Sequence+Unique.swift

extension Sequence {
    func uniqueElements(by elementsEqual: (Element, Element) -> Bool) -> [Element] {
        var result: [Element] = []
        for element in self {
            if !result.contains(where: { resultElement in elementsEqual(element, resultElement) }) {
                result.append(element)
            }
        }
        return result
    }

    func uniqueElements<T: Hashable>(byProperty propertyAccessor: (Element) -> T) -> [Element] {
        var seen: Set<T> = []
        var result: [Element] = []
        for element in self {
            let property = propertyAccessor(element)
            if !seen.contains(property) {
                result.append(element)
                seen.insert(property)
            }
        }
        return result
    }

}

extension Sequence where Element: Equatable {
    func uniqueElements() -> [Element] {
        return uniqueElements(by: ==)
    }
}

extension Sequence where Element: Hashable {
    func uniqueElements() -> [Element] {
        var seen: Set<Element> = []
        var result: [Element] = []
        for element in self {
            if !seen.contains(element) {
                result.append(element)
                seen.insert(element)
            }
        }
        return result
    }
}

[1, 2, 1, 2, 3].uniqueElements()