memoize.swift

///
/// ```swift
/// let fibonacci = memoize { (function: (Int) -> Int, n: Int) -> Int in
///     if n < 2 {
///         return n
///     }
///     return function(n - 1) + function(n - 2)
/// }
/// ```
///
/// ```swift
/// let factorial = memoize { (function: (Int) -> Int, n: Int) -> Int in
///     if n < 2 {
///         return 1
///     }
///     return n * function(n-1)
/// }
/// ```
func memoize<T: Hashable, U>(body: @escaping(((T) -> U), T) -> U) -> (T) -> U {
    var memo = [T: U](), function: ((T) -> U)!
    function = { x in
        memo[x] ?? {
            let r = body(function, x)
            memo[x] = r
            return r
        }()
    }
    return function
}