Add implementation for Sequence iterator

Author: ollieatkinson

Guides/Stride.md

@@ -5,7 +5,7 @@
 
 A type that steps over a collection’s elements by the specified amount.
 
-This is available through the `striding(by:)` method on any `Collection`.
+This is available through the `striding(by:)` method on any `Sequence`.
 
 ```swift
 (0...10).striding(by: 2) // == [0, 2, 4, 6, 8, 10]
@@ -18,11 +18,11 @@ The stride amount must be a positive value.
 
 ## Detailed Design
 
-The `striding(by:)` method is declared as a `Collection` extension, and returns a 
+The `striding(by:)` method is declared as a `Sequence` extension, and returns a 
 `Stride` type:
 
 ```swift
-extension Collection {
+extension Sequence {
   public func striding(by step: Int) -> Stride<Self>
 }
 ```

Sources/Algorithms/Stride.swift

@@ -13,7 +13,7 @@
 // striding(by:)
 //===----------------------------------------------------------------------===//
 
-extension Collection {
+extension Sequence {
   /// Returns a collection stepping through the elements every `step` starting
   /// at the first value. Any remainders of the stride will be trimmed.
   ///
@@ -32,7 +32,7 @@ extension Collection {
   }
 }
 
-public struct Stride<Base: Collection> {
+public struct Stride<Base: Sequence> {
 
   let base: Base
   let stride: Int
@@ -50,7 +50,29 @@ extension Stride {
   }
 }
 
-extension Stride: Collection {
+extension Stride: Sequence {
+  
+  public struct Iterator: IteratorProtocol {
+    
+    var iterator: Base.Iterator
+    let stride: Int
+    
+    public mutating func next() -> Base.Element? {
+      defer {
+        for _ in 0..<(stride - 1) {
+          guard iterator.next() != nil else { break }
+        }
+      }
+      return iterator.next()
+    }
+  }
+  
+  public func makeIterator() -> Stride<Base>.Iterator {
+    Iterator(iterator: base.makeIterator(), stride: stride)
+  }
+}
+
+extension Stride: Collection where Base: Collection {
 
   public struct Index: Comparable {
 

Tests/SwiftAlgorithmsTests/StrideTests.swift

@@ -15,7 +15,7 @@ import Algorithms
 final class StridingTests: XCTestCase {
 
   func testStride() {
-    let a = (0...10)
+    let a = 0...10
     XCTAssertEqualSequences(a.striding(by: 1), (0...10))
     XCTAssertEqualSequences(a.striding(by: 2), [0, 2, 4, 6, 8, 10])
     XCTAssertEqualSequences(a.striding(by: 3), [0, 3, 6, 9])
@@ -48,7 +48,7 @@ final class StridingTests: XCTestCase {
     XCTAssertEqual(a[i], 2)
     a.formIndex(before: &i)
     XCTAssertEqual(a[i], 0)
-//    a.formIndex(before: &i) // Precondition failed: Advancing past start index
+//    a.formIndex(before: &i) // Precondition failed: Incrementing past start index
 //    a.index(after: a.endIndex) // Precondition failed: Advancing past end index
   }