tar: Add unit tests for helper functions

Test the underlying helper methods used to write tar headers.

Package.swift

@@ -87,6 +87,7 @@ let package = Package(
             dependencies: [.target(name: "ContainerRegistry")],
             resources: [.process("Resources")]
         ), .testTarget(name: "containertoolTests", dependencies: [.target(name: "containertool")]),
+        .testTarget(name: "TarTests", dependencies: [.target(name: "Tar")]),
     ],
     swiftLanguageModes: [.v6]
 )

Tests/TarTests/TarTests.swift

@@ -0,0 +1,78 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the SwiftContainerPlugin open source project
+//
+// Copyright (c) 2024 Apple Inc. and the SwiftContainerPlugin project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of SwiftContainerPlugin project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+import Foundation
+import Testing
+
+@testable import Tar
+
+let blocksize = 512
+let headerLen = blocksize
+let trailerLen = 2 * blocksize
+
+@Suite struct TarUnitTests {
+    @Test(arguments: [
+        (input: 0o000, expected: "000000"),
+        (input: 0o555, expected: "000555"),
+        (input: 0o750, expected: "000750"),
+        (input: 0o777, expected: "000777"),
+        (input: 0o1777, expected: "001777"),
+    ])
+    func testOctal6(input: Int, expected: String) async throws {
+        #expect(octal6(input) == expected)
+    }
+
+    @Test(arguments: [
+        (input: 0, expected: "00000000000"),
+        (input: 1024, expected: "00000002000"),
+        (input: 0o2000, expected: "00000002000"),
+        (input: 1024 * 1024, expected: "00004000000"),
+    ])
+    func testOctal11(input: Int, expected: String) async throws {
+        #expect(octal11(input) == expected)
+    }
+
+    @Test func testUInt8writeString() async throws {
+        // Fill the buffer with 0xFF to show null termination
+        var hdr = [UInt8](repeating: 255, count: 21)
+
+        // The typechecker timed out when these test cases were passed as arguments, in the style of the octal tests
+        hdr.writeString("abc", inField: 0..<5, withTermination: .none)
+        #expect(
+            hdr == [
+                97, 98, 99, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+            ]
+        )
+
+        hdr.writeString("def", inField: 3..<7, withTermination: .null)
+        #expect(
+            hdr == [97, 98, 99, 100, 101, 102, 0, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255]
+        )
+
+        hdr.writeString("ghi", inField: 7..<11, withTermination: .space)
+        #expect(
+            hdr == [97, 98, 99, 100, 101, 102, 0, 103, 104, 105, 32, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255]
+        )
+
+        hdr.writeString("jkl", inField: 11..<16, withTermination: .nullAndSpace)
+        #expect(
+            hdr == [97, 98, 99, 100, 101, 102, 0, 103, 104, 105, 32, 106, 107, 108, 0, 32, 255, 255, 255, 255, 255]
+        )
+
+        hdr.writeString("mno", inField: 16..<21, withTermination: .spaceAndNull)
+        #expect(
+            hdr == [97, 98, 99, 100, 101, 102, 0, 103, 104, 105, 32, 106, 107, 108, 0, 32, 109, 110, 111, 32, 0]
+        )
+    }
+}