Unify int to hexadecimal char conversions

src/libraries/Common/src/System/HexConverter.cs

@@ -0,0 +1,126 @@
+using System.Runtime.CompilerServices;
+
+namespace System
+{
+    internal static class HexConverter
+    {
+        public enum Casing : uint
+        {
+            // Output [ '0' .. '9' ] and [ 'A' .. 'F' ].
+            Upper = 0,
+
+            // Output [ '0' .. '9' ] and [ 'a' .. 'f' ].
+            // This works because values in the range [ 0x30 .. 0x39 ] ([ '0' .. '9' ])
+            // already have the 0x20 bit set, so ORing them with 0x20 is a no-op,
+            // while outputs in the range [ 0x41 .. 0x46 ] ([ 'A' .. 'F' ])
+            // don't have the 0x20 bit set, so ORing them maps to
+            // [ 0x61 .. 0x66 ] ([ 'a' .. 'f' ]), which is what we want.
+            Lower = 0x2020U,
+        }
+
+        // We want to pack the incoming byte into a single integer [ 0000 HHHH 0000 LLLL ],
+        // where HHHH and LLLL are the high and low nibbles of the incoming byte. Then
+        // subtract this integer from a constant minuend as shown below.
+        //
+        //   [ 1000 1001 1000 1001 ]
+        // - [ 0000 HHHH 0000 LLLL ]
+        // =========================
+        //   [ *YYY **** *ZZZ **** ]
+        //
+        // The end result of this is that YYY is 0b000 if HHHH <= 9, and YYY is 0b111 if HHHH >= 10.
+        // Similarly, ZZZ is 0b000 if LLLL <= 9, and ZZZ is 0b111 if LLLL >= 10.
+        // (We don't care about the value of asterisked bits.)
+        //
+        // To turn a nibble in the range [ 0 .. 9 ] into hex, we calculate hex := nibble + 48 (ascii '0').
+        // To turn a nibble in the range [ 10 .. 15 ] into hex, we calculate hex := nibble - 10 + 65 (ascii 'A').
+        //                                                                => hex := nibble + 55.
+        // The difference in the starting ASCII offset is (55 - 48) = 7, depending on whether the nibble is <= 9 or >= 10.
+        // Since 7 is 0b111, this conveniently matches the YYY or ZZZ value computed during the earlier subtraction.
+
+        // The commented out code below is code that directly implements the logic described above.
+
+        // uint packedOriginalValues = (((uint)value & 0xF0U) << 4) + ((uint)value & 0x0FU);
+        // uint difference = 0x8989U - packedOriginalValues;
+        // uint add7Mask = (difference & 0x7070U) >> 4; // line YYY and ZZZ back up with the packed values
+        // uint packedResult = packedOriginalValues + add7Mask + 0x3030U /* ascii '0' */;
+
+        // The code below is equivalent to the commented out code above but has been tweaked
+        // to allow codegen to make some extra optimizations.
+
+        // The low byte of the packed result contains the hex representation of the incoming byte's low nibble.
+        // The adjacent byte of the packed result contains the hex representation of the incoming byte's high nibble.
+
+        // Finally, write to the output buffer starting with the *highest* index so that codegen can
+        // elide all but the first bounds check. (This only works if 'startingIndex' is a compile-time constant.)
+
+        // The JIT can elide bounds checks if 'startingIndex' is constant and if the caller is
+        // writing to a span of known length (or the caller has already checked the bounds of the
+        // furthest access).
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static void ToBytesBuffer(byte value, Span<byte> buffer, int startingIndex = 0, Casing casing = Casing.Upper)
+        {
+            uint difference = (((uint)value & 0xF0U) << 4) + ((uint)value & 0x0FU) - 0x8989U;
+            uint packedResult = ((((uint)(-(int)difference) & 0x7070U) >> 4) + difference + 0xB9B9U) | (uint)casing;
+
+            buffer[startingIndex + 1] = (byte)packedResult;
+            buffer[startingIndex] = (byte)(packedResult >> 8);
+        }
+
+#if ALLOW_PARTIALLY_TRUSTED_CALLERS
+        [System.Security.SecuritySafeCriticalAttribute]
+#endif
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static void ToCharsBuffer(byte value, Span<char> buffer, int startingIndex = 0, Casing casing = Casing.Upper)
+        {
+            uint difference = (((uint)value & 0xF0U) << 4) + ((uint)value & 0x0FU) - 0x8989U;
+            uint packedResult = ((((uint)(-(int)difference) & 0x7070U) >> 4) + difference + 0xB9B9U) | (uint)casing;
+
+            buffer[startingIndex + 1] = (char)(packedResult & 0xFF);
+            buffer[startingIndex] = (char)(packedResult >> 8);
+        }
+
+#if ALLOW_PARTIALLY_TRUSTED_CALLERS
+        [System.Security.SecuritySafeCriticalAttribute]
+#endif
+        public static unsafe string ToString(ReadOnlySpan<byte> bytes, Casing casing = Casing.Upper)
+        {
+#if NET45 || NET46 || NET461 || NET462 || NET47 || NET471 || NET472 || NETSTANDARD1_0 || NETSTANDARD1_3 || NETSTANDARD2_0
+            System.Diagnostics.Debug.Assert(bytes.Length <= 16);
+            Span<char> result = stackalloc char[bytes.Length * 2];
+            int pos = 0;
+            foreach (byte b in bytes)
+            {
+                ToCharsBuffer(b, result, pos, casing);
+                pos += 2;
+            }
+            return result.ToString();
+#else
+            fixed (byte* bytesPtr = bytes)
+            {
+                return string.Create(bytes.Length * 2, (Ptr: (IntPtr)bytesPtr, bytes.Length, casing), (chars, args) =>
+                {
+                    var ros = new ReadOnlySpan<byte>((byte*)args.Ptr, args.Length);
+                    for (int pos = 0; pos < ros.Length; ++pos)
+                    {
+                        ToCharsBuffer(ros[pos], chars, pos * 2, args.casing);
+                    }
+                });
+            }
+#endif
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static char ToCharUpper(int value)
+        {
+            value &= 0xF;
+            return (char)(value > 9 ? value - 10 + 'A' : value + '0');
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static char ToCharLower(int value)
+        {
+            value &= 0xF;
+            return (char)(value > 9 ? value - 10 + 'a' : value + '0');
+        }
+    }
+}

