Fix explanation of num_rows, #32

CHANGELOG.md

@@ -8,7 +8,7 @@ This change log follows the conventions of
 
 ### Fixed
 
-- An error in interpreting a value in the database export file format could lead to some rows that were actually present in tables not being found. Thanks to [@IvanOnishchenko](https://github.com/IvanOnishchenko) for [pointing this out](https://github.com/Deep-Symmetry/crate-digger/issues/32).
+- An error in interpreting a value in the database export file format could lead to some rows that were actually present in tables not being found. Thanks to [@IvanOnishchenko](https://github.com/IvanOnishchenko) for [pointing this out](https://github.com/Deep-Symmetry/crate-digger/issues/32), and [Jan Holthuis (@Holzhaus)](https://github.com/Holzhaus) for identifying a flaw in the first attempt at correcting it.
 
 ## [0.2.0] - 2024-05-04
 

doc/modules/ROOT/pages/exports.adoc

@@ -255,7 +255,7 @@ playlist entries) have a lot of very small rows, too many to count
 with a single byte. But then why not just always use
 __num_rows_large__?
 
-NOTE: The row counter entries represent the number of actually-present rows in the page. To find them, you need to scan all 16 entries of each of the row groups present in the page, ignoring any whose <<#row-presence-bits,row presence bit>> is zero.
+NOTE: The row counter entries represent the number of rows that have ever been allocated in the page, but some will no longer be valid due to deletion or updates. To find the actual rows, you need to scan all 16 entries of each of the row groups present in the page, ignoring any whose <<#row-presence-bits,row presence bit>> is zero.
 
 The purpose of the next two bytes are is also unclear. Of _u~3~_ Mr.
 Flesniak said “a bitmask (first track: 32)”, and he described _u~4~_

src/main/kaitai/rekordbox_pdb.ksy

@@ -236,16 +236,17 @@ types:
         value: |
           (num_rows_large > num_rows_small) and (num_rows_large != 0x1fff) ? num_rows_large : num_rows_small
         doc: |
-          The number of rows on this page (controls the number of row
-          index entries there are, but some of those may not be marked
-          as present in the table due to deletion).
+          The number of rows that have ever been allocated on this
+          page (controls the number of row groups there are, but some
+          entries in each group may not be marked as present in the
+          table due to deletion or updates).
         -webide-parse-mode: eager
       num_row_groups:
         value: '(num_rows - 1) / 16 + 1'
         doc: |
           The number of row groups that are present in the index. Each
-          group can hold up to sixteen rows. All but the final one
-          will hold sixteen rows.
+          group can hold up to sixteen rows, but `row_present_flags`
+          must be consulted to determine whether each is valid.
       row_groups:
         type: 'row_group(_index)'
         repeat: expr