こんなんでどう？read_bits, write_bitsは適当に作って。
long read_bits(size_t nbits);
int write_bits(unsigned long bitvector, size_t nbits);
int flush_bits(void)
int sss_read(int start, int step, int stop)
{
　int bits = start;
　int base = 0;
　int code;
　while (bits < stop && (code = read_bits(1)) != EOF && code == 0) {
　　base += (1 << bits);
　　bits += step;
　}
　if (code == EOF || (code = read_bits(bits)) == EOF) return EOF;
　return code + base;
}
int sss_write(int start, int step, int stop, int code)
{
　int bits = start;
　int base = 0;
　int limit = (1 << start);
　while (bits < stop && code >= limit) {
　　write_bits(0, 1);
　　base = limit;
　　bits += step;
　　limit += (1 << bits);
　}
　if (bits < stop) write_bits(1, 1);
　return write_bits(code - base, bits);
}