src/libraries/System.Diagnostics.DiagnosticSource/src/System.Diagnostics.DiagnosticSource.csproj

@@ -25,6 +25,9 @@
     <None Include="DiagnosticSourceUsersGuide.md" />
   </ItemGroup>
   <ItemGroup Condition=" '$(TargetFramework)' != 'netstandard1.1'">
+    <Compile Include="$(CommonPath)System\HexConverter.cs">
+      <Link>Common\System\HexConverter.cs</Link>
+    </Compile>
     <Compile Include="System\Diagnostics\Activity.cs" />
     <Compile Include="System\Diagnostics\DiagnosticSourceActivity.cs" />
     <Reference Include="System.Memory" />

src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/Activity.cs

@@ -131,7 +131,7 @@ public string? Id
                 {
                     // Convert flags to binary.
                     Span<char> flagsChars = stackalloc char[2];
-                    ActivityTraceId.ByteToHexDigits(flagsChars, (byte)((~ActivityTraceFlagsIsSet) & _w3CIdFlags));
+                    HexConverter.ToCharsBuffer((byte)((~ActivityTraceFlagsIsSet) & _w3CIdFlags), flagsChars, 0, HexConverter.Casing.Lower);
                     string id = "00-" + _traceId + "-" + _spanId + "-" + flagsChars.ToString();
 
                     Interlocked.CompareExchange(ref _id, id, null);
@@ -1018,7 +1018,7 @@ public static ActivityTraceId CreateFromBytes(ReadOnlySpan<byte> idData)
             if (idData.Length != 16)
                 throw new ArgumentOutOfRangeException(nameof(idData));
 
-            return new ActivityTraceId(SpanToHexString(idData));
+            return new ActivityTraceId(HexConverter.ToString(idData, HexConverter.Casing.Lower));
         }
         public static ActivityTraceId CreateFromUtf8String(ReadOnlySpan<byte> idData) => new ActivityTraceId(idData);
 
@@ -1097,7 +1097,7 @@ private ActivityTraceId(ReadOnlySpan<byte> idData)
                 span[1] = BinaryPrimitives.ReverseEndianness(span[1]);
             }
 
