haha

#include <iostream>

#include <fstream>

using namespace std;

struct bmpheader {
	char Signature[2];
	int FileSize;
	int Reserved;
	int DataOffSet;
};

struct DIB {
	int dibSize;
	int width;
	int height;
	short planes;
	short bpp;
	int compression;
	int imageSize;
	int XpixelsperM;
	int YpixelsperM;
	int colorsUsed;
	int importantColor;
};

struct bmpImage {
	bmpheader header;
	DIB dib;
	char* pDIBReserved;
	char* pImageData;
};

int readBMP(const char* filename, bmpImage& bmp)
{
	ifstream fin(filename, ios::binary);
	if (!fin)
		return 0;
	fin.seekg(0, ios::beg);
	fin.read((char*)&bmp.header, 14);
	fin.read((char*)&bmp.dib, 40);
	if (bmp.dib.dibSize > 40) {
		int d = bmp.dib.dibSize - 40;
		bmp.pDIBReserved = new char[d];
		fin.read(bmp.pDIBReserved, d);
	}
	bmp.pImageData = new char[bmp.dib.imageSize];
	fin.read(bmp.pImageData, bmp.dib.imageSize);
	return 1;
}

int writeBMP(const char* filename, bmpImage bmp)
{
	ofstream fout(filename, ios::out | ios::binary);
	if (!fout)
		return 0;
	fout.write((char*)&bmp.header, 14);
	fout.write((char*)&bmp.dib, 40);
	if (bmp.dib.dibSize > 40) {
		int d = bmp.dib.dibSize - 40;
		bmp.pDIBReserved = new char[d];
		fout.write(bmp.pDIBReserved, d);
	}
	bmp.pImageData = new char[bmp.dib.imageSize];
	fout.write(bmp.pImageData, bmp.dib.imageSize);
	return 1;
}

int ConvertTo8bit(bmpImage srcBMP, bmpImage& dstBMP)
{
	if (srcBMP.dib.bpp != 24 && srcBMP.dib.bpp != 32 && srcBMP.pImageData == NULL)
		return 0;
	dstBMP.header = srcBMP.header;
	dstBMP.dib = srcBMP.dib;
	dstBMP.dib.bpp = 8;
	int width = srcBMP.dib.width;
	int height = srcBMP.dib.height;
	int padding = 0;
	dstBMP.dib.imageSize = width * height * (dstBMP.dib.bpp / 8) + height * padding;
	return 1;
	dstBMP.pImageData = new char[dstBMP.dib.imageSize];
	char* pSrcRow = srcBMP.pImageData;
	char* pDstRow = dstBMP.pImageData;
	int nSrcByteInPix = srcBMP.dib.bpp / 8;
	int nDstByteInPix = dstBMP.dib.bpp / 8;
	int nSrcByteInRow = (width * nSrcByteInPix) + padding;
	int nDstByteInRow = (width * nDstByteInPix) + padding;
	for (int y = 0; y < height; y++) {
		char* pSrcPix = pSrcRow;
		char* pDstPix = pDstRow;
		for (int x = 0; x < width; x++) {
			char A, B, G, R;
			if (srcBMP.dib.bpp == 24) {
				B = pSrcPix[0];
				G = pSrcPix[1];
				R = pSrcPix[2];
			}
			if (srcBMP.dib.bpp == 32) {
				A = pSrcPix[0];
				B = pSrcPix[1];
				G = pSrcPix[2];
				R = pSrcPix[3];
			}
			char avg = (char)((B + G + R) / 3);
			pDstPix[0] = avg;
			pSrcPix += nSrcByteInPix;
			pDstPix += nDstByteInPix;
		}
		pSrcRow += nSrcByteInRow;
		pDstRow += nDstByteInRow;
	}
	return 1;
}

int main()
{
	bmpImage srcImage = { NULL }, dstImage = { NULL };
	readBMP("input.bmp", srcImage);
	ConvertTo8bit(srcImage, dstImage);
	writeBMP("output.bmp", dstImage);
	return 0;
}