		if (truncatedResult < redistributionSupply) {
			redistributionPeriod = toRedistributionPeriod(_redistribution); // since we reuse period here, can possibly be optimized by passing period instead
			redistributions[redistributionPeriod-1] &= bytes32(~maskRedistributionParticipants); // just to be safe, zero out all participant count data, in this case there will be only one
			redistributions[redistributionPeriod-1] |= bytes32(maskRedistributionIsFractional | (1 << shiftRedistributionParticipants));
		}

		increaseBaseBalance(sinkAddress, unit / ppmDivider);
		return unit;
	}

	// sets the remainder bit for the given period and books the remainder to the sink address balance
	// returns false if no change was made
	function applyRemainderOnPeriod(uint256 _remainder, uint256 _period) private returns (bool) {
		uint256 periodSupply;

		if (_remainder == 0) {
			return false;
		}

		// TODO: is this needed?
		redistributions[_period-1] |= bytes32(maskRedistributionIsFractional);

		periodSupply = toRedistributionSupply(redistributions[_period-1]);
		increaseBaseBalance(sinkAddress, periodSupply - _remainder);
		return true;
	}


	// Calculate and cache the demurrage value corresponding to the (period of the) time of the method call
	function applyDemurrage() public returns (bool) {
		uint128 epochPeriodCount;
		uint128 periodCount;
		uint256 lastDemurrageAmount;
		uint256 newDemurrageAmount;

		epochPeriodCount = actualPeriod();
		periodCount = epochPeriodCount - demurragePeriod;
		if (periodCount == 0) {
			return false;
		}
		lastDemurrageAmount = demurrageAmount;
		demurrageAmount = uint128(decayBy(lastDemurrageAmount, periodCount));
		demurragePeriod = epochPeriodCount; 
		emit Decayed(epochPeriodCount, periodCount, lastDemurrageAmount, demurrageAmount);
		return true;
	}

	// Return timestamp of start of period threshold
	function getPeriodTimeDelta(uint256 _periodCount) public view returns (uint256) {
		return periodStart + (_periodCount * periodDuration);
	}

	// Amount of demurrage cycles inbetween the current timestamp and the given target time
	function demurrageCycles(uint256 _target) public view returns (uint256) {
		return (block.timestamp - _target) / 60;
	}

	// Recalculate the demurrage modifier for the new period
	// After this, all REPORTED balances will have been reduced by the corresponding ratio (but the effecive totalsupply stays the same)
	function changePeriod() public returns (bool) {
		bytes32 currentRedistribution;
		bytes32 nextRedistribution;
		uint256 currentPeriod;
		uint256 currentParticipants;
		uint256 currentRemainder;
		uint256 currentDemurrageAmount;
		uint256 nextRedistributionDemurrage;
		uint256 demurrageCounts;
		uint256 periodTimestamp;
		uint256 nextPeriod;

		currentRedistribution = checkPeriod();
		if (currentRedistribution == bytes32(0x00)) {
			return false;
		}

		currentPeriod = toRedistributionPeriod(currentRedistribution);
		nextPeriod = currentPeriod + 1;
		periodTimestamp = getPeriodTimeDelta(currentPeriod);

		applyDemurrage();
		currentDemurrageAmount = demurrageAmount; 

		demurrageCounts = demurrageCycles(periodTimestamp);
		if (demurrageCounts > 0) {
			nextRedistributionDemurrage = growBy(currentDemurrageAmount, demurrageCounts) / ppmDivider;
		} else {
			nextRedistributionDemurrage = currentDemurrageAmount / ppmDivider;
		}
		
		nextRedistribution = toRedistribution(0, nextRedistributionDemurrage, totalSupply, nextPeriod);
		redistributions.push(nextRedistribution);

		currentParticipants = toRedistributionParticipants(currentRedistribution);
		if (currentParticipants == 0) {
			currentRemainder = applyDefaultRedistribution(currentRedistribution);
		} else {
			currentRemainder = remainder(currentParticipants, totalSupply); // we can use totalSupply directly because it will always be the same as the recorded supply on the current redistribution
			applyRemainderOnPeriod(currentRemainder, currentPeriod);
		}
		emit Period(nextPeriod);
		return true;
	}