-            _hexString = ActivityTraceId.SpanToHexString(MemoryMarshal.AsBytes(span));
+            _hexString = HexConverter.ToString(MemoryMarshal.AsBytes(span), HexConverter.Casing.Lower);
         }
 
         /// <summary>
@@ -1120,26 +1120,6 @@ internal static unsafe void SetToRandomBytes(Span<byte> outBytes)
             guidBytes.Slice(0, outBytes.Length).CopyTo(outBytes);
         }
 
-        // CONVERSION binary spans to hex spans, and hex spans to binary spans
-        /* It would be nice to use generic Hex number conversion routines, but there
-         * is nothing that is exposed publicly and efficient */
-        /// <summary>
-        /// Converts each byte in 'bytes' to hex (thus two characters) and concatenates them
-        /// and returns the resulting string.
-        /// </summary>
-        internal static string SpanToHexString(ReadOnlySpan<byte> bytes)
-        {
-            Debug.Assert(bytes.Length <= 16);   // We want it to not be very big
-            Span<char> result = stackalloc char[bytes.Length * 2];
-            int pos = 0;
-            foreach (byte b in bytes)
-            {
-                result[pos++] = BinaryToHexDigit(b >> 4);
-                result[pos++] = BinaryToHexDigit(b);
-            }
-            return result.ToString();
-        }
-
         /// <summary>
         /// Converts 'idData' which is assumed to be HEX Unicode characters to binary
         /// puts it in 'outBytes'
@@ -1162,20 +1142,6 @@ private static byte HexDigitToBinary(char c)
                 return (byte)(c - ('a' - 10));
             throw new ArgumentOutOfRangeException("idData");
         }
-        private static char BinaryToHexDigit(int val)
-        {
-            val &= 0xF;
-            if (val <= 9)
-                return (char)('0' + val);
-            return (char)(('a' - 10) + val);
-        }
-
-        internal static void ByteToHexDigits(Span<char> outChars, byte val)
-        {
-            Debug.Assert(outChars.Length == 2);
-            outChars[0] = BinaryToHexDigit((val >> 4) & 0xF);
-            outChars[1] = BinaryToHexDigit(val & 0xF);
-        }
 
         internal static bool IsLowerCaseHexAndNotAllZeros(ReadOnlySpan<char> idData)
         {
@@ -1232,14 +1198,14 @@ public static unsafe ActivitySpanId CreateRandom()
         {
             ulong id;
             ActivityTraceId.SetToRandomBytes(new Span<byte>(&id, sizeof(ulong)));
-            return new ActivitySpanId(ActivityTraceId.SpanToHexString(new ReadOnlySpan<byte>(&id, sizeof(ulong))));
+            return new ActivitySpanId(HexConverter.ToString(new ReadOnlySpan<byte>(&id, sizeof(ulong)), HexConverter.Casing.Lower));
         }
         public static ActivitySpanId CreateFromBytes(ReadOnlySpan<byte> idData)
         {
             if (idData.Length != 8)
                 throw new ArgumentOutOfRangeException(nameof(idData));
 
-            return new ActivitySpanId(ActivityTraceId.SpanToHexString(idData));
+            return new ActivitySpanId(HexConverter.ToString(idData, HexConverter.Casing.Lower));
         }
         public static ActivitySpanId CreateFromUtf8String(ReadOnlySpan<byte> idData) => new ActivitySpanId(idData);
 
@@ -1307,7 +1273,7 @@ private unsafe ActivitySpanId(ReadOnlySpan<byte> idData)
                 id = BinaryPrimitives.ReverseEndianness(id);
             }
 
-            _hexString = ActivityTraceId.SpanToHexString(new ReadOnlySpan<byte>(&id, sizeof(ulong)));
+            _hexString = HexConverter.ToString(new ReadOnlySpan<byte>(&id, sizeof(ulong)), HexConverter.Casing.Lower);
         }
 
