Looking for a trick: how to get the highest shared folder of two random folder items on a disk?

General
1

Basically the title: I am looking for a trick: how to get the highest shared folder of two random folder items on a disk?

example:

/Users/gijs/Documents/Stuff/test.txt
/Users/gijs/Documents/Music/Albums/song.mp4

This should give me the path: /Users/gijs/Documents
The actual use is not two files but a set off thousands so I am looking for a ‘light’ solution.

Who’s the mastermind?

2

If they are in this format, you could add all the strings to an in-memory database.
Then, with maybe a maximum of 30 passes, something like this:

{pseudocode… dont know if you are using API1 or 2…}


dim  totalrows as integer  = (number of rows in the original list)
dim leftpart as string = "/"
dim shortestpath as string
dim leftchars as integer = 1  
dim selectedrows as integer

selectedrows =  totalrows
shortestpath = {some SQL to get min(thestring)}

//now we have either some random value nothing like the rest,
//or the shortest shared value

//work through the letters until we lose a row..
//this wont take more than 30 passes at a guess

while selectedrows =  totalrows
leftpart = left(shortestpath,leftchars)
selectedrows = select count(1) where thevalue like   leftpart    //use parameterised
leftchars = leftchars+1
wend

//by this point, the last select got less than {total rows}, so we are not sharing 
//any longer
//the shared value is    all but the last character of leftpart
3

Thanks for the effort Jeff! , we could reduce the 30 passes by using fields “/” instead of characters right?

4

very likely.
If you split by / and build up the path that way, probably 2 or 3 passes would get it.

5

OK thanks, will make it work!

6

I think the easiesty way is to realize that the shared parent folder of two folderitems means that their path names (from the left) are identical:

In your example:

/Users/gijs/Documents/Stuff/test.txt
/Users/gijs/Documents/Music/Albums/song.mp4

If we compare from the left, we realize they are identical until this point:

/Users/gijs/Documents/S
/Users/gijs/Documents/M

So I think the algorithm is really simple:

Function CommonAncestor( f1 as folderItem, f2 as folderItem) as folderItem
  dim p1 as string = f1.nativePath
  dim p2 as string = f2.nativePath
  dim n1 as integer = p1.length
  dim n2 as integer = p2.length

  dim i1 as integer = 1 // string.middle is one-based
  dim i2 as integer = 1

  dim commonPath as string = ""
  while true
    if i1 > n1 then
       exit while
    end if
    if i2 > n2 then
       exit while
    end if
   dim c1 as string = p1.middle(i1,1)
   dim c2 as string = p2.middle(i2,1)
  if c1 <> c2 then
     exit while
  end if
  commonPath = commonPath + c1

  wend

  if c1 <> "" then
    return new folderItem(commonPath, folderItem.pathTypeNative)
  else
    return nil
  end if

(I wrote this from memory, may have some syntax errors, but you should be able to figure it out from this outline…)