[ulfarsson]

Reading a length from PropertyBitmaps.bitmaps can build that length's whole bitmap. That happens when the length is kept as the indices of its few good permutations. This adds a paragraph and an example to PropertyBitmaps' docstring (permuta/bisc/bisc.py), saying which reads build bitmaps and which do not. No code changes. The example: one good permutation of length 8 is kept as an index, and is read back as a bitmap of 5,040 bytes:

>>> pb = PropertyBitmaps(Perm.is_increasing)
>>> pb.add_length(8)
>>> pb.good_counts[8], len(pb.bitmaps[8])
(1, 5040)

At length 14 the same read costs 10.9 GB. So if pb.bitmaps holds length 14 as indices:

• 14 in pb.bitmaps, list(pb.bitmaps) and len(pb.bitmaps) build nothing;
• dict(pb.bitmaps), pb.bitmaps.items(), pb.bitmaps.values(), or PropertyBitmaps(bitmaps=pb.bitmaps) build the bitmap of length 14: 10.9 GB.

This matters to anyone who keeps a PropertyBitmaps' labels between runs of bisc or auto_bisc. The class docstring suggests doing that, and bitmaps is the natural way to read them out.

Details

• The paragraph comes after the one on the attributes, in the class docstring's style, and the example ends the doctests, as bitmaps had none.
• Its source: Add PropertyBitmaps: bisc and auto_bisc keep their input as bitmaps, or as indices where few are good #209's docstring had the same note until its last commit shortened it. No other docstring says it now.
• Its claims are the code's:
  • _Bitmaps.__getitem__ builds the bitmap of a length kept as indices each time it is asked for one.
  • _Bitmaps.__contains__ is overridden, so that in builds nothing.
  • _LengthKeys.__iter__ and __len__ read only the lengths.
  • The constructor reads a given bitmaps through items().
• Checked by counting the builds of each way of reading bitmaps, on a PropertyBitmaps of Perm.is_increasing with lengths 5 to 8 kept as indices:
  • in, list() and len() make none;
  • items(), values(), dict() and the constructor make one for each such length;
  • the constructor keeps those lengths as indices again.
• The size: 10.9 GB is the module's own size for length 14, (14! + 7) // 8 = 10,897,286,400 bytes.
• Tests: tox's flake8, mypy, pylint and black pass. All 696 tests pass on CPython 3.15, and 694 on PyPy 3.12, with the 2 reference-count and tracemalloc tests skipped there, bisc.py's doctests among them. Tilings' 3,722 tests pass at its develop with this Permuta, on both Pythons.
• No CHANGELOG entry: Unreleased already names PropertyBitmaps.

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AI assistance: written with Claude Code (Opus 5.5) as the Permuta developer, and reviewed before submission by Claude Code (Opus 5.5) as an automated reviewer; Henning Ulfarsson checked and opened it.