index

index.swift

struct SampleCollection<C: RandomAccessCollection>: RandomAccessCollection {
	let storage: C
	let sampleInterval: Int

	var startIndex: C.Index { storage.startIndex }
	var endIndex: C.Index { storage.endIndex }
	func index(before i: C.Index) -> C.Index {
		if i == endIndex {
  		return storage.index(endIndex, offsetBy: -storage.count.remainderReportingOverflow(dividingBy: sampleInterval).partialValue)
		} else {
			return storage.index(i, offsetBy: -sampleInterval)
		}
	}
	func index(after i: C.Index) -> C.Index { storage.index(i, offsetBy: sampleInterval, limitedBy: endIndex) ?? endIndex }
	func distance(from start: C.Index, to end: C.Index) -> Int { storage.distance(from: start, to: end) / sampleInterval }
	subscript(position: C.Index) -> C.Element { storage[position] }

	init(sampleInterval: Int, storage: C) {
		self.sampleInterval = sampleInterval
		self.storage = storage
	}
}

extension RandomAccessCollection {
	func sample(interval: Int) -> SampleCollection<Self> {
		SampleCollection(sampleInterval: interval, storage: self)
	}
}

let array = [0, 1, 2, 3, 4, 5, 6]
let sampled = array.sample(interval: 2)

let firstIdx = sampled.startIndex               // 0
let secondIdx = sampled.index(after: firstIdx)  // 2

let numericDistance = secondIdx - firstIdx     // 2
print(array.distance(from: firstIdx, to: secondIdx))   // 2
print(sampled.distance(from: firstIdx, to: secondIdx)) // 1