Glenn Maynard
252 lines
6.9 KiB
C++
252 lines
6.9 KiB
C++
/* CircBuf - A fast, thread-safe, lockless circular buffer. */
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#ifndef RAGE_UTIL_CIRCULAR_BUFFER
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#define RAGE_UTIL_CIRCULAR_BUFFER
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/* Lock-free circular buffer. This should be threadsafe if one thread is reading
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* and another is writing. */
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template<class T>
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class CircBuf
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{
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T *buf;
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/* read_pos is the position data is read from; write_pos is the position
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* data is written to. If read_pos == write_pos, the buffer is empty.
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*
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* There will always be at least one position empty, as a completely full
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* buffer (read_pos == write_pos) is indistinguishable from an empty buffer.
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*
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* Invariants: read_pos < size, write_pos < size. */
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unsigned size;
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/* These are volatile to prevent reads and writes to them from being optimized. */
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volatile unsigned read_pos, write_pos;
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public:
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CircBuf()
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{
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buf = NULL;
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clear();
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}
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~CircBuf()
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{
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delete[] buf;
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}
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/* Return the number of elements available to read. */
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unsigned num_readable() const
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{
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const int rpos = read_pos;
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const int wpos = write_pos;
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if( rpos < wpos )
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/* The buffer looks like "eeeeDDDDeeee" (e = empty, D = data). */
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return wpos - rpos;
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else if( rpos > wpos )
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/* The buffer looks like "DDeeeeeeeeDD" (e = empty, D = data). */
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return size - (rpos - wpos);
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else // if( rpos == wpos )
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/* The buffer looks like "eeeeeeeeeeee" (e = empty, D = data). */
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return 0;
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}
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/* Return the number of elements writable. Note that there must always
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* be one */
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unsigned num_writable() const
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{
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const int rpos = read_pos;
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const int wpos = write_pos;
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int ret;
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if( rpos < wpos )
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/* The buffer looks like "eeeeDDDDeeee" (e = empty, D = data). */
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ret = size - (wpos - rpos);
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else if( rpos > wpos )
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/* The buffer looks like "DDeeeeeeeeDD" (e = empty, D = data). */
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ret = rpos - wpos;
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else // if( rpos == wpos )
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/* The buffer looks like "eeeeeeeeeeee" (e = empty, D = data). */
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ret = size;
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/* Subtract one, to account for the element that we never fill. */
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return ret - 1;
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}
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unsigned capacity() const { return size; }
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void reserve( unsigned n )
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{
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clear();
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delete[] buf;
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buf = NULL;
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/* Reserve an extra byte. We'll never fill more than n bytes; the extra
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* byte is to guarantee that read_pos != write_pos when the buffer is full,
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* since that would be ambiguous with an empty buffer. */
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if( n != 0 )
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{
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buf = new T[n+1];
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size = n+1;
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}
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else
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size = 0;
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}
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void clear()
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{
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read_pos = write_pos = 0;
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}
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/* Indicate that n elements have been written. */
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void advance_write_pointer( int n )
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{
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write_pos = (write_pos + n) % size;
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}
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/* Indicate that n elements have been read. */
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void advance_read_pointer( int n )
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{
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read_pos = (read_pos + n) % size;
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}
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void get_write_pointers( T *pPointers[2], unsigned pSizes[2] )
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{
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const int rpos = read_pos;
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const int wpos = write_pos;
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if( rpos <= wpos )
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{
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/* The buffer looks like "eeeeDDDDeeee" or "eeeeeeeeeeee" (e = empty, D = data). */
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pPointers[0] = buf+wpos;
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pPointers[1] = buf;
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pSizes[0] = size - wpos;
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pSizes[1] = rpos;
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}
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else if( rpos > wpos )
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{
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/* The buffer looks like "DDeeeeeeeeDD" (e = empty, D = data). */
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pPointers[0] = buf+wpos;
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pPointers[1] = NULL;
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pSizes[0] = rpos - wpos;
