Sequence+group.swft

/**
 // Example:
 // dictionary will be a dictionary with keys for every unique value of object.propertyToGroupBy
 // the value of each being an array of objects
 var dictionary = arrayOfObjects.group { $0.propertyToGroupBy }
 
 // dictionary will be a dictionary with 2 keys 'Important' and 'Other' each containing an array of ItemType objects
 var dictionary = arrayOfObjects.group { (item:ItemType) -> String in
     if item.differentPropertyToGroupBy == "some value" {
        return "Important"
     } else {
        return "Other"
     }
 }
 
 */
public extension Sequence {
    func group<U: Hashable>(by key: (Iterator.Element) -> U) -> [U: [Iterator.Element]] {
        var categories: [U: [Iterator.Element]] = [:]
        for element in self {
            let key = key(element)
            if case nil = categories[key]?.append(element) {
                categories[key] = [element]
            }
        }
        return categories
    }
}