Fix unsafe Data Range when extracting subdata (#27)

Sources/FoundationExtensions/Data/Data+Extensions.swift

@@ -34,7 +34,9 @@ extension Numeric {
 extension Data {
 
     public func range(start: Int, length: Int) -> Data {
-        subdata(in: start..<start + length)
+        guard start < endIndex else { return Data() }
+        let end = Swift.min(endIndex, start + length)
+        return subdata(in: start ..< end)
     }
 
     public func readValue<T>() -> T {

Tests/FoundationExtensionsTests/Data/DataExtensionsTests.swift

@@ -0,0 +1,38 @@
+// Copyright © 2021 Lautsprecher Teufel GmbH. All rights reserved.
+
+#if !os(watchOS)
+import FoundationExtensions
+import XCTest
+
+class DataExtensionsTests: XCTestCase {
+    func testDataInRangeSuccessfullyExtractSubdataInTheMiddle() {
+        let fullData = Data([0xDE, 0xAD, 0xBE, 0xFF])
+        let partialData = fullData.range(start: 1, length: 1)
+        XCTAssertEqual(partialData, Data([0xAD]))
+    }
+
+    func testDataInRangeSuccessfullyExtractSubdataAtTheEnd() {
+        let fullData = Data([0xDE, 0xAD, 0xBE, 0xFF])
+        let partialData = fullData.range(start: 3, length: 1)
+        XCTAssertEqual(partialData, Data([0xFF]))
+    }
+
+    func testDataOutOfRangeBecauseOfLengthABitTooLong() {
+        let fullData = Data([0xDE, 0xAD, 0xBE, 0xFF])
+        let partialData = fullData.range(start: 3, length: 2)
+        XCTAssertEqual(partialData, Data([0xFF]))
+    }
+
+    func testDataOutOfRangeBecauseOfLengthWayTooLong() {
+        let fullData = Data([0xDE, 0xAD, 0xBE, 0xFF])
+        let partialData = fullData.range(start: 1, length: 10)
+        XCTAssertEqual(partialData, Data([0xAD, 0xBE, 0xFF]))
+    }
+
+    func testDataOutOfRangeBecauseOfStart() {
+        let fullData = Data([0xDE, 0xAD, 0xBE, 0xFF])
+        let partialData = fullData.range(start: 4, length: 1)
+        XCTAssertEqual(partialData, Data([]))
+    }
+}
+#endif