solutions/The C Programming Language/1.21.c

#include <stdio.h>

// replacing empty strings consisting of only spaces
// to tabs and spaces so it keeps initial indentation

int _getline(char s[], int lim)
{
	int c, i, j;
	for (i = 0; (c = getchar()) != '\n'; ++i)
	{
		if (i < lim - 1)
			s[i] = c;
	}

	j = i;
	if (j > lim - 1)
		j = lim - 1;
	s[j] = '\0';
	return j - 1;
}

void entab(char to[], char from[], int columns)
{
	int spaces_counter = 0;
	int from_i = 0;
	int to_i = 0;

	while (from[from_i] != '\0')
	{
		++spaces_counter;
		++from_i;
		if (spaces_counter == columns)
		{
			to[to_i] = '\t';
			++to_i;
			spaces_counter = 0;
		}
	}

	while (spaces_counter > 0)
	{
		to[to_i] = '_';
		++to_i;
		--spaces_counter;
	}

	to[to_i] = '\0';
}

main()
{
	int size = 500;
	char from[size];
	char to[size];
	while (1)
	{
		_getline(from, size);
		entab(to, from, 4);
		printf("%s\n", to);
	}
}