Exercise 1-21

Exercise 1-21  Write a program entab that replaces strings of blanks by the minimum number of tabs and blanks to achieve the same spacing.  Use the same tab stops as for detab.  When either a tab or a single blank would suffice to reach a tab stop, which should be given preference?

I guess the answer to the latter part of the question is that I would work out what my priority is: compatibility of file format, need for WYSIWYG printing, performance in printing, performance in displaying on screen, etc.  I would then make the decision accordingly.

#include <stdio.h>

#define TABSTOP 8

int column = 0;

static unsigned int blanks = 0;

void pushblank(void)

{

  blanks++;

}

void popblanks(void)

{

  unsigned int desired_column = column+blanks;

  unsigned int desired_tabspace = desired_column / TABSTOP;

  unsigned int present_tabspace = column / TABSTOP;

  while (desired_tabspace > present_tabspace)

  {

    unsigned int blanks_consumed = ((present_tabspace+1) * TABSTOP) - column;

    putchar('\t');

    column+= blanks_consumed;

    blanks-= blanks_consumed;

    present_tabspace++;

  }

  while (blanks)

  {

    putchar(' ');

    column++;

    blanks--;

  }

}

int main(void)

{

  int c;

  while (c=getchar(), c!=EOF)

    switch(c)

    {

    case ' ':

      pushblank();

      break;

    /* Should have cases for \r, \b and a few other things here too. */

    case '\n':

      column = 0;

      putchar(c);

      break;

    default:

      popblanks();

      putchar(c);

      column++;

      break;

    }

  popblanks();

  return 0;

}