# ###################################
##       Rule Specification        ##
#####################################
# 
# Rule Identifier:
#   elasticache_subnet_group_check
# 
# Description:
#   This control checks whether an Amazon ElastiCache cache cluster is configured with a custom subnet group.
# 
# Reports on:
#   AWS::ElastiCache::CacheCluster
# 
# 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 ElastiCache cluster resources
#      Then: SKIP
#   Scenario: 2
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains an ElastiCache cluster resource
#       And: 'CacheSubnetGroupName' has not been provided
#      Then: FAIL
#   Scenario: 3
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains an ElastiCache cluster resource
#       And: 'CacheSubnetGroupName' has been provided as an empty string or as a non-valid local reference
#      Then: FAIL
#   Scenario: 4
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains an ElastiCache cluster resource
#       And: 'CacheSubnetGroupName' has been provided and set to a value of 'default'
#      Then: FAIL
#   Scenario: 5
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains an ElastiCache cluster resource
#       And: 'CacheSubnetGroupName' has been provided as a non-empty string or a valid local reference
#      Then: PASS

#
# Constants
#
let ELASTICACHE_CACHE_CLUSTER_TYPE = "AWS::ElastiCache::CacheCluster"
let INPUT_DOCUMENT = this

#
# Assignments
#
let elasticache_cache_clusters = Resources.*[ Type == %ELASTICACHE_CACHE_CLUSTER_TYPE ]

#
# Primary Rules
#
rule elasticache_subnet_group_check when is_cfn_template(%INPUT_DOCUMENT)
                                                %elasticache_cache_clusters not empty {
    check(%elasticache_cache_clusters.Properties)
        <<
        [CT.ELASTICACHE.PR.6]: Require an Amazon ElastiCache cache cluster to use a custom subnet group
        [FIX]: Set the 'CacheSubnetGroupName' parameter to the name of a custom Amazon ElastiCache cache subnet group.
        >>
}

rule elasticache_subnet_group_check when is_cfn_hook(%INPUT_DOCUMENT, %ELASTICACHE_CACHE_CLUSTER_TYPE) {
    check(%INPUT_DOCUMENT.%ELASTICACHE_CACHE_CLUSTER_TYPE.resourceProperties)
        <<
        [CT.ELASTICACHE.PR.6]: Require an Amazon ElastiCache cache cluster to use a custom subnet group
        [FIX]: Set the 'CacheSubnetGroupName' parameter to the name of a custom Amazon ElastiCache cache subnet group.
        >>
}

#
# Parameterized Rules
#
rule check(elasticache_cache_cluster) {
    %elasticache_cache_cluster {
        # Scenario 2
        CacheSubnetGroupName exists

        # Scenarios 3, 4 and 5
        check_subnet_group_is_not_default(this) or
        check_local_references(%INPUT_DOCUMENT, CacheSubnetGroupName, "AWS::ElastiCache::SubnetGroup")
    }
}

rule check_subnet_group_is_not_default(elasticache_cache_cluster) {
        %elasticache_cache_cluster {
            check_is_string_and_not_empty(CacheSubnetGroupName)
            CacheSubnetGroupName != "default"
        }
    }

#
# 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
}

rule check_is_string_and_not_empty(value) {
    %value {
        this is_string
        this != /\A\s*\z/
    }
}

rule check_local_references(doc, reference_properties, referenced_resource_type) {
    %reference_properties {
        'Fn::GetAtt' {
            query_for_resource(%doc, this[0], %referenced_resource_type)
                <<Local Stack reference was invalid>>
        } or Ref {
            query_for_resource(%doc, this, %referenced_resource_type)
                <<Local Stack reference was invalid>>
        }
    }
}

rule query_for_resource(doc, resource_key, referenced_resource_type) {
    let referenced_resource = %doc.Resources[ keys == %resource_key ]
    %referenced_resource not empty
    %referenced_resource {
        Type == %referenced_resource_type
    }
}