# ###################################
##       Rule Specification        ##
#####################################
# 
# Rule Identifier:
#   cloudfront_origin_failover_enabled_check
# 
# Description:
#   This control checks whether your Amazon CloudFront distribution is configured with an origin group that contains two origin group members.
# 
# Reports on:
#   AWS::CloudFront::Distribution
# 
# 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 CloudFront distribution resources
#      Then: SKIP
#   Scenario: 2
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains a CloudFront distribution resource
#       And: 'OriginGroups' is not present on the CloudFront distribution resource
#      Then: FAIL
#   Scenario: 3
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains a CloudFront distribution resource
#       And: 'OriginGroups' is present on the CloudFront distribution resource
#       And: 'Quantity' within 'OriginGroups' is 0
#      Then: FAIL
#   Scenario: 4
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains a CloudFront distribution resource
#       And: 'OriginGroups' is present on the CloudFront distribution resource
#       And: 'Quantity' within 'OriginGroups' is >= 1
#       And: 'Quantity' within 'Members' is < 2
#      Then: FAIL
#   Scenario: 5
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains a CloudFront distribution resource
#       And: 'OriginGroups' is present on the CloudFront distribution resource
#       And: 'Quantity' within 'OriginGroups' is >= 1
#       And: 'Quantity' within 'Members' is == 2
#      Then: PASS

#
# Constants
#
let CLOUDFRONT_DISTRIBUTION_TYPE = "AWS::CloudFront::Distribution"
let INPUT_DOCUMENT = this

#
# Assignments
#
let cloudfront_distributions = Resources.*[ Type == %CLOUDFRONT_DISTRIBUTION_TYPE ]

#
# Primary Rules
#
rule cloudfront_origin_failover_enabled_check when is_cfn_template(%INPUT_DOCUMENT)
                                                   %cloudfront_distributions not empty {
    check(%cloudfront_distributions.Properties)
        <<
        [CT.CLOUDFRONT.PR.4]: Require an Amazon CloudFront distribution to have origin failover configured
        [FIX]: Configure an origin group on the Amazon CloudFront Distribution with two origin group members.
        >>
}

rule cloudfront_origin_failover_enabled_check when is_cfn_hook(%INPUT_DOCUMENT, %CLOUDFRONT_DISTRIBUTION_TYPE) {
    check(%INPUT_DOCUMENT.%CLOUDFRONT_DISTRIBUTION_TYPE.resourceProperties)
        <<
        [CT.CLOUDFRONT.PR.4]: Require an Amazon CloudFront distribution to have origin failover configured
        [FIX]: Configure an origin group on the Amazon CloudFront Distribution with two origin group members.
        >>
}

#
# Parameterized Rules
#
rule check(cloudfront_distribution) {
    %cloudfront_distribution {
        DistributionConfig exists
        DistributionConfig is_struct

        DistributionConfig {
            # Scenario 2
            OriginGroups exists
            OriginGroups is_struct

            OriginGroups {
                # Scenario 3
                Quantity exists
                Quantity >= 1

                Items exists
                Items is_list
                Items not empty

                Items[*] {
                    Members exists
                    Members is_struct
                    Members {
                        # Scenarios 4 and 5
                        Quantity == 2
                    }
                }
            }
        }
    }
}

#
# 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
}