pragma solidity >=0.8.0;

// SPDX-License-Identifier: AGPL-3.0-or-later
// File-Version: 3

contract AccountsIndex {

	uint256 constant blockedField = 1 << 128;
	address[] entryList;
	mapping(address => uint256) entryIndex;

	// Implements Writer
	mapping(address => bool) public isWriter;

	// Implements ERC173
	address public owner;

	// Implements AccountsIndex
	event AddressAdded(address _account); // AccountsIndex
	// Implements AccountsIndex
	event AddressActive(address indexed _account, bool _active);
	// Implements AccountsIndex
	event AddressRemoved(address _account);
	// Implements ERC173
	event OwnershipTransferred(address indexed previousOwner, address indexed newOwner); // EIP173
	// Implements Writer
	event WriterAdded(address _account); // AccountsIndex
	// Implements Writer
	event WriterDeleted(address _account);

	constructor() {
		owner = msg.sender;
		entryList.push(address(0));
	}

	function entryCount() external view returns (uint256) {
		return entryList.length - 1;
	}

	function addWriter(address _writer) public returns (bool) {
		require(owner == msg.sender);
		isWriter[_writer] = true;
		emit WriterAdded(_writer);
		return true;
	}

	function deleteWriter(address _writer) public returns (bool) {
		require(owner == msg.sender);
		delete isWriter[_writer];
		emit WriterDeleted(_writer);
		return true;
	}

	// Implements AccountsIndex
	function add(address _account) external returns (bool) {
		uint256 i;
		uint256 _entry;

		require(isWriter[msg.sender]);
		require(entryIndex[_account] == 0);
		require(entryList.length < (1 << 64));
		i = entryList.length;
		entryList.push(_account);
		_entry = uint64(i);
		_entry |= block.timestamp << 64;
		entryIndex[_account] = _entry;
		emit AddressAdded(_account);
		return true;
	}

	// Implements AccountsIndex
	function remove(address _account) external returns (bool) {
		uint256 i;
		uint256 l;

		require(isWriter[msg.sender]);
		require(this.have(_account));

		l = entryList.length - 1;

		i = entryIndex[_account];
		if (i < l) {
			entryList[i] = entryList[l];
		}		
		entryList.pop();
		entryIndex[_account] = 0;
		emit AddressRemoved(_account);
		return true;
	}

	// Implements AccountsIndex
	// Activate previously deactivated account. Will not affect the entry count.
	function activate(address _account) external returns (bool) {
		require(isWriter[msg.sender]);
		require(entryIndex[_account] > 0 && entryIndex[_account] & blockedField == blockedField);
		entryIndex[_account] >>= 129;
		emit AddressActive(_account, true);
		return true;
	}

	// Implements AccountsIndex
	// Deactivate account, without removing the entry. The entry will still be part of the entry count.
	function deactivate(address _account) external returns (bool) {
		require(isWriter[msg.sender]);
		require(entryIndex[_account] > 0 && entryIndex[_account] & blockedField == 0);
		entryIndex[_account] <<= 129;
		entryIndex[_account] |= blockedField;
		emit AddressActive(_account, false);
		return true;
	}

	// Implements AccountsIndex
	function entry(uint256 _i) external view returns (address) {
		return entryList[_i + 1];
	}

	// Implements AccountsIndex
	function time(address _account) external view returns (uint256) {
		require(entryIndex[_account] > 0);
		return entryIndex[_account] >> 64;
	}


	// Implements AccountsIndex
	function have(address _account) external view returns (bool) {
		return entryIndex[_account] > 0;
	}

	// Implements AccountsIndex
	function isActive(address _account) external view returns (bool) {
		return this.have(_account) && entryIndex[_account] & blockedField != blockedField;
	}

	// Implements EIP173
	function transferOwnership(address _newOwner) public returns (bool) {
		address oldOwner;
		require(msg.sender == owner);

		oldOwner = owner;
		owner = _newOwner;

		emit OwnershipTransferred(oldOwner, owner);
		return true;
	}

	// Implements EIP165
	function supportsInterface(bytes4 _sum) public pure returns (bool) {
		if (_sum == 0x23c5568b) { // AccountsIndex
			return true;
		}
		if (_sum == 0x01ffc9a7) { // EIP165
			return true;
		}
		if (_sum == 0x9493f8b2) { // EIP173
			return true;
		}
		if (_sum == 0xabe1f1f5) { // Writer
			return true;
		}
		return false;
	}
}