K&R (1.21): Replace blanks by tabs

entab.c

#include <stdio.h>
#define TABSTP 4

/* TODO: read this string from argv,
   to test different scenarios */
char* input_str = "Something    to test    it";

int nexttbstp (int position);
int printblank(int start, int end);

int main()
{
    char *ptr = NULL;
    int position = 1, blanknbr=0;
    int c=0;
    
    printf("%s\n", input_str);

    for (ptr=input_str; *ptr != '\0'; ptr++) {
        if (*ptr == ' ') {
            blanknbr=0;
            while (*ptr == ' ') {
                blanknbr++;
                ptr++;
            }
            if (printblank(position, position+blanknbr)) {
                return -1;
            }

            position += blanknbr;
        }
        
        printf("%c", *ptr);
        position++;
    }

    return 0;
}

/* Helper function that determines the next tabstop
   from current position */
int nexttbstp (int position)
{
    int relpos;
    
    if (position <= 0)
        return -1;

    relpos = position % TABSTP;

    return (position + (TABSTP - relpos));
}

int printblank(int start, int end)
{
    int next, length;

    
    if (end < start) {
        printf("Error: end argument inferior to start\n");
        return -1;
    }

    if ((next = nexttbstp (start)) == -1) {
        printf("Error: negative position argument\n");
        return -1;
    }




    /* Recursively insert as many tabs as possible,
       then fill the rest with spaces */
    if (end < next) {
        length = end - start;

        while (length--){
            printf(" ");
        }

        return 0;
    } else {
        printf("\t");
        return printblank(++next, end);
    }
}