eth-accounts-index/solidity/registry.sol

pragma solidity ^0.7.3;

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

contract CustodialAccountIndex {

	address[] public accounts;
	mapping(address => uint256) public accountsIndex;
	uint256 public count;
	mapping(address => bool) writers;
	address owner;

	event AccountAdded(address indexed addedAccount, uint256 indexed accountIndex);

	constructor() {
		owner = msg.sender;
		accounts.push(address(0));
		count = 1;
	}

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

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

	function add(address _account) public {
		require(writers[msg.sender]);
		require(accountsIndex[_account] == 0);
		accounts.push(_account);
		accountsIndex[_account] = count;
		count++;
		emit AccountAdded(_account, count-1);
	}
}