Author: Tomas Rutkauskas

How to do a binary search on an alphasorted TListView

Answer:

If you want to use a fast searching algorithm (binary search for example) the 
listview has to be sorted on the column you do the duplicate check on. If it is not 
sorted you have to use the listviews FindCaption or FindData method, which does a 
linear search. To sort a listview use its AlphaSort or CustomSort methods.

A binary search on a alphasorted listview for a caption would be something like 
this (untested!):

1   {
2   Function ListviewBinarySearch
3   
4   Parameters:
5   
6   listview:
7   listview to search, assumed to be sorted, must be <> nil.
8   
9   Item:
10  item caption to search for, cannot be empty
11  
12  index:
13  returns the index of the found item, or the index where the item should be inserted 
14  if it is not already in the list. Returns True if there is an item with the passed 
15  caption in the list, false otherwise.
16  
17  Description:
18  Uses a binary search and assumes that the listview is sorted ascending on the 
19  caption of the listitems. The search is case-sensitive, like the default alpha-sort 
20  routine used by the TListview class.
21  
22  Note:
23  We use the lstrcmp function for string comparison since it is the function used by 
24  the default alpha sort routine. If the listview is sorted by another means (e.g. 
25  OnCompare event) this needs to be changed, the comparison method used must always 
26  be the same one used to sort the listview, or the search will not work!
27  
28  Error Conditions: none
29  Created: 31.10.99 by P. Below
30  }
31  
32  function ListviewBinarySearch(listview: TListview; const Item: string; var index:
33    Integer): Boolean;
34  var
35    first, last, pivot, res: Integer;
36  begin
37    Assert(Assigned(listview));
38    Assert(Length(item) > 0);
39    Result := false;
40    index := 0;
41    if listview.items.count = 0 then
42      Exit;
43    first := 0;
44    last := listview.items.count - 1;
45    repeat
46      pivot := (first + last) div 2;
47      res := lstrcmp(PChar(item), Pchar(listview.items[pivot].caption));
48      if res = 0 then
49      begin
50        { Found the item, return its index and exit. }
51        index := pivot;
52        result := true;
53        Break;
54      end
55      else if res > 0 then
56      begin
57        { Item is larger than item at pivot }
58        first := pivot + 1;
59      end
60      else
61      begin
62        { Item is smaller than item at pivot }
63        last := pivot - 1;
64      end;
65    until
66      last < first;
67    index := first;
68  end;