bring back array extensions

Sources/Extensions/Array/Array+Extensions.swift

@@ -20,7 +20,49 @@
 //  THE SOFTWARE.
 //
 
-public extension Array {
+public extension Array where Element: Equatable {
+
+    /// Union array with another arrays, without element duplication
+    func union(values: [Array.Element]...) -> Array {
+        var result = self
+
+        for array in values {
+            for value in array {
+                if !result.contains(value) {
+                    result.append(value)
+                }
+            }
+        }
+
+        return result
+    }
+
+    /// Find common elements among arrays
+    func intersection(values: [Array.Element]...) -> Array {
+        var result = self
+        var intersection = Array()
+
+        for (index, value) in values.enumerated() {
+            if index > 0 {
+                result = intersection
+                intersection = Array()
+            }
+
+            value.forEach { item in
+                if result.contains(item) && !intersection.contains(item) {
+                    intersection.append(item)
+                }
+            }
+        }
+
+        return intersection
+    }
+
+    /// Find unique elements in array compared to other arrays
+    func subtract(values: [Array.Element]...) -> Array {
+        let allValues = values.flatMap { $0 }
+        return filter { !allValues.contains($0) }
+    }
 
     // Subscript for safe access to element by index
     subscript(safe index: Index) -> Element? {