Code:
/ DotNET / DotNET / 8.0 / untmp / whidbey / REDBITS / ndp / fx / src / Sys / System / IO / compression / InputBuffer.cs / 1 / InputBuffer.cs
namespace System.IO.Compression { using System; using System.Diagnostics; // This class can be used to read bits from an byte array quickly. // Normally we get bits from 'bitBuffer' field and bitsInBuffer stores // the number of bits available in 'BitBuffer'. // When we used up the bits in bitBuffer, we will try to get byte from // the byte array and copy the byte to appropiate position in bitBuffer. // // The byte array is not reused. We will go from 'start' to 'end'. // When we reach the end, most read operations will return -1, // which means we are running out of input. internal class InputBuffer { private byte[] buffer; // byte array to store input private int start; // start poisition of the buffer private int end; // end position of the buffer private uint bitBuffer = 0; // store the bits here, we can quickly shift in this buffer private int bitsInBuffer = 0; // number of bits available in bitBuffer // Total bits available in the input buffer public int AvailableBits { get{ return bitsInBuffer; } } // Total bytes available in the input buffer public int AvailableBytes { get{ return(end - start) + (bitsInBuffer / 8); } } // Ensure that count bits are in the bit buffer. // Returns false if input is not sufficient to make this true. // Count can be up to 16. public bool EnsureBitsAvailable(int count) { Debug.Assert( 0 < count && count <= 16, "count is invalid."); // manual inlining to improve perf if (bitsInBuffer < count) { if (NeedsInput()) { return false; } // insert a byte to bitbuffer bitBuffer |= (uint)buffer[start++] << bitsInBuffer; bitsInBuffer += 8; if (bitsInBuffer < count) { if (NeedsInput()) { return false; } // insert a byte to bitbuffer bitBuffer |= (uint)buffer[start++] << bitsInBuffer; bitsInBuffer += 8; } } return true; } // This function will try to load 16 or more bits into bitBuffer. // It returns whatever is contained in bitBuffer after loading. // The main difference between this and GetBits is that this will // never return -1. So the caller needs to check AvailableBits to // see how many bits are available. public uint TryLoad16Bits() { if(bitsInBuffer < 8) { if( start < end) { bitBuffer |= (uint)buffer[start++] << bitsInBuffer; bitsInBuffer += 8; } if( start < end) { bitBuffer |= (uint)buffer[start++] << bitsInBuffer; bitsInBuffer += 8; } } else if(bitsInBuffer < 16) { if( start < end) { bitBuffer |= (uint)buffer[start++] << bitsInBuffer; bitsInBuffer += 8; } } return bitBuffer; } private uint GetBitMask(int count) { return ((uint)1 << count) - 1; } // Gets count bits from the input buffer. Returns -1 if not enough bits available. public int GetBits(int count) { Debug.Assert( 0 < count && count <= 16, "count is invalid."); if ( !EnsureBitsAvailable(count) ) { return -1; } int result = (int)(bitBuffer & GetBitMask(count)); bitBuffer >>= count; bitsInBuffer -= count; return result; } /// Copies length bytes from input buffer to output buffer starting /// at output[offset]. You have to make sure, that the buffer is /// byte aligned. If not enough bytes are available, copies fewer /// bytes. /// Returns the number of bytes copied, 0 if no byte is available. public int CopyTo(byte[] output, int offset, int length) { Debug.Assert( output != null, ""); Debug.Assert( offset >=0 , ""); Debug.Assert( length >=0 , ""); Debug.Assert( offset <= output.Length - length, ""); Debug.Assert( (bitsInBuffer % 8) ==0, ""); // Copy the bytes in bitBuffer first. int bytesFromBitBuffer = 0; while (bitsInBuffer > 0 && length > 0) { output[offset++] = (byte) bitBuffer; bitBuffer >>= 8; bitsInBuffer -= 8; length--; bytesFromBitBuffer++; } if (length == 0) { return bytesFromBitBuffer; } int avail = end - start; if (length > avail) { length = avail; } Array.Copy(buffer, start, output, offset, length); start += length; return bytesFromBitBuffer + length; } // Return true is all input bytes are used. // This means the caller can call SetInput to add more input. public bool NeedsInput() { return start == end; } // Set the byte array to be processed. // All the bits remained in bitBuffer will be processed before the new bytes. // We don't clone the byte array here since it is expensive. // The caller should make sure after a buffer is passed in. // It will not be changed before calling this function again. public void SetInput(byte[] buffer, int offset, int length) { Debug.Assert( buffer != null, ""); Debug.Assert( offset >=0 , ""); Debug.Assert( length >=0 , ""); Debug.Assert( offset <= buffer.Length - length, ""); Debug.Assert( start == end, ""); this.buffer = buffer; start = offset; end = offset + length; } // Skip n bits in the buffer public void SkipBits(int n) { Debug.Assert(bitsInBuffer >= n, "No enough bits in the buffer, Did you call EnsureBitsAvailable?"); bitBuffer >>= n; bitsInBuffer -= n; } // Skips to the next byte boundary. public void SkipToByteBoundary() { bitBuffer >>= (bitsInBuffer % 8); bitsInBuffer = bitsInBuffer - (bitsInBuffer % 8); } } }
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