@ -36,12 +36,6 @@ namespace SixLabors.ImageSharp.Formats.Png.Zlib
private const int EofSymbol = 2 5 6 ;
// The lengths of the bit length codes are sent in order of decreasing
// probability, to avoid transmitting the lengths for unused bit length codes.
private static readonly int [ ] BitLengthOrder = { 1 6 , 1 7 , 1 8 , 0 , 8 , 7 , 9 , 6 , 1 0 , 5 , 1 1 , 4 , 1 2 , 3 , 1 3 , 2 , 1 4 , 1 , 1 5 } ;
private static readonly byte [ ] Bit4Reverse = { 0 , 8 , 4 , 1 2 , 2 , 1 0 , 6 , 1 4 , 1 , 9 , 5 , 1 3 , 3 , 1 1 , 7 , 1 5 } ;
private static readonly short [ ] StaticLCodes ;
private static readonly byte [ ] StaticLLength ;
private static readonly short [ ] StaticDCodes ;
@ -128,6 +122,13 @@ namespace SixLabors.ImageSharp.Formats.Png.Zlib
this . pinnedLiteralBuffer = ( short * ) this . literalBufferHandle . Pointer ;
}
/// <summary>
/// Gets the lengths of the bit length codes are sent in order of decreasing probability, to avoid transmitting the lengths for unused bit length codes.
/// </summary>
private static ReadOnlySpan < byte > BitLengthOrder = > new byte [ ] { 1 6 , 1 7 , 1 8 , 0 , 8 , 7 , 9 , 6 , 1 0 , 5 , 1 1 , 4 , 1 2 , 3 , 1 3 , 2 , 1 4 , 1 , 1 5 } ;
private static ReadOnlySpan < byte > Bit4Reverse = > new byte [ ] { 0 , 8 , 4 , 1 2 , 2 , 1 0 , 6 , 1 4 , 1 , 9 , 5 , 1 3 , 3 , 1 1 , 7 , 1 5 } ;
/// <summary>
/// Gets the pending buffer to use.
/// </summary>
@ -158,6 +159,7 @@ namespace SixLabors.ImageSharp.Formats.Png.Zlib
this . Pending . WriteBits ( this . literalTree . NumCodes - 2 5 7 , 5 ) ;
this . Pending . WriteBits ( this . distTree . NumCodes - 1 , 5 ) ;
this . Pending . WriteBits ( blTreeCodes - 4 , 4 ) ;
for ( int rank = 0 ; rank < blTreeCodes ; rank + + )
{
this . Pending . WriteBits ( this . blTree . Length [ BitLengthOrder [ rank ] ] , 3 ) ;
@ -250,6 +252,7 @@ namespace SixLabors.ImageSharp.Formats.Png.Zlib
this . blTree . BuildTree ( ) ;
int blTreeCodes = 4 ;
for ( int i = 1 8 ; i > blTreeCodes ; i - - )
{
if ( this . blTree . Length [ BitLengthOrder [ i ] ] > 0 )
@ -363,10 +366,30 @@ namespace SixLabors.ImageSharp.Formats.Png.Zlib
[MethodImpl(InliningOptions.ShortMethod)]
public static short BitReverse ( int toReverse )
{
return ( short ) ( Bit4Reverse [ toReverse & 0xF ] < < 1 2
| Bit4Reverse [ ( toReverse > > 4 ) & 0xF ] < < 8
| Bit4Reverse [ ( toReverse > > 8 ) & 0xF ] < < 4
| Bit4Reverse [ toReverse > > 1 2 ] ) ;
/ * Use unsafe offsetting and manually validate the input index to reduce the
* total number of conditional branches . There are two main cases to test here :
* 1. In the first 3 , the input value ( or some combination of it ) is combined
* with & 0xF , which results in a maximum value of 0xF no matter what the
* input value was . That is 1 5 , which is always in range for the target span .
* As a result , no input validation is needed at all in this case .
* 2. There are two cases where the input value might cause an invalid access :
* when it is either negative , or greater than 1 5 < < 1 2. We can test both
* conditions in a single pass by casting the input value to uint and right
* shifting it by 1 2 , which also preserves the sign . If it is a negative
* value ( 2 - complement ) , the test will fail as the uint cast will result
* in a much larger value . If the value was simply too high , the test will
* fail as expected . We can ' t simply check whether the value is lower than
* 1 5 < < 1 2 , because higher values are acceptable in the first 3 accesses .
* Doing this reduces the total number of index checks from 4 down to just 1. * /
int toReverseRightShiftBy12 = toReverse > > 1 2 ;
Guard . MustBeLessThanOrEqualTo < uint > ( ( uint ) toReverseRightShiftBy12 , 1 5 , nameof ( toReverse ) ) ;
ref byte bit4ReverseRef = ref MemoryMarshal . GetReference ( Bit4Reverse ) ;
return ( short ) ( Unsafe . Add ( ref bit4ReverseRef , toReverse & 0xF ) < < 1 2
| Unsafe . Add ( ref bit4ReverseRef , ( toReverse > > 4 ) & 0xF ) < < 8
| Unsafe . Add ( ref bit4ReverseRef , ( toReverse > > 8 ) & 0xF ) < < 4
| Unsafe . Add ( ref bit4ReverseRef , toReverseRightShiftBy12 ) ) ;
}
/// <inheritdoc/>
James Jackson-South
6 years ago