# ###################################
##       Rule Specification        ##
#####################################
# 
# Rule Identifier:
#   elb_multiple_az_check
# 
# Description:
#   This control checks whether an ELB classic load balancer has been configured with multiple Availability Zones.
# 
# Reports on:
#   AWS::ElasticLoadBalancing::LoadBalancer
# 
# Evaluates:
#   AWS CloudFormation, AWS CloudFormation hook
# 
# Rule Parameters:
#   None
# 
# Scenarios:
#   Scenario: 1
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document does not contain any ElasticLoadBalancing load balancer resources
#      Then: SKIP
#   Scenario: 2
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains an ElasticLoadBalancing load balancer resource
#       And: Neither 'AvailabilityZones' or 'Subnets' have been specified
#      Then: FAIL
#   Scenario: 3
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains an ElasticLoadBalancing load balancer resource
#       And: 'AvailabilityZones' been specified on the ElasticLoadBalancing load balancer resource
#       And: The number of entries in 'AvailabilityZones' is < 2 or the number of
#            unique 'AvailabilityZones' provided is less than 2 (< 2)
#      Then: FAIL
#   Scenario: 4
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains an ElasticLoadBalancing load balancer resource
#       And: 'Subnets' been specified on the ElasticLoadBalancing load balancer resource
#       And: The number of entries in 'Subnets' is < 2
#      Then: FAIL
#   Scenario: 5
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains an ElasticLoadBalancing load balancer resource
#       And: 'AvailabilityZones' been specified on the ElasticLoadBalancing load balancer resource
#       And: The number of entries in 'AvailabilityZones' is >= 2
#      Then: PASS
#   Scenario: 6
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains an ElasticLoadBalancing load balancer resource
#       And: 'Subnets' been specified on the ElasticLoadBalancing load balancer resource
#       And: The number of entries in 'Subnets' is >= 2
#      Then: PASS

#
# Constants
#
let ELASTIC_LOAD_BALANCER_TYPE = "AWS::ElasticLoadBalancing::LoadBalancer"
let INPUT_DOCUMENT = this

#
# Assignments
#
let classic_load_balancers = Resources.*[ Type == %ELASTIC_LOAD_BALANCER_TYPE ]

#
# Primary Rules
#
rule elb_multiple_az_check when is_cfn_template(%INPUT_DOCUMENT)
                                %classic_load_balancers not empty {

    check(%classic_load_balancers.Properties)
        <<
        [CT.ELASTICLOADBALANCING.PR.7]: Require any classic load balancer to have multiple Availability Zones configured
        [FIX]: Configure classic load balancers with two or more subnets or Availability Zones.
        >>
}

rule elb_multiple_az_check when is_cfn_hook(%INPUT_DOCUMENT, %ELASTIC_LOAD_BALANCER_TYPE) {

    check(%INPUT_DOCUMENT.%ELASTIC_LOAD_BALANCER_TYPE.resourceProperties)
        <<
        [CT.ELASTICLOADBALANCING.PR.7]: Require any classic load balancer to have multiple Availability Zones configured
        [FIX]: Configure classic load balancers with two or more subnets or Availability Zones.
        >>
}

#
# Parameterized Rules
#
rule check(classic_load_balancer) {
    %classic_load_balancer {
        # Scenario 2
        AvailabilityZones exists or
        Subnets exists

        when AvailabilityZones exists {
            # Scenarios 3 and 5
            two_or_more_entries(AvailabilityZones)
            AvailabilityZones[0] not in AvailabilityZones[1]
        }

        when Subnets exists {
            # Scenarios 4 and 6
            two_or_more_entries(Subnets)
        }
    }
}

rule two_or_more_entries(list_property) {
    %list_property {
        this is_list
        this not empty
        this[0] exists
        this[1] exists
    }
}

#
# Utility Rules
#
rule is_cfn_template(doc) {
    %doc {
        AWSTemplateFormatVersion exists  or
        Resources exists
    }
}

rule is_cfn_hook(doc, RESOURCE_TYPE) {
    %doc.%RESOURCE_TYPE.resourceProperties exists
}