Array Extension to Find All Possible Combinations of a Given Size in Swift

Array+Combinations.swift

extension Array {
    
    /**
     Returns all possible combinations of `k` elements
     - Parameter k: The size of the combinations to generate
     - Returns: All possible combinations of `k` elements
     */
    func combinations(k: Int) -> [[Element]] {
        guard self.count > 0 && self.count >= k else { return [] }
        
        var temp = Array<Element?>(repeating: nil, count: k)
        var result: [[Element]] = []
        
        findCombinations(input: self, temp: &temp, result: &result, length: k, start: 0)
        return result
    }
    
    private func findCombinations(input: [Element],
                                  temp: inout [Element?],
                                  result: inout [[Element]],
                                  length: Int,
                                  start: Int) {
        
        guard length > 0 else {
            result.append(temp.compactMap { $0 })
            return
        }
        
        for i in start...(input.count - length) {
            temp[temp.count - length] = input[i];
            findCombinations(input: input,
                             temp: &temp,
                             result: &result,
                             length: length-1,
                             start: i+1)
        }
    }
}