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140 lines
4.3 KiB
C++
140 lines
4.3 KiB
C++
/*
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Copyright (C) 2010 Perens LLC <bruce@perens.com>
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All rights reserved.
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This program is free software; you can redistribute it and/or modify
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it under the terms of the GNU Lesser General Public License version 2.1, as
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published by the Free Software Foundation. This program is
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distributed in the hope that it will be useful, but WITHOUT ANY
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WARRANTY; without even the implied warranty of MERCHANTABILITY or
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FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public
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License for more details.
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You should have received a copy of the GNU Lesser General Public License
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along with this program; if not, see <http://www.gnu.org/licenses/>.
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*/
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#include "defines.h"
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#include "quantise.h"
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#include <stdio.h>
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/* Compile-time constants */
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/* Size of unsigned char in bits. Assumes 8 bits-per-char. */
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static const unsigned int WordSize = 8;
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/* Mask to pick the bit component out of bitIndex. */
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static const unsigned int IndexMask = 0x7;
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/* Used to pick the word component out of bitIndex. */
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static const unsigned int ShiftRight = 3;
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/** Pack a bit field into a bit string, encoding the field in Gray code.
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*
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* The output is an array of unsigned char data. The fields are efficiently
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* packed into the bit string. The Gray coding is a naive attempt to reduce
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* the effect of single-bit errors, we expect to do a better job as the
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* codec develops.
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*
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* This code would be simpler if it just set one bit at a time in the string,
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* but would hit the same cache line more often. I'm not sure the complexity
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* gains us anything here.
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*
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* Although field is currently of int type rather than unsigned for
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* compatibility with the rest of the code, indices are always expected to
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* be >= 0.
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*/
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void CQuantize::pack(
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unsigned char *bitArray, /* The output bit string. */
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unsigned int *bitIndex, /* Index into the string in BITS, not bytes.*/
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int field, /* The bit field to be packed. */
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unsigned int fieldWidth /* Width of the field in BITS, not bytes. */
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)
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{
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pack_natural_or_gray(bitArray, bitIndex, field, fieldWidth, 1);
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}
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void CQuantize::pack_natural_or_gray(
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unsigned char *bitArray, /* The output bit string. */
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unsigned int *bitIndex, /* Index into the string in BITS, not bytes.*/
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int field, /* The bit field to be packed. */
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unsigned int fieldWidth, /* Width of the field in BITS, not bytes. */
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unsigned int gray /* non-zero for gray coding */
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)
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{
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if (gray)
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{
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/* Convert the field to Gray code */
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field = (field >> 1) ^ field;
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}
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do
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{
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unsigned int bI = *bitIndex;
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unsigned int bitsLeft = WordSize - (bI & IndexMask);
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unsigned int sliceWidth = bitsLeft < fieldWidth ? bitsLeft : fieldWidth;
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unsigned int wordIndex = bI >> ShiftRight;
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bitArray[wordIndex] |= ((unsigned char)((field >> (fieldWidth - sliceWidth)) << (bitsLeft - sliceWidth)));
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*bitIndex = bI + sliceWidth;
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fieldWidth -= sliceWidth;
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}
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while ( fieldWidth != 0 );
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}
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/** Unpack a field from a bit string, converting from Gray code to binary.
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*
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*/
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int CQuantize::unpack(
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const unsigned char *bitArray, /* The input bit string. */
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unsigned int *bitIndex, /* Index into the string in BITS, not bytes.*/
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unsigned int fieldWidth/* Width of the field in BITS, not bytes. */
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)
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{
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return unpack_natural_or_gray(bitArray, bitIndex, fieldWidth, 1);
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}
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/** Unpack a field from a bit string, to binary, optionally using
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* natural or Gray code.
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*
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*/
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int CQuantize::unpack_natural_or_gray(
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const unsigned char *bitArray, /* The input bit string. */
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unsigned int *bitIndex, /* Index into the string in BITS, not bytes.*/
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unsigned int fieldWidth,/* Width of the field in BITS, not bytes. */
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unsigned int gray /* non-zero for Gray coding */
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)
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{
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unsigned int field = 0;
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unsigned int t;
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do
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{
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unsigned int bI = *bitIndex;
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unsigned int bitsLeft = WordSize - (bI & IndexMask);
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unsigned int sliceWidth = bitsLeft < fieldWidth ? bitsLeft : fieldWidth;
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field |= (((bitArray[bI >> ShiftRight] >> (bitsLeft - sliceWidth)) & ((1 << sliceWidth) - 1)) << (fieldWidth - sliceWidth));
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*bitIndex = bI + sliceWidth;
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fieldWidth -= sliceWidth;
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}
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while ( fieldWidth != 0 );
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if (gray)
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{
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/* Convert from Gray code to binary. Works for maximum 8-bit fields. */
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t = field ^ (field >> 8);
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t ^= (t >> 4);
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t ^= (t >> 2);
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t ^= (t >> 1);
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}
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else
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{
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t = field;
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}
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return t;
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}
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