[tests] Ensure correct implementation of hash for ChunkedByCount (#123)

Author: LucianoPAlmeida

Sources/Algorithms/Chunked.swift

@@ -515,12 +515,13 @@ extension ChunkedByCount: Equatable where Base: Equatable {}
 // Since we have another stored property of type `Index` on the
 // collection, synthesis of `Hashble` conformace would require
 // a `Base.Index: Hashable` constraint, so we implement the hasher
-// only in terms of `base`. Since the computed index is based on it,
-// it should not make a difference here.
+// only in terms of `base` and `chunkCount`. Since the computed
+// index is based on it, it should not make a difference here.
 extension ChunkedByCount: Hashable where Base: Hashable {
   @inlinable
   public func hash(into hasher: inout Hasher) {
     hasher.combine(base)
+    hasher.combine(chunkCount)
   }
 }
 extension ChunkedByCount.Index: Hashable where Base.Index: Hashable {}

Tests/SwiftAlgorithmsTests/ChunkedTests.swift

@@ -145,4 +145,16 @@ final class ChunkedTests: XCTestCase {
       validateIndexTraversals(chunks)
     }
   }
+  
+  func testChunksOfCountHash() {
+    let collection1 = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
+    let collection2 = [1, 2, 3, 4, 5]
+    
+    XCTAssertEqualHashValue(
+      collection1.chunks(ofCount: 1), collection1.chunks(ofCount: 1))
+    XCTAssertNotEqualHashValue(
+      collection1.chunks(ofCount: 1), collection2.chunks(ofCount: 1))
+    XCTAssertNotEqualHashValue(
+      collection1.chunks(ofCount: 2), collection2.chunks(ofCount: 4))
+  }
 }

Tests/SwiftAlgorithmsTests/TestUtilities.swift

@@ -203,6 +203,19 @@ func XCTAssertEqualHashValue<T: Hashable, U: Hashable>(
   )
 }
 
+/// Asserts that two hashable instances don't produce the same hash value.
+func XCTAssertNotEqualHashValue<T: Hashable, U: Hashable>(
+  _ expression1: @autoclosure () throws -> T,
+  _ expression2: @autoclosure () throws -> U,
+  _ message: @autoclosure () -> String = "",
+  file: StaticString = #file, line: UInt = #line
+) {
+  XCTAssertNotEqual(
+    hash(try expression1()), hash(try expression2()),
+    message(), file: file, line: line
+  )
+}
+
 /// Tests that all index traversal methods behave as expected.
 ///
 /// Verifies the correctness of the implementations of `startIndex`, `endIndex`,