#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <Windows.h>

int main( void ){
	// READ IN DATA FROM FILE
	char *buf;
	{
		FILE *file = fopen( "in.bmp", "rb" );
		fseek( file, 0, SEEK_END );
		long size = ftell( file );
		rewind( file );
		buf = ( char * )malloc( size );
		fread( buf, 1, size, file );
		fclose( file );
	}

	//DISPLAY HEADER INFO
	BITMAPFILEHEADER *fh;
	BITMAPINFOHEADER *ih;
	{
		fh = ( BITMAPFILEHEADER * )buf;
		ih = (BITMAPINFOHEADER *)(fh + 1);

		char *type = (char *)&fh->bfType;
		if( strncmp( type, "BM", 2 ) )
			exit( 1 );

		printf("type: %.2s\n", type);
		printf("size: %u\n", fh->bfSize);
		printf("offbits: %u\n", fh->bfOffBits);
		printf("BITMAPINFOHEADER: %d\n", sizeof(BITMAPINFOHEADER));
		printf("BITMAPV4HEADER: %d\n", sizeof(BITMAPV4HEADER));
		printf("BITMAPV5HEADER: %d\n", sizeof(BITMAPV5HEADER));
		printf("size: %u\n", ih->biSize);
		printf("width: %u\n", ih->biWidth);
		printf("height: %u\n", ih->biHeight);
		printf("planes: %hu\n", ih->biPlanes);
		printf("bitcount: %hu\n", ih->biBitCount);
		printf("compression: %u\n", ih->biCompression);
		printf("sizeimage: %u\n", ih->biSizeImage);
		printf("xpelspermeter: %u\n", ih->biXPelsPerMeter);
		printf("ypelspermeter: %u\n", ih->biYPelsPerMeter);
		printf("clrused: %u\n", ih->biClrUsed);
		printf("clrimportant: %u\n", ih->biClrImportant);
	}

	do{
		//SANITY CHECKS (these should pass the default output of BMP3 from ImageMagick )
		if( ih->biSize != sizeof(BITMAPINFOHEADER) )
			break;
		if( ih->biBitCount != 24 )
			break;
		if( fh->bfOffBits + ih->biSizeImage != fh->bfSize )
			break;
		if( ih->biCompression )
			break;
		if( ih->biWidth * ih->biHeight * 3 != ih->biSizeImage )
			break;

		//CREATE NEW FILE
		FILE *file = fopen("out.bmp", "wb");

		//EDIT HEADERS
		fh->bfSize = sizeof(BITMAPFILEHEADER) + sizeof(BITMAPINFOHEADER) + ih->biWidth * ih->biHeight * 4;
		ih->biBitCount = 32;
		ih->biSizeImage = ih->biWidth * ih->biHeight * 4;

		//WRITE HEADERS
		fwrite(fh, sizeof(BITMAPFILEHEADER), 1, file);
		fwrite(ih, sizeof(BITMAPINFOHEADER), 1, file);

		FILE *rgbafile = fopen( "in.rgba", "rb" );
		fseek( rgbafile, 0, SEEK_END );
		long rgbasize = ftell( rgbafile );
		if( rgbasize != ih->biWidth * ih->biHeight * 4 )
			break;
		rewind( rgbafile );
		char *rgbabuf = ( char * )malloc( ih->biWidth * ih->biHeight * 4 );
		fread( rgbabuf, 1, ih->biWidth * ih->biHeight * 4, rgbafile );
		fclose( rgbafile );

		char *new_rgbabuf = ( char * )malloc( ih->biWidth * ih->biHeight * 4 );
		for( int i = 0; i != ih->biHeight; i++ )
			for( int j = 0; j != ih->biWidth; j++ ){
				char *a = &new_rgbabuf[ 4 * (ih->biWidth*i + j) ];
				char *b = &rgbabuf[ 4 * ( ih->biWidth*(ih->biHeight-i-1)+j ) ];
				a[ 0 ] = b[ 2 ];
				a[ 1 ] = b[ 1 ];
				a[ 2 ] = b[ 0 ];
				a[ 3 ] = b[ 3 ];
			}
		fwrite( new_rgbabuf, 1, ih->biWidth * ih->biHeight * 4, file );
		free( new_rgbabuf );

		free( rgbabuf );

		fclose( file );
		printf( "Wrote to out.bmp.\n" );
	}while( 0 );

	free( buf );
	system( "pause" );
	return 0;
}