Xbase: Clipper Index Files (*.ntx)

The Structure of Clipper Index files (*.ntx)

The CA-Clipper index file is a modified B+ tree. The size of a page is 1024 bytes. The first page is the header:

     _______________________  _______
 00h /   0  | Signature byte      *9|  ^
 00h /   1  | (binary)           *12|  | 
            |-----------------------|  |
 00h /   2  | Indexing version no.  |  |
 00h /   3  | (Compiler version)    |  |
            |-----------------------|  |
 00h /   4  | Offset in file for    | File
 00h /   5  | first index page.     | header
 00h /   6  | (root)                |  |
 00h /   7  |                       | (page 0)
            |-----------------------|  |
 00h /   8  | Offset to an unused   |  |
 00h /   9  | next key page.     *11|  |
 00h /  10  |                       |  |
 00h /  11  |                       |  |
            |-----------------------|  |
 00h /  12  | Key size + 8 bytes  *1|  |
 00h /  13  |                       |  |
            |-----------------------|  |
 00h /  14  | Key size              |  |
 00h /  15  |                       |  |
            |-----------------------|  |
 00h /  16  | No. of decimals in    |  |
 00h /  17  | key                   |  |
            |-----------------------|  |
 00h /  18  | Max. item           *6|  |
 00h /  19  |                       |  |
            |-----------------------|  |
 00h /  20  | Half page           *7|  |
 00h /  21  |                       |  |
            |-----------------------|  |
 00h /  22  | Key expression.     *8|  |
 00h /  23  | (NULL terminated)     |  |
            :                       :  |
 00h /  277 |                       |  |
            |-----------------------|  |
 00h /  278 | Unique index flag   *3|  |
            |-----------------------|  |
 00h /  279 | (Unused)              |  |
            :                       :  |
            :                       :  |
 00h /  1023|                       |  |
   1 x 1024 |=======================| _v_____
 00h /     0| Address of next empty |    ^
 00h /     1| page               *11|    |
 00h /     2|                       |    |
 00h /     3|                       |    |
            |-----------------------|   Empty page
            : (Reserved)            |   (in linked list)
            :                       |    |
            :                       |    |
 00h /  1023:                       |    |
   2 x 1024 |=======================| ___V___
            | No. of used entries   |    |
            | on current page (N)   |    |
            |-----------------------|   Used
            | Array of N unsigned   |   page
            : longs               *5:    |
            :                       :    |
            |-----------------------|    |
            | Array of index        |    |
            : entries               :    |
            :                       :    |         __  |=======================|
            :                       :    |        /   0| Address of left page  |
            |                       |    |       /    1| in tree             *4|
            |                       |    |      /     2|                       |
            |                       |    |     /      3|                       |
            |=======================| ___|____/        |-----------------------|
            |  Array of index       |    |            4| Record no. in DBF     |
            :                       :    |            5|                       |
            :                       :    |            6|                       |
            :                       :    |            7|                       |
            :                       :    |             |-----------------------|
            :                       :    |            8| Key field             |
            :                       : ___|____         :                       :
            |=======================|    |    \       N|                       |
            |                       |    |     \_____  |=======================|
            :                       :    |
            |__End_of_File__________| ___v____  End of file ( 1Ah )

1. Also called signature. Distance between key pages. size + 2 longs. ie.e Left pointer + record no.
2. The first, or root page of an indes has a minimum of 1 entry regardless of this value.
3. Unique index flag

   1	Unique
   0	NOT unique

4. Address of left page in tree This number will be between the minimum (half page) and maximum (max. item) defined in the header unless it is the root page.
5. Array on unsigned longs The array length is equal to the maximum number of key entries per page +1. Value 00h = No record. Other values: Record offset from start of page
6. Max. item. The maximum number of keys (with their pointers) that can fit on an index page.
7. Half page. The maximum number of keys that can fit on an index page, divided by two. This is an important value in a B-tree system as it is the minimum number of keys that must be on a page.
8. Key expression The actual expression on which the index was built. It is a NULL terminated string with at max length of 256 bytes.
9. Version no

   0003h	Clipper 87
   0006h	Clipper 5.x

10. ?? This is a reference to the first page in a linked list of empty pages in beteen valid data.
11. Offset of next unused pages Address 00 00 00 00h is the last empty page in chain.
12. Binary version no Stored as:

    Value	Description
    03h 00h	Default
    00h 03h	Clipper 87
    00h 06h	Clipper 5.x