Make progress on related objects.

Frameworks/RSDatabase/Related Objects/RelatedObjectIDsLookupTable.swift

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+//
+//  RelatedObjectIDsLookupTable.swift
+//  RSDatabase
+//
+//  Created by Brent Simmons on 9/10/17.
+//  Copyright © 2017 Ranchero Software, LLC. All rights reserved.
+//
+
+import Foundation
+
+// Maps objectIDs to Set<String> where the Strings are relatedObjectIDs.
+
+struct RelatedObjectIDsLookupTable {
+	
+	private let dictionary: [String: Set<String>] // objectID: Set<relatedObjectID>
+	
+	init(dictionary: [String: Set<String>]) {
+		
+		self.dictionary = dictionary
+	}
+	
+	init(lookupValues: Set<LookupValue>) {
+		
+		var d = [String: Set<String>]()
+		
+		for lookupValue in lookupValues {
+			let objectID = lookupValue.objectID
+			let relatedObjectID: String = lookupValue.relatedObjectID
+			if d[objectID] == nil {
+				d[objectID] = Set([relatedObjectID])
+			}
+			else {
+				d[objectID]!.insert(relatedObjectID)
+			}
+		}
+		
+		self.init(dictionary: d)
+	}
+	
+	func objectIDs() -> Set<String> {
+		
+		return Set(dictionary.keys)
+	}
+	
+	func relatedObjectIDs() -> Set<String> {
+		
+		var ids = Set<String>()
+		for (_, relatedObjectIDs) in dictionary {
+			ids.formUnion(relatedObjectIDs)
+		}
+		return ids
+	}
+	
+	subscript(_ objectID: String) -> Set<String>? {
+		get {
+			return dictionary[objectID]
+		}
+	}
+}