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/* $Id: bit_pack.c,v 1.3 2002/10/03 14:10:41 flaterco Exp $ */
#include <math.h>
#include <stdio.h>
#include "nvtypes.h"
static NV_U_BYTE mask[8] = {0x00, 0x80, 0xc0, 0xe0, 0xf0, 0xf8, 0xfc,
0xfe}, notmask[8] = {0xff, 0x7f, 0x3f, 0x1f, 0x0f,
0x07, 0x03, 0x01};
/*****************************************************************************\
DISTRIBUTION STATEMENT
This source file is unclassified, distribution unlimited, public
domain. It is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
\*****************************************************************************/
/***************************************************************************\
* *
* Function calculate_bits - Computes the number of bits needed for *
* a specified integer range. *
* buffer. *
* *
* Synopsis calculate_bits (range); *
* *
* NV_U_INT32 range integer data range *
* *
* Returns NV_U_INT32 number of bits needed to store *
* data in this range *
* *
* Author Jan C. Depner *
* *
\***************************************************************************/
NV_U_INT32 calculate_bits (NV_U_INT32 range)
{
NV_FLOAT64 result;
NV_U_INT32 bits;
result = (log10 ((NV_FLOAT64) (range)) / log10 (2.0));
bits = (NV_U_INT32)result;
if (fmod (result, 1.0) != 0.0) bits++;
return (bits);
}
/***************************************************************************\
* *
* Function bit_pack - Packs a long value into consecutive bits in *
* buffer. *
* *
* Synopsis bit_pack (buffer, start, numbits, value); *
* *
* NV_U_BYTE buffer[] address of buffer to use *
* NV_U_INT32 start start bit position in buffer *
* NV_U_INT32 numbits number of bits to store *
* NV_INT32 value value to store *
* *
* Description Packs the value 'value' into 'numbits' bits in 'buffer' *
* starting at bit position 'start'. The majority of *
* this code is based on Appendix C of Naval Ocean *
* Research and Development Activity Report #236, 'Data *
* Base Structure to Support the Production of the Digital *
* Bathymetric Data Base', Nov. 1989, James E. Braud, *
* John L. Breckenridge, James E. Current, Jerry L. *
* Landrum. *
* *
* Returns void *
* *
* Author Jan C. Depner *
* *
\***************************************************************************/
void bit_pack (NV_U_BYTE buffer[], NV_U_INT32 start, NV_U_INT32 numbits,
NV_INT32 value)
{
NV_INT32 start_byte, end_byte, start_bit, end_bit, i;
i = start + numbits;
/* Right shift the start and end by 3 bits, this is the same as */
/* dividing by 8 but is faster. This is computing the start and end */
/* bytes for the field. */
start_byte = start >> 3;
end_byte = i >> 3;
/* AND the start and end bit positions with 7, this is the same as */
/* doing a mod with 8 but is faster. Here we are computing the start */
/* and end bits within the start and end bytes for the field. */
start_bit = start & 7;
end_bit = i & 7;
/* Compute the number of bytes covered. */
i = end_byte - start_byte - 1;
/* If the value is to be stored in one byte, store it. */
if (start_byte == end_byte)
{
/* Rather tricky. We are masking out anything prior to the start */
/* bit and after the end bit in order to not corrupt data that has */
/* already been stored there. */
buffer[start_byte] &= mask[start_bit] | notmask[end_bit];
/* Now we mask out anything in the value that is prior to the */
/* start bit and after the end bit. This is, of course, after we */
/* have shifted the value left past the end bit. */
buffer[start_byte] |= (value << (8 - end_bit)) &
(notmask[start_bit] & mask[end_bit]);
}
/* If the value covers more than 1 byte, store it. */
else
{
/* Here we mask out data prior to the start bit of the first byte. */
buffer[start_byte] &= mask[start_bit];
/* Get the upper bits of the value and mask out anything prior to */
/* the start bit. As an example of what's happening here, if we */
/* wanted to store a 14 bit field and the start bit for the first */
/* byte is 3, we would be storing the upper 5 bits of the value in */
/* the first byte. */
buffer[start_byte++] |= (value >> (numbits - (8 - start_bit))) &
notmask[start_bit];
/* Loop while decrementing the byte counter. */
while (i--)
{
/* Clear the entire byte. */
buffer[start_byte] &= 0;
/* Get the next 8 bits from the value. */
buffer[start_byte++] |= (value >> ((i << 3) + end_bit)) & 255;
}
