Projects/Magstripe/CircularBitBuffer.c

   1 /*
   2              LUFA Library
   3      Copyright (C) Dean Camera, 2009.
   4
   5   dean [at] fourwalledcubicle [dot] com
   6       www.fourwalledcubicle.com
   7 */
   8
   9 /*
  10   Copyright 2009  Denver Gingerich (denver [at] ossguy [dot] com)
  11   Copyright 2009  Dean Camera (dean [at] fourwalledcubicle [dot] com)
  12
  13   Permission to use, copy, modify, and distribute this software
  14   and its documentation for any purpose and without fee is hereby
  15   granted, provided that the above copyright notice appear in all
  16   copies and that both that the copyright notice and this
  17   permission notice and warranty disclaimer appear in supporting
  18   documentation, and that the name of the author not be used in
  19   advertising or publicity pertaining to distribution of the
  20   software without specific, written prior permission.
  21
  22   The author disclaim all warranties with regard to this
  23   software, including all implied warranties of merchantability
  24   and fitness.  In no event shall the author be liable for any
  25   special, indirect or consequential damages or any damages
  26   whatsoever resulting from loss of use, data or profits, whether
  27   in an action of contract, negligence or other tortious action,
  28   arising out of or in connection with the use or performance of
  29   this software.
  30 */
  31
  32 /** Circular bit buffer library. This will allow for individual bits
  33  *  to be stored in packed form inside circular buffers, to reduce
  34  *  overall RAM usage.
  35  */
  36
  37 #include "CircularBitBuffer.h"
  38
  39 /** Function to initialize or reset a bit buffer, ready for data to be stored into it. */
  40 void BitBuffer_Init(BitBuffer_t* Buffer)
  41 {
  42         /* Reset the number of stored bits in the buffer */
  43         Buffer->Elements        = 0;
  44
  45         /* Reset the data in and out pointer structures in the buffer to the first buffer bit */
  46         Buffer->In.CurrentByte  = Buffer->Data;
  47         Buffer->In.ByteMask     = (1 << 0);
  48         Buffer->Out.CurrentByte = Buffer->Data;
  49         Buffer->Out.ByteMask    = (1 << 0);
  50 }
  51
  52 /** Function to store the given bit into the given bit buffer. */
  53 void BitBuffer_StoreNextBit(BitBuffer_t* Buffer, bool Bit)
  54 {
  55         /* If the bit to store is true, set the next bit in the buffer */
  56         if (Bit)
  57           *Buffer->In.CurrentByte |= Buffer->In.ByteMask;
  58
  59         /* Increment the number of stored bits in the buffer counter */
  60         Buffer->Elements++;
  61
  62         /* Check if the current buffer byte is full of stored bits */
  63         if (Buffer->In.ByteMask == (1 << 7))
  64         {
  65                 /* Check if the end of the buffer has been reached, if so reset to start of buffer, otherwise advance to next bit */
  66                 if (Buffer->In.CurrentByte != &Buffer->Data[sizeof(Buffer->Data) - 1])
  67                   Buffer->In.CurrentByte++;
  68                 else
  69                   Buffer->In.CurrentByte = Buffer->Data;
  70
  71                 /* Reset the storage bit mask in the current buffer byte to the first bit */
  72                 Buffer->In.ByteMask = (1 << 0);
  73         }
  74         else
  75         {
  76                 /* Shift the current storage bit mask to the next bit in the current byte */
  77                 Buffer->In.ByteMask <<= 1;
  78         }
  79 }
  80
  81 /** Function to retrieve the next bit stored in the given bit buffer. */
  82 bool BitBuffer_GetNextBit(BitBuffer_t* Buffer)
  83 {
  84         /* Retrieve the value of the next bit stored in the buffer */
  85         bool Bit = ((*Buffer->Out.CurrentByte & Buffer->Out.ByteMask) != 0);
  86
  87         /* Clear the buffer bit */
  88         *Buffer->Out.CurrentByte &= ~Buffer->Out.ByteMask;
  89
  90         /* Decrement the number of stored bits in the buffer counter */
  91         Buffer->Elements--;
  92
  93         /* Check if the current buffer byte is empty of stored bits */
  94         if (Buffer->Out.ByteMask == (1 << 7))
  95         {
  96                 /* Check if the end of the buffer has been reached, if so reset to start of buffer, otherwise advance to next bit */
  97                 if (Buffer->Out.CurrentByte != &Buffer->Data[sizeof(Buffer->Data) - 1])
  98                   Buffer->Out.CurrentByte++;
  99                 else
 100                   Buffer->Out.CurrentByte = Buffer->Data;
 101
 102                 /* Reset the retrieval bit mask in the current buffer byte to the first bit */
 103                 Buffer->Out.ByteMask = (1 << 0);
 104         }
 105         else
 106         {
 107                 /* Shift the current retrieval bit mask to the next bit in the current byte */
 108                 Buffer->Out.ByteMask <<= 1;
 109         }
 110
 111         /* Return the retrieved bit from the buffer */
 112         return Bit;
 113 }