console.log('Loading function');

const AWS = require('aws-sdk');

AWS.config.update({
	region: 'ap-southeast-1'
});

var elb = new AWS.ELB();

var sampleEvent = {
	"version": "0",
	"id": "1e5527d7-bb36-4607-3370-4164db56a40e",
	"detail-type": "EC2 Spot Instance Interruption Warning",
	"source": "aws.ec2",
	"account": "123456789012",
	"time": "1970-01-01T00:00:00Z",
	"region": "us-east-1",
	"resources": [
		"arn:aws:ec2:us-east-1b:instance/i-0b662ef9931388ba0"
	],
	"detail": {
		"instance-id": "i-0b662ef9931388ba123",
		"instance-action": "terminate"
	}
}

exports.handler = (event, context, callback) => {
	var instanceToDeregister;

	//get instance id from event
	if (event && event.detail && event.detail['instance-action'] == 'terminate' && event.detail['instance-id']) {
		instanceToDeregister = event.detail['instance-id'];
		console.log(`got instance to deregister ${instanceToDeregister}`);
	} else {
		return callback(`no instance-id found to deregister in event ${JSON.stringify(event)}`);
	}

	elb.describeLoadBalancers({}, function(err, data) {
		if (err || !data) {
			console.log(err || 'no data in describeLoadBalancers'); // an error occurred
			return callback(err || 'no data in describeLoadBalancers');
		} else {
			if (data.LoadBalancerDescriptions) {
				//scan for matching lb for instance
				// console.log(data.LoadBalancerDescriptions);
				var toDeregister = {};
				for (var i = 0; i < data.LoadBalancerDescriptions.length; i++) {
					var loadBalancer = data.LoadBalancerDescriptions[i];
					// console.log(loadBalancer.LoadBalancerName, "->", loadBalancer.Instances);

					if (loadBalancer.Instances && loadBalancer.Instances.length > 0) {
						for (var j = 0; j < loadBalancer.Instances.length; j++) {
							var entry = loadBalancer.Instances[j];
							if (entry.InstanceId == instanceToDeregister) {
								toDeregister.instanceId = entry.InstanceId;
								toDeregister.loadBalancerName = loadBalancer.LoadBalancerName;
								break;
							}
						}
					}

					if (toDeregister.instanceId && toDeregister.loadBalancerName) {
						break;
					}
				}

				if (toDeregister.instanceId && toDeregister.loadBalancerName) {
					// deregister the instance
					console.log(`deregistering ${toDeregister.instanceId} from ${toDeregister.loadBalancerName}`)
					deregisterInstance(toDeregister.instanceId, toDeregister.loadBalancerName, callback);
				} else {
					callback(`did not find instance attached to any load balancer ${JSON.stringify(event)}`)
				}
			} else {
				callback('no lb found');
			}
		}
	});
};

function deregisterInstance(instanceId, loadBalancerName, callback) {
	var params = {
		Instances: [{
			InstanceId: instanceId
		}],
		LoadBalancerName: loadBalancerName
	};
	elb.deregisterInstancesFromLoadBalancer(params, function(err, data) {
		if (err) {
			console.log(err, err.stack);
			callback(err);
		} else {
			console.log(`deregistered ${JSON.stringify(data)}`);
			callback(null, 'Success');
		}
	});
}