[CBOR] Make Half Read/Write methods public and add unit tests (#38946)

* Make Cbor Half methods public and add unit tests

* address feedback

Author: eiriktsarpalis

src/libraries/System.Formats.Cbor/ref/System.Formats.Cbor.cs

@@ -44,6 +44,7 @@ public void ReadEndArray() { }
         public void ReadEndIndefiniteLengthByteString() { }
         public void ReadEndIndefiniteLengthTextString() { }
         public void ReadEndMap() { }
+        public System.Half ReadHalf() { throw null; }
         public int ReadInt32() { throw null; }
         public long ReadInt64() { throw null; }
         public void ReadNull() { }
@@ -144,6 +145,7 @@ public void WriteEndArray() { }
         public void WriteEndIndefiniteLengthByteString() { }
         public void WriteEndIndefiniteLengthTextString() { }
         public void WriteEndMap() { }
+        public void WriteHalf(System.Half value) { }
         public void WriteInt32(int value) { }
         public void WriteInt64(long value) { }
         public void WriteNull() { }

src/libraries/System.Formats.Cbor/src/System/Formats/Cbor/Reader/CborReader.Simple.cs

@@ -21,7 +21,7 @@ public partial class CborReader
         ///   there was an unexpected end of CBOR encoding data. -or-
         ///   the next value uses a CBOR encoding that is not valid under the current conformance mode.
         /// </exception>
-        internal Half ReadHalf()
+        public Half ReadHalf()
         {
             CborInitialByte header = PeekInitialByte(expectedType: CborMajorType.Simple);
             ReadOnlySpan<byte> buffer = GetRemainingBytes();

src/libraries/System.Formats.Cbor/src/System/Formats/Cbor/Writer/CborWriter.Simple.cs

@@ -19,7 +19,7 @@ public partial class CborWriter
         ///   The major type of the encoded value is not permitted in the parent data item. -or-
         ///   The written data is not accepted under the current conformance mode
         /// </exception>
-        internal void WriteHalf(Half value)
+        public void WriteHalf(Half value)
         {
             EnsureWriteCapacity(1 + HalfHelpers.SizeOfHalf);
             WriteInitialByte(new CborInitialByte(CborMajorType.Simple, CborAdditionalInfo.Additional16BitData));

src/libraries/System.Formats.Cbor/tests/Reader/CborReaderTests.Simple.cs

@@ -11,6 +11,28 @@ public partial class CborReaderTests
         // Data points taken from https://tools.ietf.org/html/rfc7049#appendix-A
         // Additional pairs generated using http://cbor.me/
 
+        [Theory]
+        [InlineData(0.0, "f90000")]
+        [InlineData(-0.0, "f98000")]
+        [InlineData(1.0, "f93c00")]
+        [InlineData(1.5, "f93e00")]
+        [InlineData(65504.0, "f97bff")]
+        [InlineData(5.960464477539063e-8, "f90001")]
+        [InlineData(0.00006103515625, "f90400")]
+        [InlineData(-4.0, "f9c400")]
+        [InlineData(double.PositiveInfinity, "f97c00")]
+        [InlineData(double.NaN, "f97e00")]
+        [InlineData(double.NegativeInfinity, "f9fc00")]
+        public static void ReadHalf_SingleValue_HappyPath(float expectedResult, string hexEncoding)
+        {
+            byte[] encoding = hexEncoding.HexToByteArray();
+            var reader = new CborReader(encoding);
+            Assert.Equal(CborReaderState.HalfPrecisionFloat, reader.PeekState());
+            Half actualResult = reader.ReadHalf();
+            AssertHelpers.Equal((Half)expectedResult, actualResult);
+            Assert.Equal(CborReaderState.Finished, reader.PeekState());
+        }
+
         [Theory]
         [InlineData(100000.0, "fa47c35000")]
         [InlineData(3.4028234663852886e+38, "fa7f7fffff")]
@@ -225,6 +247,25 @@ public static void ReadNull_InvalidTypes_ShouldThrowInvalidOperationException(st
             Assert.Equal(encoding.Length, reader.BytesRemaining);
         }
 
+        [Theory]
+        [InlineData("01")] // integer
+        [InlineData("40")] // empty text string
+        [InlineData("60")] // empty byte string
+        [InlineData("80")] // []
+        [InlineData("a0")] // {}
+        [InlineData("f6")] // null
+        [InlineData("f4")] // false
+        [InlineData("c202")] // tagged value
+        [InlineData("fa47c35000")] // single-precision float encoding
+        [InlineData("fb7ff0000000000000")] // double-precision float encoding
+        public static void ReadHalf_InvalidTypes_ShouldThrowInvalidOperationException(string hexEncoding)
+        {
+            byte[] encoding = hexEncoding.HexToByteArray();
+            var reader = new CborReader(encoding);
+            Assert.Throws<InvalidOperationException>(() => reader.ReadHalf());
+            Assert.Equal(encoding.Length, reader.BytesRemaining);
+        }
+
         [Theory]
         [InlineData("01")] // integer
         [InlineData("40")] // empty text string
@@ -259,5 +300,21 @@ public static void ReadDouble_InvalidTypes_ShouldThrowInvalidOperationException(
             Assert.Throws<InvalidOperationException>(() => reader.ReadDouble());
             Assert.Equal(encoding.Length, reader.BytesRemaining);
         }
+
+        public static class AssertHelpers
+        {
+            // temporary workaround for xunit's lack of support for Half equality assertions
+            public static void Equal(Half expected, Half actual)
+            {
+                if (Half.IsNaN(expected))
+                {
+                    Assert.True(Half.IsNaN(actual), $"Expected: {expected}\nActual:  {actual}");
+                }
+                else
+                {
+                    Assert.Equal(expected, actual);
+                }
+            }
+        }
     }
 }

src/libraries/System.Formats.Cbor/tests/Writer/CborWriterTests.Simple.cs

@@ -11,6 +11,26 @@ public partial class CborWriterTests
         // Data points taken from https://tools.ietf.org/html/rfc7049#appendix-A
         // Additional pairs generated using http://cbor.me/
 
+        [Theory]
+        [InlineData(0.0, "f90000")]
+        [InlineData(-0.0, "f98000")]
+        [InlineData(1.0, "f93c00")]
+        [InlineData(1.5, "f93e00")]
+        [InlineData(65504.0, "f97bff")]
+        [InlineData(5.960464477539063e-8, "f90001")]
+        [InlineData(0.00006103515625, "f90400")]
+        [InlineData(-4.0, "f9c400")]
+        [InlineData(float.PositiveInfinity, "f97c00")]
+        [InlineData(float.NaN, "f9fe00")]
+        [InlineData(float.NegativeInfinity, "f9fc00")]
+        public static void WriteHalf_SingleValue_HappyPath(float input, string hexExpectedEncoding)
+        {
+            byte[] expectedEncoding = hexExpectedEncoding.HexToByteArray();
+            var writer = new CborWriter();
+            writer.WriteHalf((Half)input);
+            AssertHelper.HexEqual(expectedEncoding, writer.Encode());
+        }
+
         [Theory]
         [InlineData(100000.0, "fa47c35000")]
         [InlineData(3.4028234663852886e+38, "fa7f7fffff")]