Develop a more efficient ordered-union function in VBA - excel

Question

I would like to make a user-defined function union in VBA, such that:
it could take variable parameters
each parameter is a one-column range like A1, A2:A10; we don't need to consider passing constant values to parameters
we could consider, within one input range, there are no duplicates; but it is very possible to have duplicates among input ranges.
union combines the input ranges, and keeps the order of the elements. For instance, =union(A1:A5, C1:C2, E1:E3) has the following expected output in Column I:
I wrote the following code which works. However, the problem is that it is slow. A union over a list of 4000 rows and a list of 20 rows takes already several seconds. First, I don't know the way I coded arrays could be improved. Second, the algorithm just consists in comparing each new element against the accumulating result list; there is no sort, no other techniques. Third, I don't know if there are any existing functions we could use in other objects of VBA (eg, VBA FILTER function, Collection, ArrayLists, Scripting.Dictionary).
Could anyone propose a more efficient code?
Function getDimension(var As Variant) As Long
On Error GoTo Err
Dim i As Long
Dim tmp As Long
i = 0
Do While True
i = i + 1
tmp = UBound(var, i)
Loop
Err:
getDimension = i - 1
End Function
Function exists(v As Variant, arr As Variant, resCount As Long) As Boolean
If resCount = 0 Then
exists = False
Else
exists = False
i = LBound(arr, 1)
Do While (i <= resCount) And (Not exists)
If arr(i) = v Then
exists = True
End If
i = i + 1
Loop
End If
End Function
' assumption: every input is a range (eg, A1, A1:A2)
' assumption: each input range has only one column
Function union(ParamArray arr() As Variant) As Variant
Dim res As Variant
ReDim res(1 To 100000)
Dim resCount As Long
resCount = 0
For k = LBound(arr) To UBound(arr)
Dim arrk As Variant
Dim v
arrk = arr(k).Value2
If getDimension(arrk) = 0 Then 'case of A1, B1
v = arrk
If Not exists(v, res, resCount) Then
resCount = resCount + 1
res(resCount) = v
End If
ElseIf getDimension(arrk) = 2 Then 'case of A1:A10, B1:B10
For i = LBound(arrk, 1) To UBound(arrk, 1)
v = arrk(i, 1)
If Not exists(v, res, resCount) Then
resCount = resCount + 1
res(resCount) = v
End If
Next i
End If
Next k
ReDim Preserve res(1 To resCount)
union = Application.WorksheetFunction.Transpose(res)
End Function

Answer 1

Something like this should work, using a Dictionary to eliminate duplicates.
Function UniqueValues(ParamArray arr() As Variant)
Dim r, c As Range, v, dict
Set dict = CreateObject("scripting.dictionary")
For Each r In arr
For Each c In r.Cells
v = c.Value
If Len(v) > 0 Then dict(v) = 1
Next c
Next r
UniqueValues = ToColumn(dict.keys)
End Function
Function ToColumn(arr)
Dim arrOut, i As Long
ReDim arrOut(1 To UBound(arr) + 1, 1 To 1)
For i = 1 To UBound(arr) + 1
arrOut(i, 1) = arr(i - 1)
Next i
ToColumn= arrOut
End Function