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HexConverter.cs
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HexConverter.cs
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// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
using System.Diagnostics;
using System.Runtime.CompilerServices;
#if SYSTEM_PRIVATE_CORELIB
using System.Runtime.InteropServices;
using System.Runtime.Intrinsics;
using System.Runtime.Intrinsics.X86;
using Internal.Runtime.CompilerServices;
#endif
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 SYSTEM_PRIVATE_CORELIB
private static void EncodeToUtf16_Ssse3(ReadOnlySpan<byte> bytes, Span<char> chars, Casing casing)
{
Debug.Assert(bytes.Length >= 4);
nint pos = 0;
Vector128<byte> shuffleMask = Vector128.Create(
0xFF, 0xFF, 0, 0xFF, 0xFF, 0xFF, 1, 0xFF,
0xFF, 0xFF, 2, 0xFF, 0xFF, 0xFF, 3, 0xFF);
Vector128<byte> asciiTable = (casing == Casing.Upper) ?
Vector128.Create((byte)'0', (byte)'1', (byte)'2', (byte)'3',
(byte)'4', (byte)'5', (byte)'6', (byte)'7',
(byte)'8', (byte)'9', (byte)'A', (byte)'B',
(byte)'C', (byte)'D', (byte)'E', (byte)'F') :
Vector128.Create((byte)'0', (byte)'1', (byte)'2', (byte)'3',
(byte)'4', (byte)'5', (byte)'6', (byte)'7',
(byte)'8', (byte)'9', (byte)'a', (byte)'b',
(byte)'c', (byte)'d', (byte)'e', (byte)'f');
do
{
// Read 32bits from "bytes" span at "pos" offset
uint block = Unsafe.ReadUnaligned<uint>(
ref Unsafe.Add(ref MemoryMarshal.GetReference(bytes), pos));
// Calculate nibbles
Vector128<byte> lowNibbles = Ssse3.Shuffle(
Vector128.CreateScalarUnsafe(block).AsByte(), shuffleMask);
Vector128<byte> highNibbles = Sse2.ShiftRightLogical(
Sse2.ShiftRightLogical128BitLane(lowNibbles, 2).AsInt32(), 4).AsByte();
// Lookup the hex values at the positions of the indices
Vector128<byte> indices = Sse2.And(
Sse2.Or(lowNibbles, highNibbles), Vector128.Create((byte)0xF));
Vector128<byte> hex = Ssse3.Shuffle(asciiTable, indices);
// The high bytes (0x00) of the chars have also been converted
// to ascii hex '0', so clear them out.
hex = Sse2.And(hex, Vector128.Create((ushort)0xFF).AsByte());
// Save to "chars" at pos*2 offset
Unsafe.WriteUnaligned(
ref Unsafe.As<char, byte>(
ref Unsafe.Add(ref MemoryMarshal.GetReference(chars), pos * 2)), hex);
pos += 4;
} while (pos < bytes.Length - 3);
// Process trailing elements (bytes.Length % 4)
for (; pos < bytes.Length; pos++)
{
ToCharsBuffer(Unsafe.Add(ref MemoryMarshal.GetReference(bytes), pos), chars, (int)pos * 2, casing);
}
}
#endif
public static void EncodeToUtf16(ReadOnlySpan<byte> bytes, Span<char> chars, Casing casing = Casing.Upper)
{
Debug.Assert(chars.Length >= bytes.Length * 2);
#if SYSTEM_PRIVATE_CORELIB
if (Ssse3.IsSupported && bytes.Length >= 4)
{
EncodeToUtf16_Ssse3(bytes, chars, casing);
return;
}
#endif
for (int pos = 0; pos < bytes.Length; pos++)
{
ToCharsBuffer(bytes[pos], chars, pos * 2, casing);
}
}
#if ALLOW_PARTIALLY_TRUSTED_CALLERS
[System.Security.SecuritySafeCriticalAttribute]
#endif
public static unsafe string ToString(ReadOnlySpan<byte> bytes, Casing casing = Casing.Upper)
{
#if NETFRAMEWORK || NETSTANDARD2_0
Span<char> result = stackalloc char[0];
if (bytes.Length > 16)
{
var array = new char[bytes.Length * 2];
result = array.AsSpan();
}
else
{
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), static (chars, args) =>
{
var ros = new ReadOnlySpan<byte>((byte*)args.Ptr, args.Length);
EncodeToUtf16(ros, chars, args.casing);
});
}
#endif
}
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public static char ToCharUpper(int value)
{
value &= 0xF;
value += '0';
if (value > '9')
{
value += ('A' - ('9' + 1));
}
return (char)value;
}
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public static char ToCharLower(int value)
{
value &= 0xF;
value += '0';
if (value > '9')
{
value += ('a' - ('9' + 1));
}
return (char)value;
}
public static bool TryDecodeFromUtf16(ReadOnlySpan<char> chars, Span<byte> bytes)
{
return TryDecodeFromUtf16(chars, bytes, out _);
}
public static bool TryDecodeFromUtf16(ReadOnlySpan<char> chars, Span<byte> bytes, out int charsProcessed)
{
Debug.Assert(chars.Length % 2 == 0, "Un-even number of characters provided");
Debug.Assert(chars.Length / 2 == bytes.Length, "Target buffer not right-sized for provided characters");
int i = 0;
int j = 0;
int byteLo = 0;
int byteHi = 0;
while (j < bytes.Length)
{
byteLo = FromChar(chars[i + 1]);
byteHi = FromChar(chars[i]);
// byteHi hasn't been shifted to the high half yet, so the only way the bitwise or produces this pattern
// is if either byteHi or byteLo was not a hex character.
