detab.c

/* 
 * Exercise 5-11. Modify the program and detab (written in exercises in Chapter
 * 1) to accept a list of tab stops as arguments. Use the default tab setting
 * if there are no arguments.
 *
 * By Faisal Saadatmand
 */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define MAXLEN 1000 /* max length of input/output line */
#define MAXTABS 100 /* max number of tab stop positions */
#define N 4 /* default tabstops (every n columns) */

/* functions */
int getLine(char *, int);
int setTabList(int, char **, int *);
int ToTabStop(int, int *, int);
void detab(char *, char *, int *, int);

/* getLine function: read a line into s, return length */
int getLine(char *s, int lim)
{
	int c;
	char *len;

	len = s;
	while (--lim > 0 && (c = getchar()) != EOF && c != '\n')
		*s++ = c;
	if (c == '\n')
		*s++ = c;
	*s = '\0';
	return strlen(len);
}

/* setTabStops: extracts topstops positions from command line arguments and
 * store them in tabList. Return size of tabList, 0 if no tabstop list was
 * provided, or -1 if arg is an invalid tabstop value. */
int setTabList(int count, char **arg, int *tabList)
{
	int i;

	for (i = 0; --count > 0; ++i)
		if (!(tabList[i] = atoi(*++arg)) || tabList[i] < 0) {
			printf("Invalid tabstop: %s\n", *arg);
			return -1; 
		}
	return i; /* i is 0 if no tablist */
}

/* toNextTab: return the number of blanks to the next tab stop position. */
int ToTabStop(int column, int *tabList, int tlSize)
{
	if (!tlSize)
		return N - (column % N);
	/* if list exists, find nearest tab stop position in it */
	while (tlSize != 0 && column >= *tabList) {
		++tabList;
		--tlSize;
	}
	return *tabList - column;
}

/* detab function: replaces tabs with the proper number of blanks */
void detab(char *in, char *out, int *tabList, int tlSize)
{
	int i; /* index for read line */
	int j; /* index for modified (written) line */
	int nblanks; /* number of required blanks */

	for (i = j = 0; in[i] != '\0'; ++i)
		if (in[i] == '\t') {
			nblanks = ToTabStop(j, tabList, tlSize);
			while (nblanks-- > 0)
				out[j++] = ' ';
		} else 
			out[j++] = in[i];
	out[j] = '\0';
}

int main(int argc, char *argv[])
{
	char in[MAXLEN]; /* currently read line */
	char out[MAXLEN]; /* modified line */
	int tabList[MAXTABS]; /* a list of tab stop positions */
	int tlSize; /* size of tablist */

	if ((tlSize = setTabList(argc, argv, tabList)) >= 0)
		while (getLine(in, MAXLEN) > 0) {
			detab(in, out, tabList, tlSize);
			printf("%s", out);
		}
	return 0;
}