/* For the last byte we mask out anything after the end bit. */
buffer[start_byte] &= notmask[end_bit];
/* Get the last part of the value and stuff it in the end byte. */
/* The left shift effectively erases anything above 8 - end_bit */
/* bits in the value so that it will fit in the last byte. */
buffer[start_byte] |= (value << (8 - end_bit));
}
}
/***************************************************************************\
* *
* Function bit_unpack - Unpacks a long value from consecutive bits *
* in buffer. *
* *
* Synopsis bit_unpack (buffer, start, numbits); *
* *
* NV_U_BYTE buffer[] address of buffer to use *
* NV_U_INT32 start start bit position in buffer *
* NV_U_INT32 numbits number of bits to retrieve *
* *
* Description Unpacks the value from 'numbits' bits in 'buffer' *
* starting at bit position 'start'. The value is assumed *
* to be unsigned. The majority of this code is based on *
* Appendix C of Naval Ocean Research and Development *
* Activity Report #236, 'Data Base Structure to Support *
* the Production of the Digital Bathymetric Data Base', *
* Nov. 1989, James E. Braud, John L. Breckenridge, James *
* E. Current, Jerry L. Landrum. *
* *
* Returns NV_U_INT32 value retrieved from buffer *
* *
* Author Jan C. Depner *
* *
\***************************************************************************/
NV_U_INT32 bit_unpack (NV_U_BYTE buffer[], NV_U_INT32 start, NV_U_INT32 numbits)
{
NV_INT32 start_byte, end_byte, start_bit, end_bit, i;
NV_U_INT32 value;
i = start + numbits;
/* Right shift the start and end by 3 bits, this is the same as */
/* dividing by 8 but is faster. This is computing the start and end */
/* bytes for the field. */
start_byte = start >> 3;
end_byte = i >> 3;
/* AND the start and end bit positions with 7, this is the same as */
/* doing a mod with 8 but is faster. Here we are computing the start */
/* and end bits within the start and end bytes for the field. */
start_bit = start & 7;
end_bit = i & 7;
/* Compute the number of bytes covered. */
i = end_byte - start_byte - 1;
/* If the value is stored in one byte, retrieve it. */
if (start_byte == end_byte)
{
/* Mask out anything prior to the start bit and after the end bit. */
value = (NV_U_INT32) buffer[start_byte] & (notmask[start_bit] &
mask[end_bit]);
/* Now we shift the value to the right. */
value >>= (8 - end_bit);
}
/* If the value covers more than 1 byte, retrieve it. */
else
{
/* Here we mask out data prior to the start bit of the first byte */
/* and shift to the left the necessary amount. */
value = (NV_U_INT32) (buffer[start_byte++] & notmask[start_bit]) <<
(numbits - (8 - start_bit));
/* Loop while decrementing the byte counter. */
while (i--)
{
/* Get the next 8 bits from the buffer. */
value += (NV_U_INT32) buffer[start_byte++] << ((i << 3) + end_bit);
}
/* For the last byte we mask out anything after the end bit and */
/* then shift to the right (8 - end_bit) bits. */
value += (NV_U_INT32) (buffer[start_byte] & mask[end_bit]) >>
(8 - end_bit);
}
return (value);
}
/***************************************************************************\
* *
* Function signed_bit_unpack - Unpacks a signed long value from *
* consecutive bits in buffer. *
* *
* Synopsis signed_bit_unpack (buffer, start, numbits); *
* *
* NV_U_BYTE buffer[] address of buffer to use *
* NV_U_INT32 start start bit position in buffer *
* NV_U_INT32 numbits number of bits to retrieve *
* *
* Description Unpacks the value from 'numbits' bits in 'buffer' *
* starting at bit position 'start'. The value is assumed *
* to be signed. The majority of this code is based on *
* Appendix C of Naval Ocean Research and Development *
* Activity Report #236, 'Data Base Structure to Support *
* the Production of the Digital Bathymetric Data Base', *
* Nov. 1989, James E. Braud, John L. Breckenridge, James *
* E. Current, Jerry L. Landrum. *
* *
* Returns NV_INT32 value retrieved from buffer *
* *
* Author Jan C. Depner *
* *
\***************************************************************************/
NV_INT32 signed_bit_unpack (NV_U_BYTE buffer[], NV_U_INT32 start,
NV_U_INT32 numbits)
{
static NV_INT32 extend_mask = 0x7fffffff;
NV_INT32 value;
value = bit_unpack (buffer, start, numbits);
if (value & (1 << (numbits - 1))) value |= (extend_mask << numbits);
return (value);
}
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