if ((byteLo | byteHi) == 0xFF)
break;
bytes[j++] = (byte)((byteHi << 4) | byteLo);
i += 2;
}
if (byteLo == 0xFF)
i++;
charsProcessed = i;
return (byteLo | byteHi) != 0xFF;
}
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public static int FromChar(int c)
{
return c >= CharToHexLookup.Length ? 0xFF : CharToHexLookup[c];
}
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public static int FromUpperChar(int c)
{
return c > 71 ? 0xFF : CharToHexLookup[c];
}
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public static int FromLowerChar(int c)
{
if ((uint)(c - '0') <= '9' - '0')
return c - '0';
if ((uint)(c - 'a') <= 'f' - 'a')
return c - 'a' + 10;
return 0xFF;
}
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public static bool IsHexChar(int c)
{
if (IntPtr.Size == 8)
{
// This code path, when used, has no branches and doesn't depend on cache hits,
// so it's faster and does not vary in speed depending on input data distribution.
// We only use this logic on 64-bit systems, as using 64 bit values would otherwise
// be much slower than just using the lookup table anyway (no hardware support).
// The magic constant 18428868213665201664 is a 64 bit value containing 1s at the
// indices corresponding to all the valid hex characters (ie. "0123456789ABCDEFabcdef")
// minus 48 (ie. '0'), and backwards (so from the most significant bit and downwards).
// The offset of 48 for each bit is necessary so that the entire range fits in 64 bits.
// First, we subtract '0' to the input digit (after casting to uint to account for any
// negative inputs). Note that even if this subtraction underflows, this happens before
// the result is zero-extended to ulong, meaning that `i` will always have upper 32 bits
// equal to 0. We then left shift the constant with this offset, and apply a bitmask that
// has the highest bit set (the sign bit) if and only if `c` is in the ['0', '0' + 64) range.
// Then we only need to check whether this final result is less than 0: this will only be
// the case if both `i` was in fact the index of a set bit in the magic constant, and also
// `c` was in the allowed range (this ensures that false positive bit shifts are ignored).
ulong i = (uint)c - '0';
ulong shift = 18428868213665201664UL << (int)i;
ulong mask = i - 64;
return (long)(shift & mask) < 0 ? true : false;
}
return FromChar(c) != 0xFF;
}
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public static bool IsHexUpperChar(int c)
{
return (uint)(c - '0') <= 9 || (uint)(c - 'A') <= ('F' - 'A');
}
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public static bool IsHexLowerChar(int c)
{
return (uint)(c - '0') <= 9 || (uint)(c - 'a') <= ('f' - 'a');
}
/// <summary>Map from an ASCII char to its hex value, e.g. arr['b'] == 11. 0xFF means it's not a hex digit.</summary>
public static ReadOnlySpan<byte> CharToHexLookup => new byte[]
{
0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, // 15
0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, // 31
0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, // 47
0x0, 0x1, 0x2, 0x3, 0x4, 0x5, 0x6, 0x7, 0x8, 0x9, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, // 63
0xFF, 0xA, 0xB, 0xC, 0xD, 0xE, 0xF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, // 79
0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, // 95
0xFF, 0xa, 0xb, 0xc, 0xd, 0xe, 0xf, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, // 111
0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, // 127
0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, // 143
0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, // 159
0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, // 175
0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, // 191
0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, // 207
0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, // 223
0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, // 239
0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF // 255
};
}
}