Tracker.js

// assume this is node.js/browserify land so this is actually private
var next_server_number = function( servers ){
	var current_int = 1;
	servers = servers.sort( function( a, b ){
		return a - b;
	});
	for( var i = 0; i < servers.length; i++ ){
		if( current_int === servers[i] ){
			current_int++;
		}
		else {
			break;
		}
	}
	return current_int;
};

var Tracker = function(){
	// internally store server ids
	this.servers = {};
};

Tracker.prototype.allocate = function( server_type ){
	var server_id = 1;
	// does server type exist
	// if it does exist, find lowest server id
	// add server id lowest
	if( this.servers[server_type] ){
		server_id = next_server_number( this.servers[server_type] );
		this.servers[server_type].push( server_id );
	}
	// if it doesn't exist, add array for server ids
	// add server id 1
	else {
		this.servers[server_type] = [ server_id ];
	}
	return server_type + server_id;
};

Tracker.prototype.deallocate = function( server_type_id ){
	// split server name and id
	// find server id in server ids
	// if server exists, splice it out
	var server_id_bits = /([a-z]+)(\d+)$/.exec( server_type_id );
	if( !server_id_bits ){
		throw new Error('Invalid argument. Must be format "server1", "server2", etc');
	}
	var server_type = server_id_bits[1];
	var server_id = parseInt( server_id_bits[2], 10 );
	var server_id_index = this.servers[server_type].indexOf( server_id );
	if( server_id_index >= 0 ){
		this.servers[server_type].splice( server_id_index, 1 );
	}
	return this.servers[server_type];
};

module.exports = Tracker;