import re
import boto3

def is_valid_host(host):
    if len(host) > 255:
        return False
    if host[-1] == ".":
        host = host[:-1]
    allowed = re.compile("(?!-)[A-Z\d-]{1,63}(?<!-)$", re.IGNORECASE)
    return all(allowed.match(x) for x in host.split("."))

def is_valid_ip(ip):
    return re.match(r"^\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}$",ip)

def lambda_handler(event, context):
    host = event["host"]
    cname = host[:host.find(".")]
    hosted_zone = host[host.find(".")+1:]
    new_ip = event["ip"]
    print "Processing request to update %s with ip %s" % (host, new_ip)
    route53 = boto3.client('route53')
    try:
        print ("Validating inputs... "),
        if not (is_valid_host(host) and is_valid_ip(new_ip)):
            raise
        print ("Success")
        print ("Retrieving hosted zone..."),
        response = route53.list_hosted_zones_by_name(
            DNSName=hosted_zone,
            MaxItems='1'
        )
        r53hosted_zone = route53.get_hosted_zone(
            Id=response["HostedZones"][0]["Id"]
        )
        print ("Success")
        print ("Checking against existing IP..."),
        r53resource_record_set = route53.list_resource_record_sets(
            HostedZoneId = r53hosted_zone["HostedZone"]["Id"],
            StartRecordName = host,
            MaxItems = "1"
        )
        retMsg = ""
        if r53resource_record_set["ResourceRecordSets"][0]["ResourceRecords"][0]["Value"]==new_ip:
            print ("IP hasn't changed. Exiting.")
            return "nochg %s" % new_ip
        print("Success")
        print ("Updating route53 record for %s..." % (host)),
        dns_changes = {
            'Changes': [
                {
                    'Action': 'UPSERT',
                    'ResourceRecordSet': {
                        'Name': host ,
                        'Type': 'A',
                        'ResourceRecords': [
                            {
                                'Value': new_ip
                            }
                        ],
                        'TTL': 300
                    }
                }
            ]
        }
        response = route53.change_resource_record_sets(
            HostedZoneId=r53hosted_zone["HostedZone"]["Id"],
            ChangeBatch=dns_changes
        )
        print("Success")
        return "good %s" % (new_ip)
    except:
        print ("Failed")
        return "dnserr"