add precondition to index(before:) and index(after:)

Author: ollieatkinson

Sources/Algorithms/Stride.swift

@@ -66,7 +66,8 @@ extension Stride: Collection {
   }
 
   public func index(after i: Index) -> Index {
-    base.index(i.base, offsetBy: stride, limitedBy: base.endIndex)
+    precondition(i.base < base.endIndex, "Advancing past end index")
+    return base.index(i.base, offsetBy: stride, limitedBy: base.endIndex)
       .map(Index.init) ?? endIndex
   }
 
@@ -87,8 +88,10 @@ extension Stride: BidirectionalCollection
   where Base: RandomAccessCollection {
 
   public func index(before i: Index) -> Index {
+    precondition(i.base > base.startIndex, "Advancing past start index")
     if i == endIndex {
       let count = base.count
+      precondition(count > 0, "Advancing index when count is zero")
       return Index(
         base.index(
           base.endIndex,

Tests/SwiftAlgorithmsTests/StrideTests.swift

@@ -46,6 +46,10 @@ final class StridingTests: XCTestCase {
     XCTAssertEqual(a[i], 4)
     a.formIndex(before: &i)
     XCTAssertEqual(a[i], 2)
+    a.formIndex(before: &i)
+    XCTAssertEqual(a[i], 0)
+//    a.formIndex(before: &i) // Precondition failed: Advancing past start index
+//    a.index(after: a.endIndex) // Precondition failed: Advancing past end index
   }
 
   func testStrideCompositionEquivalence() {
@@ -60,5 +64,6 @@ final class StridingTests: XCTestCase {
     XCTAssertEqual((1...10).stride(by: 5).last, 6) // 1, 6
     XCTAssertEqual((1...100).stride(by: 50).last, 51) // 1, 51
     XCTAssertEqual((1...5).stride(by: 2).last, 5) // 1, 3, 5
+    XCTAssertEqual([Int]().stride(by: 2).last, nil) // 1, 3, 5
   }
 }