Convert Compuserve RLE to Portable Bitmap format

rletopbm.c

/* Convert Compuserve RLE images to Portable Bitmap format
 * by William McBrine <wmcbrine@gmail.com>
 * Placed in the public domain
 *
 * Version 2.0 - April 15, 2018
 *               - make P4 type of PBM instead of P1
 *               - only stop parsing on ESC or EOF
 *               - "medium" res are 96 lines
 * Version 1.1 - October 16, 2005
 * Version 1.0 - November 12, 2000
 */

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

int main(int argc, char **argv)
{
    FILE *inp;
    int c, width, pixels, maxwidth, maxlines, maxpix, last, done;
    unsigned char acc;

    if (argc > 1) {
        inp = fopen(argv[1], "rb");
        if (!inp)
            return EXIT_FAILURE;
    } else
        inp = stdin;

    c = fgetc(inp);

    /* Normally first char is ESC, sometimes omitted */
    if (27 == c)
        c = fgetc(inp);

    if ('G' == c)
        c = fgetc(inp);
    else
        return EXIT_FAILURE;

    switch (c) {
    case 'H':           /* High resolution (normal) */
        maxwidth = 256;
        maxlines = 192;
        break;
    case 'M':           /* Medium resolution -- I've never seen
                           one of these, but just in case */
        maxwidth = 128;
        maxlines =  96;
        break;
    default:
        return EXIT_FAILURE;
    }

    maxpix = maxwidth * maxlines;

    width = 0;
    pixels = 0;
    last = 0;
    done = 0;
    acc = 0;

    printf("P4\n%d %d\n", maxwidth, maxlines);

    while (!done && ((c = fgetc(inp)) != EOF))
        if (c >= ' ') {
            c -= ' ';
            last = !last;
            while (c-- && !done) {
                acc |= last;
                width++;
                pixels++;
                if (8 == width) {
                    putchar(acc);
                    acc = 0;
                    width = 0;
                    if (pixels >= maxpix)
                        done = 1;
                }
                else
                    acc <<= 1;
            }
        } else
            if (27 == c)
                done = 1;

    if (width)
        putchar(acc << (7 - width));

    if (argc > 1)
        fclose(inp);

    return EXIT_SUCCESS;
}