         /// <summary>

src/libraries/System.Net.Http/src/System.Net.Http.csproj

@@ -118,6 +118,9 @@
     <Compile Include="$(CommonPath)System\Text\SimpleRegex.cs">
       <Link>Common\System\Text\SimpleRegex.cs</Link>
     </Compile>
+    <Compile Include="$(CommonPath)System\HexConverter.cs">
+      <Link>Common\System\HexConverter.cs</Link>
+    </Compile>
   </ItemGroup>
   <!-- SocketsHttpHandler implementation -->
   <ItemGroup>

src/libraries/System.Net.Http/src/System/Net/Http/Headers/HeaderUtilities.cs

@@ -27,8 +27,6 @@ internal static class HeaderUtilities
         // Validator
         internal static readonly Action<HttpHeaderValueCollection<string>, string> TokenValidator = ValidateToken;
 
-        private static readonly char[] s_hexUpperChars = { '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'A', 'B', 'C', 'D', 'E', 'F' };
-
         internal static void SetQuality(ObjectCollection<NameValueHeaderValue> parameters, double? value)
         {
             Debug.Assert(parameters != null);
@@ -125,8 +123,8 @@ private static void AddHexEscaped(byte c, StringBuilder destination)
             Debug.Assert(destination != null);
 
             destination.Append('%');
-            destination.Append(s_hexUpperChars[(c & 0xf0) >> 4]);
-            destination.Append(s_hexUpperChars[c & 0xf]);
+            destination.Append(HexConverter.ToCharUpper(c >> 4));
+            destination.Append(HexConverter.ToCharUpper(c));
         }
 
         internal static double? GetQuality(ObjectCollection<NameValueHeaderValue> parameters)

src/libraries/System.Net.Http/tests/UnitTests/System.Net.Http.Unit.Tests.csproj

@@ -11,6 +11,9 @@
     <DefaultReferenceExclusions Include="System.Net.Mail" />
   </ItemGroup>
   <ItemGroup>
+    <Compile Include="$(CommonPath)System\HexConverter.cs">
+      <Link>Common\System\HexConverter.cs</Link>
+    </Compile>
     <Compile Include="$(CommonPath)System\NotImplemented.cs">
       <Link>ProductionCode\Common\System\NotImplemented.cs</Link>
     </Compile>

src/libraries/System.Net.Mail/src/System.Net.Mail.csproj

@@ -143,6 +143,9 @@
     <Compile Include="$(CommonPath)System\Net\Security\NetEventSource.Security.cs">
       <Link>Common\System\Net\Security\NetEventSource.Security.cs</Link>
     </Compile>
+    <Compile Include="$(CommonPath)System\HexConverter.cs">
+      <Link>Common\System\HexConverter.cs</Link>
+    </Compile>
   </ItemGroup>
   <!-- Unix specific files -->
   <ItemGroup Condition="'$(TargetsUnix)'=='true'">

src/libraries/System.Net.Mail/src/System/Net/Mime/QEncodedStream.cs

@@ -40,9 +40,6 @@ internal class QEncodedStream : DelegatedStream, IEncodableStream
              255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, // F
         };
 
-        //bytes that correspond to the hex char representations in ASCII (0-9, A-F)
-        private static readonly byte[] s_hexEncodeMap = new byte[] { 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 65, 66, 67, 68, 69, 70 };
-
         private ReadStateInfo _readState;
         private readonly WriteStateInfoBase _writeState;
 
@@ -248,9 +245,9 @@ public int EncodeBytes(byte[] buffer, int offset, int count)
                     //append an = to indicate an encoded character
                     WriteState.Append((byte)'=');
                     //shift 4 to get the first four bytes only and look up the hex digit
-                    WriteState.Append(s_hexEncodeMap[buffer[cur] >> 4]);
+                    WriteState.Append((byte)HexConverter.ToCharUpper(buffer[cur] >> 4));
                     //clear the first four bytes to get the last four and look up the hex digit
-                    WriteState.Append(s_hexEncodeMap[buffer[cur] & 0xF]);
+                    WriteState.Append((byte)HexConverter.ToCharUpper(buffer[cur]));
                 }
             }
             WriteState.AppendFooter();

src/libraries/System.Net.Mail/tests/Unit/System.Net.Mail.Unit.Tests.csproj

@@ -219,6 +219,9 @@
     <Compile Include="$(CommonPath)System\Net\Security\NetEventSource.Security.cs">
       <Link>Common\System\Net\Security\NetEventSource.Security.cs</Link>
     </Compile>
+    <Compile Include="$(CommonPath)System\HexConverter.cs">
+      <Link>Common\System\HexConverter.cs</Link>
+    </Compile>
   </ItemGroup>
   <!-- Unix specific files -->
   <ItemGroup Condition="'$(TargetsUnix)'=='true'">

src/libraries/System.Net.NetworkInformation/src/System.Net.NetworkInformation.csproj

