Collection Interpolation in Swift

Collection+.swift

extension Collection {
    
    func interpolated(by interpolation: (_ previous: Element, _ current: Element) throws -> Element) rethrows -> [Element] {
        
        var iterator = makeIterator()
        var result = [Element]()
        
        guard var previous = iterator.next() else {
            return []
        }
        
        result.append(previous)
        
        while let current = iterator.next() {
            let interpolated = try interpolation(previous, current)
            result.append(interpolated)
            result.append(current)
            previous = current
        }
        
        return result
        
    }
    
}

Usage:

let a: [Double] = [1, 2, 3]
let b = a.interpolated { ($0 + $1) / 2 }
print(b) // [1.0, 1.5, 2.0, 2.5, 3.0]