Finding anagrams in Swift

anagrams.swift

func findAnagramsIn(words: [String]) -> [[String]] {
    var signatureToAnagrams = [String: [String]]()
    for word in words {
        let signature = String(word.characters.sort())
        if let anagrams = signatureToAnagrams[signature] {
            signatureToAnagrams[signature] = anagrams + [word]
        }
        else {
            signatureToAnagrams[signature] = [word]
        }
    }
    return signatureToAnagrams.flatMap { (_, anagrams) in
        anagrams.count > 1 ? anagrams : nil
    }
}