@@ -52,6 +52,9 @@
     <Compile Include="$(CommonPath)System\Net\NetworkInformation\NetworkInformationException.cs">
       <Link>Common\System\Net\NetworkInformation\NetworkInformationException.cs</Link>
     </Compile>
+    <Compile Include="$(CommonPath)System\HexConverter.cs">
+      <Link>Common\System\HexConverter.cs</Link>
+    </Compile>
   </ItemGroup>
   <ItemGroup Condition="'$(TargetsWindows)' == 'true'">
     <!-- Logging -->

src/libraries/System.Net.NetworkInformation/src/System/Net/NetworkInformation/PhysicalAddress.cs

@@ -91,16 +91,7 @@ public override bool Equals(object comparand)
 
         public override string ToString()
         {
-            return string.Create(_address.Length * 2, _address, (span, addr) =>
-            {
-                int p = 0;
-                foreach (byte value in addr)
-                {
-                    byte upper = (byte)(value >> 4), lower = (byte)(value & 0xF);
-                    span[p++] = (char)(upper + (upper < 10 ? '0' : 'A' - 10));
-                    span[p++] = (char)(lower + (lower < 10 ? '0' : 'A' - 10));
-                }
-            });
+            return HexConverter.ToString(_address.AsSpan(), HexConverter.Casing.Upper);
         }
 
         public byte[] GetAddressBytes()

src/libraries/System.Net.NetworkInformation/tests/FunctionalTests/PhysicalAddressTest.cs

@@ -259,7 +259,7 @@ public void TryParseSpan_Invalid_ReturnsFalse(string address)
         public void ToString_NullAddress_NullReferenceException()
         {
             var pa = new PhysicalAddress(null);
-            Assert.Throws<NullReferenceException>(() => pa.ToString());
+            Assert.Equal("", pa.ToString());
         }
 
         [Theory]

src/libraries/System.Net.Primitives/src/System.Net.Primitives.csproj

@@ -94,6 +94,9 @@
     <Compile Include="$(CommonPath)System\Runtime\CompilerServices\PreserveDependencyAttribute.cs">
       <Link>Common\System\Runtime\CompilerServices\PreserveDependencyAttribute.cs</Link>
     </Compile>
+    <Compile Include="$(CommonPath)System\HexConverter.cs">
+      <Link>Common\System\HexConverter.cs</Link>
+    </Compile>
     <!-- Logging -->
     <Compile Include="$(CommonPath)System\Net\Logging\NetEventSource.Common.cs">
       <Link>Common\System\Net\Logging\NetEventSource.Common.cs</Link>

src/libraries/System.Net.Primitives/src/System/Net/IPAddressParser.cs

@@ -279,22 +279,18 @@ private static void AppendSections(ushort[] address, int fromInclusive, int toEx
         }
 
         /// <summary>Appends a number as hexadecimal (without the leading "0x") to the StringBuilder.</summary>
-        private static unsafe void AppendHex(ushort value, StringBuilder buffer)
+        private static void AppendHex(ushort value, StringBuilder buffer)
         {
-            const int MaxLength = sizeof(ushort) * 2; // two hex chars per byte
-            char* chars = stackalloc char[MaxLength];
-            int pos = MaxLength;
+            if ((value & 0xF000) != 0)
+                buffer.Append(HexConverter.ToCharLower(value >> 12));
 
-            do
-            {
-                int rem = value % 16;
-                value /= 16;
-                chars[--pos] = rem < 10 ? (char)('0' + rem) : (char)('a' + (rem - 10));
-                Debug.Assert(pos >= 0);
-            }
-            while (value != 0);
+            if ((value & 0xFF00) != 0)
+                buffer.Append(HexConverter.ToCharLower(value >> 8));
+
+            if ((value & 0xFFF0) != 0)
+                buffer.Append(HexConverter.ToCharLower(value >> 4));
 
-            buffer.Append(chars + pos, MaxLength - pos);
+            buffer.Append(HexConverter.ToCharLower(value));
         }
 
         /// <summary>Extracts the IPv4 address from the end of the IPv6 address byte array.</summary>