Packing 7 bits into 8 ("PUUUSSSSHHH!")

[SMurf]

Hello,

I'm trying to write two small functions that compresses/decompresses 7-bit data that is packed together into 8 bits. These are my alogrithms:-

int compress(unsigned char *in, unsigned char *out)
{
	unsigned char *src;
	unsigned char *dest;
	int lshift, rshift;

	src = in;
	dest = out;
	while (src - in < 17280) //size of imput data
	{
		rshift = 6;
		for (lshift=1;lshift<8;lshift++)
		{
			*dest |= (*(src++) & 0x7F) << lshift;
			*(dest++) |= (*src & 0x7F) >> rshift--;
		}
	}
	return 0;
}

int decompress(unsigned char *in, unsigned char *out)
{
	unsigned char *src;
	unsigned char *dest;
	int lshift, rshift;

	src = in;
	dest = out;
	while (src - in < 15120) //size of imput data
	{
		rshift = 6;
		for (lshift=1;lshift<8;lshift++)
		{
			*(dest++) |= (*src & (0x7F << lshift)) >> lshift;
			*dest |= (*(src++) & (0x7F >> rshift)) << rshift--;
		}
	}
	return 0;
}

I'm not sure this is right though (The decompressed data certainly isnt...) Help?

[SMurf]

I had drawn the process out previously and now how since revised the iterations to look like this:-

(Compress/pack)
mask1 = 0x7F, mask2 = 0x40;
*(dest++) |= ((*(src++) & mask1) << 1) | ((*src & mask2) >> 6);

mask1 = 0x3F, mask2 = 0x60;
*(dest++) |= ((*(src++) & mask1) << 2) | ((*src & mask2) >> 5);

mask1 = 0x1F, mask2 = 0x70;
*(dest++) |= ((*(src++) & mask1) << 3) | ((*src & mask2) >> 4);

mask1 = 0x0F, mask2 = 0x78;
*(dest++) |= ((*(src++) & mask1) << 4) | ((*src & mask2) >> 3);

mask1 = 0x07, mask2 = 0x7C;
*(dest++) |= ((*(src++) & mask1) << 5) | ((*src & mask2) >> 2);

mask1 = 0x03, mask2 = 0x7E;
*(dest++) |= ((*(src++) & mask1) << 6) | ((*src & mask2) >> 1);

mask1 = 0x01, mask2 = 0x7F;
*(dest++) |= ((*(src++) & mask1) << 7) | ((*src & mask2) >> 0);


(Decompress/unpack)
mask1 = 0x00, mask2 = 0xFE;
*(dest++) |= ((*src & mask2) >> 1);

mask1 = 0x01, mask2 = 0xFC;
*(dest++) |= ((*(src++) & mask1) << 6) | ((*src & mask2) >> 2);

mask1 = 0x03, mask2 = 0xF8;
*(dest++) |= ((*(src++) & mask1) << 5) | ((*src & mask2) >> 3);

mask1 = 0x07, mask2 = 0xF0;
*(dest++) |= ((*(src++) & mask1) << 4) | ((*src & mask2) >> 4);

mask1 = 0x0F, mask2 = 0xE0;
*(dest++) |= ((*(src++) & mask1) << 3) | ((*src & mask2) >> 5);

mask1 = 0x1F, mask2 = 0xC0;
*(dest++) |= ((*(src++) & mask1) << 2) | ((*src & mask2) >> 6);

mask1 = 0x3F, mask2 = 0x80;
*(dest++) |= ((*(src++) & mask1) << 1) | ((*src & mask2) >> 7);

mask1 = 0x7F, mask2 = 0x00;
*(dest++) |= ((*(src++) & mask1) << 0) | ((*src & mask2) >> 8);

