/*  Only works with LF, not CRLF or CR
 *  Characters are presumed to be 7-bits.
 *  Tabs only count as one character.
 */

#include <stdio.h>
#include <ctype.h>  /* isspace() */

#define COLUMNS 10

int main()
{
    FILE *infile, *outfile;
    long last_white_in = 0;
    int  in_char, line_length, word_length;

    /* Open files... */
    if ((infile = fopen("infile.txt", "rb")) != NULL) {
        puts("Successfully opened input file.");
    } else {
        puts("Unable to open input file.");
        return 1;
    }

    if ((outfile = fopen("outfile.txt", "wb")) != NULL) {
        puts("Successfully opened output file.");
    } else {
        puts("Unable to open output file.");
        return 2;
    }

    /* Iterate over files while inserting newlines */
    for (in_char = fgetc(infile), line_length = 0;
        !feof(infile);
        in_char = fgetc(infile))
    {

        /* Input at end of line ? */
        if (in_char == '\n') {
            fputc('\n', outfile);
            line_length = 0;
            continue;
        }

        /* Wrap long output line ? */
        if (++line_length > COLUMNS) {
            word_length = ftell(infile) - last_white_in;

            /* No whitespace to break on ? */
            if (COLUMNS < word_length) {
                fputc('\n', outfile);
                fseek(infile, -1, SEEK_CUR);

            /* Whitespace to break on ? */
            } else {
                fseek(infile, word_length*-1, SEEK_CUR);
                fseek(outfile, word_length*-1, SEEK_CUR);
                fputc('\n', outfile);
            }

            last_white_in = ftell(infile);
            line_length = 0;
            continue;
        }

        /* Mark last occurance of whitespace */
        if (isspace(in_char)) last_white_in = ftell(infile);

        /* Copy input character to output */
        fputc(in_char, outfile);
    }

    fclose(infile);
    fclose(outfile);
    return 0;
}