Map, FlatMap, Reduce & More

Performance & inout


Apart from the higher flexibility that reduce offers, it has another advantage: Oftentimes, chaining map and filter induces a performance penalty as Swift has to iterate over your collection multiple times in order to generate the required data. Imagine the following code:

[0, 1, 2, 3, 4].map({ $0 + 3})
    .filter({ $0 % 2 == 0})
    .reduce(0, +)

Apart from being nonsensical, it is also wasting CPU cycles. The initial sequence will be iterated over 3 times. First to map it, then to filter it, and finally to sum up the contents. Instead, all of this can just as well be implemented as one reduce call, which greatly improves the performance:

[0, 1, 2, 3, 4].reduce(0, { (ac: Int, r: Int) -> Int in 
   if (r + 3) % 2 == 0 {
     return ac + r + 3
   } else {
     return ac

Here's a quick benchmark of running both versions and the for-loop version below over an list with 100.000 items:

var ux = 0
for i in Array(0...100000) {
    if (i + 3) % 2 == 0 {
        ux += (i + 3)
For Loop: 0.030 seconds
Reduce: 0.034 seconds
Map/Filter: 0.072 seconds

As you can see, the reduce version' performance is very close to the mutating for loop and more than twice as fast as the chaining operation.

However, in other situations, chained operation can greatly outperform reduce. Consider the following example where we add 3 to each entry in the array.

Array(0...100000).map({ $0 + 3}).reverse().prefix(3)
// 0.027 Seconds

And the reduce version:

Array(0...100000).reduce([], { (ac: [Int], r: Int) -> [Int] in
    return ac + [r + 3]

// 2.927 Seconds
For Loop: 0.027 seconds
Reduce: 2.927 seconds

Here, we have a stark performance difference of 0.027s for the chained operation vs. 2.927s for the reduce operation, what's happening here?

Arrays in Swift are value types with so-called copy on write semantics.

Copy on Write

Imagine you had a struct User:

struct User {
  var username: String
var benedikt = User(username: "Benedikt")
var secondBenedikt = benedikt
var thirdBenedikt = benedikt

struct types in Swift are value types. Value type means that each copy is a new distinct value. So if I were to change the username value of secondBenedikt to Klaus, then the username value of the other two benedikts (benedikt, thirdBenedikt) would still be Benedikt and not Klaus. So, everytime you do a a = b, b is copied to a.

Copy operations, however, are expensive. All that memory has to be copied from a to b. So Swift employs a smart trick: As long as you don't mutate / modify a variable, it will just not copy it.

So in our example above, benedikt, secondBenedikt, and thirdBenedikt are the same thing, they point to the same memory. Only once you change one of them (say benedikt.username = 'Hans') will they be copied into distinct types.

So what's all that to do with our reduce issue here?

Array Value Types

Arrays are value types, too. This means that whenever an array is mutated, a new copy is created. So in our reduce function:

Array(0...100000).reduce([], { (ac: [Int], r: Int) -> [Int] in
    return ac + [r + 3]

This will copy the array 100000 times. That's why the performance is so abysmal. So how do we fix this?

The power of inout

There's another version of reduce with slightly different parameters. Its function signature looks like this (simplified):

func reduce<Result>(into initialResult: Result, 
  _ updateAccumulatingResult: (inout Result, Element) throws -> ()) rethrows -> Result

The magic is the inout Result. Inout is a special attribute that you can use in function signatures to denote to Swift that you wish to refer to the same instance of a type without making copies. The name implies how it works: When the function is called, the value is moved in to the function, when the function is done, the value is moved out again.

In the case of our arrays, instead of making multiple copies, we will always modify the same array.

So if we rewrite our reduce from above with reduce(into:) what is the performance?

Here is the updated code:

Array(0...100000).reduce(into: [Int](), { ac, r in
                return ac.append(r + 3)

And this is the new performance:

Map: 0.0295 seconds
Reduce Into: 0.0376 seconds
Reduce: 1.49 seconds

We're almost reached the speed of the simpler map implementation. It is much faster now. Awesome!