This in turn led me to revise the functions like so:-

int compress(unsigned char *in, unsigned char *out)
{
	unsigned char *src, *dest, mask1, mask2;
	int lshift, rshift;

	src = in;
	dest = out;
	while (src - in < 17280) //size of imput data
	{
		mask1 = 0x7F;
		mask2 = 0x40;
		rshift = 6;
		for (lshift=1;lshift<8;lshift++)
		{
			*(dest++) |=( (*(src++) & mask1) << lshift) | ((*src & mask2) >> rshift--);
			mask1 >>= 1;
			mask2 += (mask2 >> 1);
		}
	}
	return 0;
}

int decompress(unsigned char *in, unsigned char *out)
{
	unsigned char *src, *dest, mask1, mask2;
	int lshift, rshift;

	src = in;
	dest = out;
	while (src - in < 17280) //size of imput data
	{
		mask1 = 0x00;
		mask2 = 0xFE;
		rshift = 1;
		for (lshift=7;lshift>0;lshift--)
		{
			if (lshift == 7)
				*(dest++) |= ((*src & mask2) >> rshift++);
			else
				*(dest++) |= ((*(src++) & mask1) << lshift) | ((*src & mask2) >> rshift++);

			mask1 <<= 1;
			mask1++;
			mask2 <<= 1;
		}
	}
	return 0;
}

I still don't seem to be any closer to outputting meaningful data.

[swoopy]

Try this.

int compress(unsigned char *in, unsigned char *out)
{
	unsigned char *src, *dest, mask1, mask2;
	int lshift, rshift;

	src = in;
	dest = out;
	while (src - in < 17280) //size of imput data
	{
		mask1 = 0x7F;
		mask2 = 0x40;
		rshift = 6;
		for (lshift=1;lshift<8;lshift++)
		{
			*dest = ( (*(src++) & mask1) << lshift);
			*dest++ |= ((*src & mask2) >> rshift--);
			mask1 >>= 1;
			mask2 |= (mask2 >> 1);
		}
		src++;
	}
	return 0;
}

int decompress(unsigned char *in, unsigned char *out)
{
	unsigned char *src, *dest, mask1, mask2;
	int lshift, rshift;

	src = in;
	dest = out;
	while (src - in < 17280) //size of imput data
	{
		mask1 = 0x00;
		mask2 = 0xFE;
		rshift = 1;
		for (lshift=7;lshift>=0;lshift--)
		{
			if (lshift == 7)
				*dest++ |= ((*src & mask2) >> rshift++);
			else
			{
				*dest = ((*(src++) & mask1) << lshift);
				*dest++ |= ((*src & mask2) >> rshift++);
			}

			mask1 <<= 1;
			mask1++;
			mask2 <<= 1;
		}
	}
	return 0;
}

[sara101]

THIS code is of SMurf & corrected by swoopy

plzzz can some one give me whole program means its main function & also comments on each line which help me in understanding this prog as i cant understand
why
mask1 = 0x7F;
mask2 = 0x40;
rshift = 6;
this take & while (src - in < 17280)this also why only 17280?
& also not understand some function also so plzzzzzz help me in this regard & plzz give me main function with comments if u can
thanks in advance.
its urgent plzz.

Hello,

I'm trying to write two small functions that compresses/decompresses 7-bit data that is packed together into 8 bits. These are my alogrithms:-

insert

Code:

int compress(unsigned char *in, unsigned char *out)
{
	unsigned char *src, *dest, mask1, mask2;
	int lshift, rshift;

	src = in;
	dest = out;
	while (src - in < 17280) //size of imput data
	{
		mask1 = 0x7F;
		mask2 = 0x40;
		rshift = 6;
		for (lshift=1;lshift<8;lshift++)
		{
			*dest = ( (*(src++) & mask1) << lshift);
			*dest++ |= ((*src & mask2) >> rshift--);
			mask1 >>= 1;
			mask2 |= (mask2 >> 1);
		}
		src++;
	}
	return 0;
}

int decompress(unsigned char *in, unsigned char *out)
{
	unsigned char *src, *dest, mask1, mask2;
	int lshift, rshift;

	src = in;
	dest = out;
	while (src - in < 17280) //size of imput data
	{
		mask1 = 0x00;
		mask2 = 0xFE;
		rshift = 1;
		for (lshift=7;lshift>=0;lshift--)
		{
			if (lshift == 7)
				*dest++ |= ((*src & mask2) >> rshift++);
			else
			{
				*dest = ((*(src++) & mask1) << lshift);
				*dest++ |= ((*src & mask2) >> rshift++);
			}

			mask1 <<= 1;
			mask1++;
			mask2 <<= 1;
		}
	}
	return 0;
}

[Salem]

4 years - impressive.
Still closed though.