The fractal-heap reader split direct vs indirect block rows using the FRHP
"Starting # of Rows in Root Indirect Block" field (a constant, typically 1),
mislabeled as starting_row_of_indirect_blocks. For any heap whose data spans
more than one direct block — common in libhdf5 files with a large group or
many dense attributes — this treated direct blocks as indirect and walked into
garbage, failing with InvalidFractalHeapSignature.
Derive the split from the heap geometry instead: max_direct_rows =
log2(max_direct_block_size / starting_block_size) + 2. Rows below it hold
direct blocks; rows at/above hold child indirect blocks.
Validated against an h5py-written group with 400 dense attributes (root
indirect block, 4 rows, 13 direct blocks): all values now read correctly.
Regression fixture covers an 80-attribute multi-block heap.
Co-Authored-By: Claude Opus 4.8 <[email protected]>
