text: /* -----------------------------------------------------------------------------
  
  	Copyright (c) 2006 Simon Brown                          si@sjbrown.co.uk
  
  	Permission is hereby granted, free of charge, to any person obtaining
  	a copy of this software and associated documentation files (the 
  	"Software"), to	deal in the Software without restriction, including
  	without limitation the rights to use, copy, modify, merge, publish,
  	distribute, sublicense, and/or sell copies of the Software, and to 
  	permit persons to whom the Software is furnished to do so, subject to 
  	the following conditions:
  
  	The above copyright notice and this permission notice shall be included
  	in all copies or substantial portions of the Software.
  
  	THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
  	OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF 
  	MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
  	IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY 
  	CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, 
  	TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE 
  	SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
  	
     -------------------------------------------------------------------------- */
     
  #include "alpha.h"
  #include <algorithm>
  
  namespace squish {
  
  static int FloatToInt( float a, int limit )
  {
  	// use ANSI round-to-zero behaviour to get round-to-nearest
  	int i = ( int )( a + 0.5f );
  
  	// clamp to the limit
  	if( i < 0 )
  		i = 0;
  	else if( i > limit )
  		i = limit; 
  
  	// done
  	return i;
  }
  
  void CompressAlphaDxt3( u8 const* rgba, int mask, void* block )
  {
  	u8* bytes = reinterpret_cast< u8* >( block );
  	
  	// quantise and pack the alpha values pairwise
  	for( int i = 0; i < 8; ++i )
  	{
  		// quantise down to 4 bits
  		float alpha1 = ( float )rgba[8*i + 3] * ( 15.0f/255.0f );
  		float alpha2 = ( float )rgba[8*i + 7] * ( 15.0f/255.0f );
  		int quant1 = FloatToInt( alpha1, 15 );
  		int quant2 = FloatToInt( alpha2, 15 );
  		
  		// set alpha to zero where masked
  		int bit1 = 1 << ( 2*i );
  		int bit2 = 1 << ( 2*i + 1 );
  		if( ( mask & bit1 ) == 0 )
  			quant1 = 0;
  		if( ( mask & bit2 ) == 0 )
  			quant2 = 0;
  
  		// pack into the byte
  		bytes[i] = ( u8 )( quant1 | ( quant2 << 4 ) );
  	}
  }
  
  void DecompressAlphaDxt3( u8* rgba, void const* block )
  {
  	u8 const* bytes = reinterpret_cast< u8 const* >( block );
  	
  	// unpack the alpha values pairwise
  	for( int i = 0; i < 8; ++i )
  	{
  		// quantise down to 4 bits
  		u8 quant = bytes[i];
  		
  		// unpack the values
  		u8 lo = quant & 0x0f;
  		u8 hi = quant & 0xf0;
  
  		// convert back up to bytes
  		rgba[8*i + 3] = lo | ( lo << 4 );
  		rgba[8*i + 7] = hi | ( hi >> 4 );
  	}
  }
  
  static void FixRange( int& min, int& max, int steps )
  {
  	if( max - min < steps )
  		max = std::min( min + steps, 255 );
  	if( max - min < steps )
  		min = std::max( 0, max - steps );
  }
  
  static int FitCodes( u8 const* rgba, int mask, u8 const* codes, u8* indices )
  {
  	// fit each alpha value to the codebook
  	int err = 0;
  	for( int i = 0; i < 16; ++i )
  	{
  		// check this pixel is valid
  		int bit = 1 << i;
  		if( ( mask & bit ) == 0 )
  		{
  			// use the first code
  			indices[i] = 0;
  			continue;
  		}
  		
  		// find the least error and corresponding index
  		int value = rgba[4*i + 3];
  		int least = 2147483647; //INT_MAX
  		int index = 0;
  		for( int j = 0; j < 8; ++j )
  		{
  			// get the squared error from this code
  			int dist = ( int )value - ( int )codes[j];
  			dist *= dist;
  			
  			// compare with the best so far
  			if( dist < least )
  			{
  				least = dist;
  				index = j;
  			}
  		}
  		
  		// save this index and accumulate the error
  		indices[i] = ( u8 )index;
  		err += least;
  	}
  	
  	// return the total error
  	return err;
  }
  
  static void WriteAlphaBlock( int alpha0, int alpha1, u8 const* indices, void* block )
  {
  	u8* bytes = reinterpret_cast< u8* >( block );
  	
  	// write the first two bytes
  	bytes[0] = ( u8 )alpha0;
  	bytes[1] = ( u8 )alpha1;
  	
  	// pack the indices with 3 bits each
  	u8* dest = bytes + 2;
  	u8 const* src = indices;
  	for( int i = 0; i < 2; ++i )
  	{
  		// pack 8 3-bit values
  		int value = 0;
  		for( int j = 0; j < 8; ++j )
  		{
  			int index = *src++;
  			value |= ( index << 3*j );
  		}
  			
  		// store in 3 bytes
  		for( int j = 0; j < 3; ++j )
  		{
  			int byte = ( value >> 8*j ) & 0xff;
  			*dest++ = ( u8 )byte;
  		}
  	}
  }
  
  static void WriteAlphaBlock5( int alpha0, int alpha1, u8 const* indices, void* block )
  {
  	// check the relative values of the endpoints
  	if( alpha0 > alpha1 )
  	{
  		// swap the indices
  		u8 swapped[16];
  		for( int i = 0; i < 16; ++i )
  		{
  			u8 index = indices[i];
  			if( index == 0 )
  				swapped[i] = 1;
  			else if( index == 1 )
  				swapped[i] = 0;
  			else if( index <= 5 )
  				swapped[i] = 7 - index;
  			else 
  				swapped[i] = index;
  		}
  		
