code: private void fvswap(int[] fmap, int yyp1, int yyp2, int yyn) {
          while (yyn > 0) {
              fswap(fmap, yyp1, yyp2);
              yyp1++; yyp2++; yyn--;
          }
      }
docs: swaps two intervals starting at yyp1 and yyp2 of length yyn inside fmap.