How does Wave Function Collapse generate a tilemap or bitmap, what does it guarantee, and when does it fail (contradiction)?

Problem: Generate locally-consistent tilemaps or textures from an example. Conditions: input is a small bitmap (overlapping model, NxN patterns, typically N=3) or a tile list with adjacency rules (simple tiled model). Method: count NxN patterns (optionally with rotations and reflections); start with every output cell in an all-allowed superposition; loop: observe (pick the cell with the lowest non-zero Shannon entropy and collapse it using the pattern frequencies), then propagate the removed options through neighbours; stop when everything is collapsed. Guarantees in the README: (C1) output contains only NxN patterns present in the input; (weak C2) pattern distribution is similar to the input. Failure modes: if propagation zeroes every option in a cell, the run hits a contradiction and returns nothing; the README says deciding whether other valid bitmaps exist is NP-hard, so no fast always-finishing solver exists, yet contradictions are rare in practice (author's statement, not measured here). Limitations: local similarity only; no global structure (a connected critical path, reachability) is guaranteed, so a level generator must add constraints or validation. Version/date: undated README as fetched 2026-10-23. Curator qualifications: All cited documentation excerpts located on the live pages by curator GET; no video sources. Correction folded 2026-10-24 from C3-B724-04 (curator-reviewed discovery audit): Problem. C3-B705-01 says the overlapping model uses NxN patterns 'typically N=3'. Correction. The cited README never states a typical N; it gives the general NxN description, calls the simplest case NxN=1x2, and cites a voxel example made with N=2. The value N=3 appears nowhere in the fetched text. Other claims in that finding (C1 and weak C2 guarantees, contradiction handling, NP-hard note) match the README. Limitations. Common usage of N=3 may be true elsewhere (for example in the repository samples) but is not in the cited README. Version/date. Undated README. Limitations: Rarity of contradictions is the author's claim without data in the README. I did not run the code. Excerpt 3 is two adjacent sentences joined by the omitted subject of the first. Corrected by C3-B724-04; where the correction disagrees, the correction governs. Evidence status: statements located in the cited sources by a correlated desk review (Claude); not executed; not independently reviewed.

{
  "citations": [
    {
      "url": "https://raw.githubusercontent.com/mxgmn/WaveFunctionCollapse/master/README.md",
      "note": "Intro: local similarity; Intro: observation-propagation; Intro: contradiction; Algorithm steps 4-5"
    }
  ]
}