Fix disaggregate(limiting_variable) silently losing a zone's value (#3403)

.claude/sweep-accuracy-state.csv

@@ -6,7 +6,7 @@ contour,2026-05-01,,,,"Marching squares correct: NaN check uses self-inequality,
 corridor,2026-05-01,,LOW,1,"LOW: corridor inherits float32 from cost_distance; for very large accumulated costs, normalized = corridor - corridor_min loses precision near min (intrinsic to upstream dtype, not corridor itself). NaN handling correct (skipna min, np.isfinite check before normalize). All 4 backends route through pure xarray arithmetic; threshold uses dask/cupy/numpy where with try/except dispatch. No CRIT/HIGH issues."
 cost_distance,2026-06-16,3369,CRITICAL,5,"CRITICAL heap overflow (#3369/this PR): numba Dijkstra kernels _cost_distance_kernel + _cost_distance_tile_kernel sized the binary min-heap at height*width, but a lazy-deletion heap pushes a pixel on every improving relaxation, so push count exceeds h*w on non-uniform friction. _heap_push then writes OOB -> heap corruption, SIGABRT (exit 134, 'corrupted size vs prev_size') on iterative dask path; UB on numpy path. Reference heapq Dijkstra hits 44 pushes on a 6x6=36 grid. Fix: max_heap = h*w*(n_neighbors+1), tile kernel adds +2*(w+h)+4 for phase-2 boundary seeds. Verified: cupy relax kernel (parallel Bellman-Ford) does NOT use this heap, GPU path unaffected. CUDA available; numpy/cupy/dask+numpy/dask+cupy all agree post-fix over 30+40 random adversarial grids; 88 module tests pass (4 new regression tests). Cats 1-4 clean: dist float64 / out float32 fine; inf/nan/zero friction all impassable (tested); bounds guards use >=h/>=w; planar algorithm, no curvature expected. Supersedes prior #1191 (cupy max_iterations h+w->h*w, fixed PR #1192)."
 curvature,2026-03-30T15:00:00Z,,,,Formula matches ArcGIS reference. Backends consistent. No issues found.
-dasymetric,2026-04-14T12:00:00Z,,,,Mass conservation correct. Weighted/binary/limiting_variable all verified. Pycnophylactic Tobler algorithm correct.
+dasymetric,2026-06-20,3403,MEDIUM,2;5,"Cat2/Cat5: disaggregate(limiting_variable) silently dropped a zone's whole value when no pixel could absorb it (all pixels in a cap-0 class, e.g. all zero-weight) - returned finite zeros summing to 0 instead of NaN, violating the documented conservation property; weighted method returns NaN for the same input. Fix #3403/this PR: set zone result to NaN when n_overflow==0, matching weighted. Also flagged (LOW, not fixed, documented only): non-finite +Inf weights are not sanitized like negatives are - an Inf weight poisons the whole zone (wsum=inf -> NaN/0, mass lost), consistent across numpy/cupy/dask backends (not a divergence). Cat1/Cat3/Cat4 clean: weighted division is by sum of positives (no cancellation), no neighborhood stencil off-by-one (pycnophylactic shifts are symmetric and bounds-guarded), no geodesic/curvature math. CUDA available; cupy + dask+cupy parity verified (62 tests pass incl. cross-backend). Backends: cupy + dask paths delegate to the numpy core so the fix covers all four."
 diffusion,2026-05-01,,LOW,1;2;5,"LOW: no Kahan summation across long iterations (drift over 100k steps, standard for explicit Euler); lap=n+s+w+e-4*val has catastrophic cancellation for nearly-uniform large values; res=0 in attrs causes div-by-zero (no guard); dask+cupy boundary='nan' relies on dask accepting cp.nan as fill. CPU/GPU NaN handling consistent (np.isnan vs val!=val). depth=1 matches stencil radius. Memory guards, CFL check, step cap all in place. No CRIT/HIGH."
 edge_detection,2026-05-01,,,,Thin wrappers around convolve_2d with fixed Sobel/Prewitt/Laplacian kernels; no issues found
 emerging_hotspots,2026-04-30,,MEDIUM,2;3,MEDIUM: threshold_90 uses int() (truncation) instead of ceil() so n_times=11 requires only 9/11 (81.8%) instead of 90%. MEDIUM: NaN time steps produce gi_bin=0 which classifier counts as 'non-significant' rather than missing; threshold_90 uses full n_times not valid count. LOW: 'global_std == 0' check does not catch NaN std for fully/mostly NaN inputs.

xrspatial/dasymetric.py

@@ -391,6 +391,12 @@ def _disaggregate_limiting_numpy(zones, weight, values_dict, nodata_zone,
             n_overflow = int(overflow_mask.sum())
             if n_overflow > 0:
                 result[overflow_mask] += remaining / n_overflow
+            else:
+                # No pixel can absorb the leftover (e.g. every pixel in the
+                # zone is uninhabitable under the caps).  Match the weighted
+                # method: a value with nowhere to go becomes NaN rather than
+                # silently vanishing into zeros.
+                result[zmask] = np.nan
 
     return result
 

xrspatial/tests/test_dasymetric.py

@@ -258,6 +258,29 @@ def test_custom_density_caps(self):
         # conservation
         assert np.nansum(data) == pytest.approx(100.0)
 
+    def test_uninhabitable_zone_is_nan_not_zero(self):
+        """A zone with no habitable pixel must not silently lose its value.
+
+        When every pixel falls in a cap-0 class (e.g. all zero-weight),
+        the value has nowhere to go.  The result must be NaN (matching the
+        weighted method), not finite zeros that drop the value (#3403).
+        """
+        zones_data = np.array([[1, 1], [1, 1]], dtype=np.float64)
+        weight_data = np.zeros((2, 2), dtype=np.float64)
+        zones = create_test_raster(zones_data, backend='numpy')
+        weight = create_test_raster(weight_data, backend='numpy')
+
+        result = disaggregate(zones, {1: 500.0}, weight,
+                              method='limiting_variable')
+        data = result.values
+        assert np.all(np.isnan(data))
+        # the value is not silently zeroed away
+        assert np.nansum(data) == pytest.approx(0.0)
+
+        # weighted method agrees: same input yields all-NaN
+        weighted = disaggregate(zones, {1: 500.0}, weight, method='weighted')
+        assert np.all(np.isnan(weighted.values))
+
     @pytest.mark.skipif(not has_dask_array(), reason="dask not available")
     def test_raises_for_dask(self, simple_zones_data, simple_weight_data,
                              simple_values):