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pSizes[1] = 0;
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}
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/* Subtract one, to account for the element that we never fill. */
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if( pSizes[1] )
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--pSizes[1];
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else
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--pSizes[0];
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}
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void get_read_pointers( T *pPointers[2], unsigned pSizes[2] )
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{
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const int rpos = read_pos;
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const int wpos = write_pos;
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if( rpos < wpos )
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{
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/* The buffer looks like "eeeeDDDDeeee" (e = empty, D = data). */
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pPointers[0] = buf+rpos;
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pPointers[1] = NULL;
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pSizes[0] = wpos - rpos;
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pSizes[1] = 0;
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}
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else if( rpos > wpos )
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{
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/* The buffer looks like "DDeeeeeeeeDD" (e = empty, D = data). */
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pPointers[0] = buf+rpos;
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pPointers[1] = buf;
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pSizes[0] = size - rpos;
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pSizes[1] = wpos;
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}
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else
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{
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/* The buffer looks like "eeeeeeeeeeee" (e = empty, D = data). */
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pPointers[0] = NULL;
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pPointers[1] = NULL;
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pSizes[0] = 0;
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pSizes[1] = 0;
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}
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}
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/* Write buffer_size elements from buffer, and advance the write pointer. If
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* the data will not fit entirely, the write pointer will be unchanged
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* and false will be returned. */
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bool write( const T *buffer, unsigned buffer_size )
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{
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T *p[2];
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unsigned sizes[2];
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get_write_pointers( p, sizes );
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if( buffer_size > sizes[0] + sizes[1] )
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return false;
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const int from_first = min( buffer_size, sizes[0] );
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memcpy( p[0], buffer, from_first*sizeof(T) );
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if( buffer_size > sizes[0] )
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memcpy( p[1], buffer+from_first, max(buffer_size-sizes[0], 0u)*sizeof(T) );
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advance_write_pointer( buffer_size );
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return true;
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}
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/* Read buffer_size elements from buffer, and advance the read pointer. If
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* the buffer can not be filled completely, the read pointer will be unchanged
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* and false will be returned. */
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bool read( T *buffer, unsigned buffer_size )
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{
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T *p[2];
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unsigned sizes[2];
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get_read_pointers( p, sizes );
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if( buffer_size > sizes[0] + sizes[1] )
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return false;
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const int from_first = min( buffer_size, sizes[0] );
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memcpy( buffer, p[0], from_first*sizeof(T) );
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if( buffer_size > sizes[0] )
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memcpy( buffer+from_first, p[1], max(buffer_size-sizes[0], 0u)*sizeof(T) );
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/* Set the data that we just read to 0xFF. This way, if we're passing pointesr
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* through, we can tell if we accidentally get a stale pointer. */
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memset( p[0], 0xFF, from_first*sizeof(T) );
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if( buffer_size > sizes[0] )
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memset( p[1], 0xFF, max(buffer_size-sizes[0], 0u)*sizeof(T) );
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advance_read_pointer( buffer_size );
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return true;
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}
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};
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#endif
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/*
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* Copyright (c) 2004 Glenn Maynard
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* All rights reserved.
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*
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* Permission is hereby granted, free of charge, to any person obtaining a
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* copy of this software and associated documentation files (the
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* "Software"), to deal in the Software without restriction, including
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* without limitation the rights to use, copy, modify, merge, publish,
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* distribute, and/or sell copies of the Software, and to permit persons to
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* whom the Software is furnished to do so, provided that the above
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* copyright notice(s) and this permission notice appear in all copies of
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* the Software and that both the above copyright notice(s) and this
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* permission notice appear in supporting documentation.
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*
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* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
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* OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
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* MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT OF
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* THIRD PARTY RIGHTS. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR HOLDERS
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* INCLUDED IN THIS NOTICE BE LIABLE FOR ANY CLAIM, OR ANY SPECIAL INDIRECT
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* OR CONSEQUENTIAL DAMAGES, OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS
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* OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
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* OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
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* PERFORMANCE OF THIS SOFTWARE.
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*/
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