  		// write the block
  		WriteAlphaBlock( alpha1, alpha0, swapped, block );
  	}
  	else
  	{
  		// write the block
  		WriteAlphaBlock( alpha0, alpha1, indices, block );
  	}	
  }
  
  static void WriteAlphaBlock7( int alpha0, int alpha1, u8 const* indices, void* block )
  {
  	// check the relative values of the endpoints
  	if( alpha0 < alpha1 )
  	{
  		// swap the indices
  		u8 swapped[16];
  		for( int i = 0; i < 16; ++i )
  		{
  			u8 index = indices[i];
  			if( index == 0 )
  				swapped[i] = 1;
  			else if( index == 1 )
  				swapped[i] = 0;
  			else
  				swapped[i] = 9 - index;
  		}
  		
  		// write the block
  		WriteAlphaBlock( alpha1, alpha0, swapped, block );
  	}
  	else
  	{
  		// write the block
  		WriteAlphaBlock( alpha0, alpha1, indices, block );
  	}	
  }
  
  void CompressAlphaDxt5( u8 const* rgba, int mask, void* block )
  {
  	// get the range for 5-alpha and 7-alpha interpolation
  	int min5 = 255;
  	int max5 = 0;
  	int min7 = 255;
  	int max7 = 0;
  	for( int i = 0; i < 16; ++i )
  	{
  		// check this pixel is valid
  		int bit = 1 << i;
  		if( ( mask & bit ) == 0 )
  			continue;
  
  		// incorporate into the min/max
  		int value = rgba[4*i + 3];
  		if( value < min7 )
  			min7 = value;
  		if( value > max7 )
  			max7 = value;
  		if( value != 0 && value < min5 )
  			min5 = value;
  		if( value != 255 && value > max5 )
  			max5 = value;
  	}
  	
  	// handle the case that no valid range was found
  	if( min5 > max5 )
  		min5 = max5;
  	if( min7 > max7 )
  		min7 = max7;
  		
  	// fix the range to be the minimum in each case
  	FixRange( min5, max5, 5 );
  	FixRange( min7, max7, 7 );
  	
  	// set up the 5-alpha code book
  	u8 codes5[8];
  	codes5[0] = ( u8 )min5;
  	codes5[1] = ( u8 )max5;
  	for( int i = 1; i < 5; ++i )
  		codes5[1 + i] = ( u8 )( ( ( 5 - i )*min5 + i*max5 )/5 );
  	codes5[6] = 0;
  	codes5[7] = 255;
  	
  	// set up the 7-alpha code book
  	u8 codes7[8];
  	codes7[0] = ( u8 )min7;
  	codes7[1] = ( u8 )max7;
  	for( int i = 1; i < 7; ++i )
  		codes7[1 + i] = ( u8 )( ( ( 7 - i )*min7 + i*max7 )/7 );
  		
  	// fit the data to both code books
  	u8 indices5[16];
  	u8 indices7[16];
  	int err5 = FitCodes( rgba, mask, codes5, indices5 );
  	int err7 = FitCodes( rgba, mask, codes7, indices7 );
  	
  	// save the block with least error
  	if( err5 <= err7 )
  		WriteAlphaBlock5( min5, max5, indices5, block );
  	else
  		WriteAlphaBlock7( min7, max7, indices7, block );
  }
  
  void DecompressAlphaDxt5( u8* rgba, void const* block )
  {
  	// get the two alpha values
  	u8 const* bytes = reinterpret_cast< u8 const* >( block );
  	int alpha0 = bytes[0];
  	int alpha1 = bytes[1];
  	
  	// compare the values to build the codebook
  	u8 codes[8];
  	codes[0] = ( u8 )alpha0;
  	codes[1] = ( u8 )alpha1;
  	if( alpha0 <= alpha1 )
  	{
  		// use 5-alpha codebook
  		for( int i = 1; i < 5; ++i )
  			codes[1 + i] = ( u8 )( ( ( 5 - i )*alpha0 + i*alpha1 )/5 );
  		codes[6] = 0;
  		codes[7] = 255;
  	}
  	else
  	{
  		// use 7-alpha codebook
  		for( int i = 1; i < 7; ++i )
  			codes[1 + i] = ( u8 )( ( ( 7 - i )*alpha0 + i*alpha1 )/7 );
  	}
  	
  	// decode the indices
  	u8 indices[16];
  	u8 const* src = bytes + 2;
  	u8* dest = indices;
  	for( int i = 0; i < 2; ++i )
  	{
  		// grab 3 bytes
  		int value = 0;
  		for( int j = 0; j < 3; ++j )
  		{
  			int byte = *src++;
  			value |= ( byte << 8*j );
  		}
  		
  		// unpack 8 3-bit values from it
  		for( int j = 0; j < 8; ++j )
  		{
  			int index = ( value >> 3*j ) & 0x7;
  			*dest++ = ( u8 )index;
  		}
  	}
  	
  	// write out the indexed codebook values
  	for( int i = 0; i < 16; ++i )
  		rgba[4*i + 3] = codes[indices[i]];
  }
  
  } // namespace squish
  