Compact Index File Structure (.idx)
Compact Index Header Record

Byte offset Description
00 – 03
Pointer to root node

04 – 07
Pointer to free node list ( -1 if not present)

08 – 11
Reserved for internal use

12 – 13
Length of key

14
Index options (any of the following numeric values or their sums): 1 – a unique index 8 – index has FOR clause 32 – compact index format 64 – compound index header

15
Index signature

16 – 19
Reserved for internal use

20 – 23
Reserved for internal use

24 – 27
Reserved for internal use

28 – 31
Reserved for internal use

32 – 35
Reserved for internal use

36 – 501
Reserved for internal use

502 – 503
Ascending or descending: 0 = ascending 1 = descending

504 – 505
Reserved for internal use

506 – 507
FOR expression pool length1

508 – 509
Reserved for internal use

510 – 511
Key expression pool length1

512 – 1023
Key expression pool (uncompiled)


1 This information tracks the space used in the key expression pool.

Compact Index Interior Node Record
Byte offset Description
00 – 01
Node attributes (any of the following numeric values or their sums): a. 0 – index node b. 1 – root node c. 2 – leaf node

02 – 03
Number of keys present (0, 1 or many)

04 – 07
Pointer to node directly to left of current node (on same level, -1 if not present)

08 – 11
Pointer to node directly to right of current node (on same level; -1 if not present)

12 – 511
Up to 500 characters containing the key value for the length of the key with a four-byte hexadecimal number (stored in normal left-to-right format):

This node always contains the index key, record number and intra-index pointer.2

The key/four-byte hexadecimal number combinations will occur the number of times indicated in bytes 02 – 03.


Compact Index Exterior Node Record
00 – 01
Node attributes (any of the following numeric values or their sums): 0 – index node 1 – root node 2 – leaf node

02 – 03
Number of keys present (0, 1 or many)

04 – 07
Pointer to the node directly to the left of current node (on same level; -1 if not present)

08 – 11
Pointer to the node directly to right of the current node (on same level; -1 if not present)

12 – 13
Available free space in node

14 – 17
Record number mask

18
Duplicate byte count mask

19
Trailing byte count mask

20
Number of bits used for record number

21
Number of bits used for duplicate count

22
Number of bits used for trail count

23
Number of bytes holding record number, duplicate count and trailing count

24 – 511
Index keys and information2


2 Each entry consists of the record number, duplicate byte count and trailing byte count, all compacted. The key text is placed at the logical end of the node, working backwards, allowing for previous key entries.

Вот описание файла .cdx из МСДН. Структура описана для 9 фокса, но думаю. что для она подойдет и для предыдущих версий.