Functional Reduce By Key

Array+ReduceByKey.swift

extension Array {
    /// GroupBy that puts each element into a group of similiar elements denoted by an Equatable attribute, complexity: O(n)
    func reduce<G: Equatable>(by key: (Element) -> (G)) -> [G: [Element]] {
        return reduce([:], { (result, element) in
            var groups = result
            let group = key(element)
                            
            if let existingGroupElements = groups[group] {
                groups[group] = existingGroupElements + [element]
            } else {
                groups[group] = [element]
            }         
                            
            return groups
        })
    }
}

CategorizeExample.swift

struct Post {
  var title: String
  var author: String
}

let posts = [
  Post(title: "Post 1", author: "Peter Parker"),
  Post(title: "Post 3", author: "Clark Kent"),
  Post(title: "Post 2", author: "Peter Parker")
]

let postsByAuthor = posts.reduce(by: { $0.author })

/*
postsByAuthor:

[
  "Peter Parker": [
    Post(title: "Post 1", author: "Peter Parker"),
    Post(title: "Post 2", author: "Peter Parker")
  ],
  "Clark Kent": [
    Post(title: "Post 3", author: "Clark Kent")
  ]
]
*/