	// Reverse a value reduced by demurrage by the given period to its original value
	function growBy(uint256 _value, uint256 _period) public view returns (uint256) {
		uint256 valueFactor;
		uint256 truncatedTaxLevel;
	      
		valueFactor = 1000000;
		truncatedTaxLevel = taxLevel / ppmDivider;

		for (uint256 i = 0; i < _period; i++) {
			valueFactor = valueFactor + ((valueFactor * truncatedTaxLevel) / 1000000);
		}
		return (valueFactor * _value) / 1000000;
	}

	// Calculate a value reduced by demurrage by the given period
	// TODO: higher precision if possible
	function decayBy(uint256 _value, uint256 _period) public view returns (uint256) {
		uint256 valueFactor;
		uint256 truncatedTaxLevel;
	      
		valueFactor = 1000000;
		truncatedTaxLevel = taxLevel / ppmDivider;

		for (uint256 i = 0; i < _period; i++) {
			valueFactor = valueFactor - ((valueFactor * truncatedTaxLevel) / 1000000);
		}
		return (valueFactor * _value) / 1000000;
	}

	// If the given account is participating in a period and that period has been crossed
	// THEN increase the base value of the account with its share of the value reduction of the period
	function applyRedistributionOnAccount(address _account) public returns (bool) {
		bytes32 periodRedistribution;
		uint256 supply;
		uint256 participants;
		uint256 baseValue;
		uint256 value;
		uint256 period;
		uint256 demurrage;
	       
		period = accountPeriod(_account);
		if (period == 0 || period >= actualPeriod()) {
			return false;
		}
		periodRedistribution = redistributions[period-1];
		participants = toRedistributionParticipants(periodRedistribution);
		if (participants == 0) {
			return false;
		}

		supply = toRedistributionSupply(periodRedistribution);
		demurrage = toRedistributionDemurrageModifier(periodRedistribution);
		baseValue = ((supply / participants) * (taxLevel / 1000000)) / ppmDivider;
		value = (baseValue * demurrage) / 1000000;

		// zero out period for the account
		account[_account] &= bytes32(~maskAccountPeriod); 
		increaseBaseBalance(_account, value);

		emit Redistribution(_account, period, value);
		return true;
	}

	// Inflates the given amount according to the current demurrage modifier
	function toBaseAmount(uint256 _value) public view returns (uint256) {
		//return (_value * ppmDivider * 1000000) / toDemurrageAmount(demurrageModifier);
		return (_value * ppmDivider * 1000000) / demurrageAmount;
	}

	// ERC20, triggers tax and/or redistribution
	function approve(address _spender, uint256 _value) public returns (bool) {
		uint256 baseValue;

		changePeriod();
		applyRedistributionOnAccount(msg.sender);

		baseValue = toBaseAmount(_value);
		allowance[msg.sender][_spender] += baseValue;
		emit Approval(msg.sender, _spender, _value);
		return true;
	}

	// ERC20, triggers tax and/or redistribution
	function transfer(address _to, uint256 _value) public returns (bool) {
		uint256 baseValue;
		bool result;

		changePeriod();
		applyRedistributionOnAccount(msg.sender);

		baseValue = toBaseAmount(_value);
		result = transferBase(msg.sender, _to, baseValue);
		emit Transfer(msg.sender, _to, _value);
		return result;
	}


	// ERC20, triggers tax and/or redistribution
	function transferFrom(address _from, address _to, uint256 _value) public returns (bool) {
		uint256 baseValue;
		bool result;

		changePeriod();
		applyRedistributionOnAccount(msg.sender);

		baseValue = toBaseAmount(_value);
		require(allowance[_from][msg.sender] >= baseValue);

		result = transferBase(_from, _to, baseValue);
		emit Transfer(_from, _to, _value);
		return result;
	}

	// ERC20 transfer backend for transfer, transferFrom
	function transferBase(address _from, address _to, uint256 _value) private returns (bool) {
		uint256 period;

		decreaseBaseBalance(_from, _value);
		increaseBaseBalance(_to, _value);

		period = actualPeriod();
		if (_value >= minimumParticipantSpend && accountPeriod(_from) != period && _from != _to) {
			registerAccountPeriod(_from, period);
		}
		return true;
	}
}