Add Base16.isValidCount(_:) method

Sources/Hex/Base16.swift

@@ -2,8 +2,14 @@ import Foundation
 
 /// A Base-16 codec that translates between Byte sequence and Hexadecimal (also known as Base-16) strings.
 public enum Base16 {
+    /// Validates the length of hex characters.
+    @inlinable
+    public static func isValidCount(_ count: Int) -> Bool {
+        (0 <= count) && count.isMultiple(of: 2)
+    }
+
     /// The result of one Base-16 decoding step.
-    public enum DecodingResult: Sendable {
+    public enum DecodingResult: Equatable, Sendable {
         /// An indication that no more hex characters are available in the input.
         case emptyInput
         /// An indication of a decoding error.

Sources/Hex/Data+Hex.swift

@@ -5,6 +5,9 @@ extension Data {
     ///
     /// - Parameter hexData: The Hexadecimal encoded data.
     public init?(hexEncoded hexData: Data) {
+        guard Base16.isValidCount(hexData.count) else {
+            return nil
+        }
         guard let bytes = Data.decode(hexData) else {
             return nil
         }
@@ -15,6 +18,9 @@ extension Data {
     ///
     /// - Parameter hexString: The Hexadecimal encoded string.
     public init?(hexEncoded hexString: String) {
+        guard Base16.isValidCount(hexString.count) else {
+            return nil
+        }
         var hexString = hexString
         guard let bytes = hexString.withUTF8(Data.decode(_:)) else {
             return nil

Tests/HexTests/Base16Tests.swift

@@ -0,0 +1,38 @@
+#if canImport(Testing)
+
+import Foundation
+import Hex
+import Testing
+
+struct Base16Tests {
+    @Test(
+        // Arrange
+        arguments: [
+            (-2, false),
+            (-1, false),
+            (0, true),
+            (1, false),
+            (2, true),
+        ]
+    )
+    func isValidCount(argument: (count: Int, expected: Bool)) {
+        // Act
+        let isValid = Base16.isValidCount(argument.count)
+
+        // Assert
+        #expect(isValid == argument.expected)
+    }
+
+    @Test func decodeOneDigitString() {
+        // Arrange
+        var it = "0".utf8.makeIterator()
+
+        // Act
+        let result = Base16.decode(&it)
+
+        // Assert
+        #expect(result == .error)
+    }
+}
+
+#endif

Tests/HexTests/Data+HexTests.swift

@@ -60,6 +60,17 @@ struct DataHexTests {
         #expect(data.isEmpty)
     }
 
+    @Test func decodeOneDigitData() {
+        // Arrange
+        let hexData = Data("4".utf8)
+
+        // Act
+        let data = Data(hexEncoded: hexData)
+
+        // Assert
+        #expect(data == nil)
+    }
+
     @Test func decodeOneDigitString() {
         // Arrange
         let hexString = "4"
@@ -93,9 +104,20 @@ struct DataHexTests {
         #expect(data == nil)
     }
 
+    @Test func decodeNonHexData() {
+        // Arrange
+        let hexData = Data("G0".utf8)
+
+        // Act
+        let data = Data(hexEncoded: hexData)
+
+        // Assert
+        #expect(data == nil)
+    }
+
     @Test func decodeNonHexString() {
         // Arrange
-        let hexString = "4G"
+        let hexString = "0G"
 
         // Act
         let data = Data(hexEncoded: hexString)