This stuff dates way, way before 2004.

For non-power of two, just checked our own very old circular byte buffer library code and using the notation from this article, it is:

  entriesAllocated() { return ((wrPtr-rdPtr+2*bufSize) % (2*bufSize)); }
  remainingSpace() { return bufSize - entriesAllocated(); }
  isEmpty() { return (entriesAllocated()==0); }
  isFull() { return (entriesAllocated()==bufSize); }
  incWr(int n) { wrPtr = (wrPtr+n) % (2*bufSize); }
  incRd(int n) { rdPtr = (rdPtr+n) % (2*bufSize); }