/* bitstreams.cpp
   A demonstration of writing individual bits to a file,
   and reading individual bits from a file.

   S. Tanimoto, Oct. 18, 1999.
   */
 
#include <stdio.h>

/* These static ints are used to buffer the input and output
   and to keep a count of the number of bits read and written. */

static int currentByte = 0;
static int numBitsUsedInByte = 0;
static int totalNumBitsWritten = 0;
static int totalNumBitsRead = 0;

/* Take a string like "00101" and write it to the output file.
   Since we can only really write 8 bits at a time, we have
   to buffer up the bits until 8 bits are ready, and then
   write out that byte.
   Any character other than 1 or NULL is written out as 0.
   */

void writeBits(char *bitString, FILE *filePointer) {
  while (*bitString != NULL) {
    char thisDigit = *bitString++;
    currentByte <<= 1; /* shift left one place */
    if (thisDigit == '1') currentByte++; /* Insert new bit */
    numBitsUsedInByte++;
    totalNumBitsWritten++;
    /* If we have 8 good bits, write them out */
    if (numBitsUsedInByte == 8) {
      putc((char)currentByte, filePointer);
      currentByte = 0;
      numBitsUsedInByte = 0;
    }
  }
}

/* print the remaining bits in a full byte by themselves. */
void flushBits(FILE *filePointer) {
  currentByte <<= (8 - numBitsUsedInByte);
  putc((char)currentByte, filePointer);
  currentByte = 0;
  numBitsUsedInByte = 0;
}

/* Gets the next bit from the input stream and
   returns it as an int.  If it has to read
   a new byte, but cannot because end-of-file
   is reached, it returns -1.
   */
int readOneBit(FILE *filePointer) {
  int temp = 0;
  int bit = 0;
  if (numBitsUsedInByte == 0) {
    currentByte = getc(filePointer);
    if (currentByte == EOF) return -1;
  }
  temp = currentByte & 0x80; /* access bit 7 */
  currentByte &= 0x7F; /* mask off bit 7 */
  currentByte <<= 1;   /* shif remaining bits left */
  numBitsUsedInByte++;
  numBitsUsedInByte = numBitsUsedInByte % 8;
  totalNumBitsRead++;
  if (temp > 0) bit = 1;
  return bit;
}

/* Write out some bits to one file.
   Then read in some bits from another file. */
int main() {
  FILE *infile, *outfile;
  int bit;
  outfile = fopen("outBits.dat", "w");
  writeBits("1010101010101", outfile);  
  writeBits("111", outfile);
  writeBits("001", outfile);
  flushBits(outfile);
  fclose(outfile);
  printf("Total number of bits written to the stream is %d.\n",
          totalNumBitsWritten);

  /* Now let's read the bits back in. */
  infile = fopen("inBits.dat", "rb");
  if (infile == NULL) {
    printf("Could not open the file for reading.\n");
    exit (0);
  }
  printf("Here are the bits read back in: \n");
  while ( (bit = readOneBit(infile)) != -1) {
    printf("%d", bit);
  }
  fclose(infile);
  printf("\n");
  printf("Total number of bits read from the stream is %d.\n",
          totalNumBitsRead);
  exit (0);
}