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/*
Copyright (C) 2010 Perens LLC <bruce@perens.com>
This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.
This program 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. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see <http://www.gnu.org/licenses/>.
*/
#include "defines.h"
#include "quantise.h"
#include <stdio.h>
/* Compile-time constants */
/* Size of unsigned char in bits. Assumes 8 bits-per-char. */
static const unsigned int WordSize = 8;
/* Mask to pick the bit component out of bitIndex. */
static const unsigned int IndexMask = 0x7;
/* Used to pick the word component out of bitIndex. */
static const unsigned int ShiftRight = 3;
/** Pack a bit field into a bit string, encoding the field in Gray code.
*
* The output is an array of unsigned char data. The fields are efficiently
* packed into the bit string. The Gray coding is a naive attempt to reduce
* the effect of single-bit errors, we expect to do a better job as the
* codec develops.
*
* This code would be simpler if it just set one bit at a time in the string,
* but would hit the same cache line more often. I'm not sure the complexity
* gains us anything here.
*
* Although field is currently of int type rather than unsigned for
* compatibility with the rest of the code, indices are always expected to
* be >= 0.
*/
void
pack(
unsigned char * bitArray, /* The output bit string. */
unsigned int * bitIndex, /* Index into the string in BITS, not bytes.*/
int field, /* The bit field to be packed. */
unsigned int fieldWidth/* Width of the field in BITS, not bytes. */
)
{
/* Convert the field to Gray code */
field = (field >> 1) ^ field;
do {
unsigned int bI = *bitIndex;
unsigned int bitsLeft = WordSize - (bI & IndexMask);
unsigned int sliceWidth =
bitsLeft < fieldWidth ? bitsLeft : fieldWidth;
unsigned int wordIndex = bI >> ShiftRight;
bitArray[wordIndex] |=
((unsigned char)((field >> (fieldWidth - sliceWidth))
<< (bitsLeft - sliceWidth)));
*bitIndex = bI + sliceWidth;
fieldWidth -= sliceWidth;
} while ( fieldWidth != 0 );
}
/** Unpack a field from a bit string, converting from Gray code to binary.
*
*/
int
unpack(
const unsigned char * bitArray, /* The input bit string. */
unsigned int * bitIndex, /* Index into the string in BITS, not bytes.*/
unsigned int fieldWidth/* Width of the field in BITS, not bytes. */
)
{
unsigned int field = 0;
unsigned int t;
do {
unsigned int bI = *bitIndex;
unsigned int bitsLeft = WordSize - (bI & IndexMask);
unsigned int sliceWidth =
bitsLeft < fieldWidth ? bitsLeft : fieldWidth;
field |= (((bitArray[bI >> ShiftRight] >> (bitsLeft - sliceWidth)) & ((1 << sliceWidth) - 1)) << (fieldWidth - sliceWidth));
*bitIndex = bI + sliceWidth;
fieldWidth -= sliceWidth;
} while ( fieldWidth != 0 );
/* Convert from Gray code to binary. Works for maximum 8-bit fields. */
t = field ^ (field >> 8);
t ^= (t >> 4);
t ^= (t >> 2);
t ^= (t >> 1);
return t;
}
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