Group By

Go over a list and return a new list with the previous list' items grouped by a discriminator function. The function in question needs to return a Hashable type so that we can differentiate keys. The order of the items will be preserved while the order of the groups won't necessarily be preserved.

func groupby<T, H: Hashable>(_ items: [T], f: (T) -> H) -> [H: [T]] {

    return items.reduce([:], { (var ac: [H: [T]], o: T) -> [H: [T]] in 

        let h = f(o)

        if var c = ac[h] {

            c.append(o)

            ac.updateValue(c, forKey: h)

        } else {

            ac.updateValue([o], forKey: h)

        }

        return ac

    })

}

print(groupby([1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12], f: { $0 % 3 }))

// prints: [2: [2, 5, 8, 11], 0: [3, 6, 9, 12], 1: [1, 4, 7, 10]]

print(groupby([\"Carl\", \"Cozy\", \"Bethlehem\", \"Belem\", \"Brand\", \"Zara\"], f: { $0.characters.first! }))

// prints: [\"C\" : [\"Carl\" , \"Cozy\"] , \"B\" : [\"Bethlehem\" , \"Belem\" , \"Brand\"] , \"Z\" : [\"Zara\"]]