如何编写一个小数转换分数的函数？

public function xtof(str as currency, optional fenm as integer = 32) as string

' 只限于整除分数.
  dim cfm as currency
  dim cfmmod as integer
  on error goto erroreof

  cfm = 1 / fenm
  xtof = ""
  if str = 0 then xtof = "": exit function

  dim point as integer
  dim dint as string
  dim dpoint as currency
  dim fint, fint1, fint2 as integer
  if str <> 0 then
        if str > 1 then
            point = instr(1, str, ".", 1)
            if point = 0 then
                xtof = str:
                exit function
            else
                dint = mid(str, 1, point - 1)
                dpoint = ccur("0." & mid(str, point + 1))
                fint = instr(1, xtof(dpoint), "/", 1)
                fint1 = cint(mid(xtof(dpoint), 1, fint - 1))
                fint2 = cint(mid(xtof(dpoint), fint + 1))

                xtof = cstr(dint * fint2 + fint1) & "/" & cstr(fint2)

            end if
        else
          if fenm mod cint(str / cfm) = 0 then
            xtof = "1/" + cstr(fenm / cint(str / cfm))
          else
              cfmmod = maxgys(fenm, cint(str / cfm))
            xtof = cstr(cint(str / cfm / cfmmod)) + "/" + cstr(cint(fenm / cfmmod))
          end if
        end if
  else
      xtof = "0"
  end if
  exit function
erroreof:
  xtof = ""
end function
function maxgys(num1 as integer, num2 as integer) as integer
    dim minnum, i as integer
    minnum = num1
    if num1 > num2 then minnum = num2
    for i = 1 to minnum
    if ((num1 mod i) = 0) and ((num2 mod i) = 0) then maxgys = i

    next i
end function