pragma solidity >0.6.12;

// SPDX-License-Identifier: GPL-3.0-or-later
// File-Version: 2

contract CustodialAccountIndex {

	uint256 constant blockedField = 1 << 64;
	uint64 public entryCount;
	address[] entryList;
	mapping(address => uint256) public entryIndex;
	mapping(address => bool) writers;
	address public owner;
	address newOwner;

	event AddressAdded(address indexed _executor, uint256 indexed _accountIndex, address _account); // AccountsIndex
	event AddressActive(address indexed _executor, address _account);
	event AddressInactive(address indexed _executor, address _account);
	event AddressRemoved(address indexed _executor, address _account);
	event OwnershipTransferred(address indexed previousOwner, address indexed newOwner); // EIP173

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

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

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

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

		require(writers[msg.sender]);
		require(entryIndex[_account] == 0);
		require(entryList.length < (1 << 128));
		i = entryList.length;
		entryList.push(_account);
		entryIndex[_account] = i;
		entryCount += 1;
		emit AddressAdded(msg.sender, i, _account);
		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(writers[msg.sender]);
		require(entryIndex[_account] > 0 && entryIndex[_account] & blockedField == 0);
		entryIndex[_account] <<= 65;
		entryIndex[_account] |= blockedField;
		emit AddressInactive(msg.sender, _account);
	}

	// Implements AccountsIndex
	// Activate previously deactivated account. Will not affect the entry count.
	function activate(address _account) external returns (bool) {
		require(writers[msg.sender]);
		require(entryIndex[_account] > 0 && entryIndex[_account] & blockedField == blockedField);
		entryIndex[_account] >>= 65;
		emit AddressActive(msg.sender, _account);
	}

	// Implements AccountsIndex
	// (An account can remove itself).
	function remove(address _account) external returns (bool) {
		require(writers[msg.sender] || msg.sender == _account);
		require(entryIndex[_account] > 0);
		entryIndex[_account] = 0;
		entryCount = entryCount - 1;
		emit AddressRemoved(msg.sender, _account);
		return true;
	}

	// 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;
	}

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

		oldOwner = owner;
		owner = _newOwner;

		emit OwnershipTransferred(oldOwner, owner);
	}

	// Implements AccountsIndex
	function entry(uint256 _idx) public returns(address) {
		address r;

		r = entryList[_idx + 1];
		if (entryIndex[r] > 0) {
			return r;
		}
		return address(0);
	}

	// Implements EIP165
	function supportsInterface(bytes4 _sum) public pure returns (bool) {
		if (_sum == 0xcbdb05c7) { // AccountsIndex
			return true;
		}
		if (_sum == 0x01ffc9a7) { // EIP165
			return true;
		}
		if (_sum == 0x9493f8b2) { // EIP173
			return true;
		}
		if (_sum == 0x80c84bd6) { // Writer
			return true;
		}
		return false;
	}
}