JSONAPI/Sources/JSONAPITesting/EntityCheck.swift

//
//  EntityCheck.swift
//  JSONAPITesting
//
//  Created by Mathew Polzin on 11/27/18.
//

import JSONAPI

public enum EntityCheckError: Swift.Error {
	/// The attributes should live in a struct, not
	/// another type class.
	case attributesNotStruct

	/// The relationships should live in a struct, not
	/// another type class.
	case relationshipsNotStruct

	/// All stored properties on an Attributes struct should
	/// be one of the supplied Attribute types.
	case nonAttribute(named: String)

	/// All stored properties on a Relationships struct should
	/// be one of the supplied Relationship types.
	case nonRelationship(named: String)

	/// It is explicitly stated by the JSON spec
	/// a "none" value for arrays is an empty array, not `nil`.
	case nullArray(named: String)
}

public struct EntityCheckErrors: Swift.Error {
	let problems: [EntityCheckError]
}

private protocol OptionalAttributeType {}
extension Optional: OptionalAttributeType where Wrapped: AttributeType {}

private protocol OptionalArray {}
extension Optional: OptionalArray where Wrapped: ArrayType {}

private protocol AttributeTypeWithOptionalArray {}
extension TransformedAttribute: AttributeTypeWithOptionalArray where RawValue: OptionalArray {}
extension Attribute: AttributeTypeWithOptionalArray where RawValue: OptionalArray {}

private protocol OptionalRelationshipType {}
extension Optional: OptionalRelationshipType where Wrapped: RelationshipType {}

private protocol _RelationshipType {}
extension ToOneRelationship: _RelationshipType {}
extension ToManyRelationship: _RelationshipType {}

private protocol _AttributeType {}
extension TransformedAttribute: _AttributeType {}
extension Attribute: _AttributeType {}

public extension Entity {
	static func check(_ entity: Entity) throws {
		var problems = [EntityCheckError]()

		let attributesMirror = Mirror(reflecting: entity.attributes)

		if attributesMirror.displayStyle != .`struct` {
			problems.append(.attributesNotStruct)
		}

		for attribute in attributesMirror.children {
			if attribute.value as? _AttributeType == nil,
				attribute.value as? OptionalAttributeType == nil {
				problems.append(.nonAttribute(named: attribute.label ?? "unnamed"))
			}
			if attribute.value as? AttributeTypeWithOptionalArray != nil {
				problems.append(.nullArray(named: attribute.label ?? "unnamed"))
			}
		}

		let relationshipsMirror = Mirror(reflecting: entity.relationships)

		if relationshipsMirror.displayStyle != .`struct` {
			problems.append(.relationshipsNotStruct)
		}

		for relationship in relationshipsMirror.children {
			if relationship.value as? _RelationshipType == nil,
				relationship.value as? OptionalRelationshipType == nil {
				problems.append(.nonRelationship(named: relationship.label ?? "unnamed"))
			}
		}

		guard problems.count == 0 else {
			throw EntityCheckErrors(problems: problems)
		}
	}
}