solutions/The C Programming Language/1.20.c

#include <stdio.h>

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 is_placing_spaces = 0;
	int remaining_spaces = columns;
	int i_to = 0;
	int i_from = 0;

	while (from[i_from] != '\0')
	{
		if (from[i_from] == '\t')
			is_placing_spaces = 1;
		
		if (is_placing_spaces)
		{
			if (remaining_spaces == 0)
			{
				is_placing_spaces = 0;
				remaining_spaces = columns + 1;
				++i_from;
			}
			else
			{
				to[i_to] = '_';
				++i_to;
			}
		}
		else
		{
			if (remaining_spaces == 0)
				remaining_spaces = columns;

			to[i_to] = from[i_from];
			++i_to;
			++i_from;
		}

		--remaining_spaces;
	}

	to[i_to] = '\0';
}

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