Set operations (union, intersection) on Swift array?

Are there any standard library calls I can use to either perform set operations on two arrays, or implement such logic myself (ideally as functionally and also efficiently as possible)?

Yes, Swift has the Set class.

let array1 = ["a", "b", "c"]
let array2 = ["a", "b", "d"]

let set1:Set<String> = Set(array1)
let set2:Set<String> = Set(array2)

Swift 3.0+ can do operations on sets as:

firstSet.union(secondSet)// Union of two sets
firstSet.intersection(secondSet)// Intersection of two sets
firstSet.symmetricDifference(secondSet)// exclusiveOr

Swift 2.0 can calculate on array arguments:

set1.union(array2)       // {"a", "b", "c", "d"} 
set1.intersect(array2)   // {"a", "b"}
set1.subtract(array2)    // {"c"}
set1.exclusiveOr(array2) // {"c", "d"}

Swift 1.2+ can calculate on sets:

set1.union(set2)        // {"a", "b", "c", "d"}
set1.intersect(set2)    // {"a", "b"}
set1.subtract(set2)     // {"c"}
set1.exclusiveOr(set2)  // {"c", "d"}

If you're using custom structs, you need to implement Hashable.

Thanks to Michael Stern in the comments for the Swift 2.0 update.

Thanks to Amjad Husseini in the comments for the Hashable info.