[Sad Catapella N1][Correctness] Big-endian int8/int16 histogram blocks are reinterpreted as native-endian pixel values #25

Scope / classification

Normalization adversarial audit, Batch 1, main f784c95e3049483f87466150ffc5764375fa631d. Confirmed source-level defect with isolated NumPy byte-order probe; full package tests not executed.

Evidence

• _measure, lines 399–411 normalizes the declared dtype to native order via check_supported, but passes original blocks unconverted.
• _small_integer_histogram, lines 443–456 uses values.view(f'u{dtype.itemsize}'), which reads bytes as native-endian unsigned values, without converting from a block's actual byte order. Both whole-plane and blockwise APIs use this fast path for 8-/16-bit integers.
• check_supported, lines 22–42 deliberately admits supported nonnative dtypes by returning the native dtype.
• Existing test_histogram.py examples and block parity tests 224–235 use native-endian inputs only.

Minimal counterexample

np.array([1,2,3],dtype='>u2') has numeric values 1,2,3, but arr.view('u2') on a little-endian host produces [256,512,768]. The current fast path builds the histogram over those wrong values and calls it the numeric original. An isolated NumPy probe confirmed the reinterpretation; this is not a repository test run.

Impact

Wrong minimum/maximum, wrong bin counts and consequently wrong normalization statistics for big-endian 16-bit images, an important microscopy storage format. No explicit warning or refusal.

Fix / acceptance

Normalize values to the declared native dtype by value, not by raw bytes, before reinterpretation (or use a byte-order-aware bit mapping). Preserve signed integer order, streaming and existing speed. Test >i2, >u2, <i2, <u2 on native and non-native hosts, 8-bit controls, strided blocks and real TIFF big-endian fixture; assert numerical minima/limits/counts against an independent reference, whole-block parity and no extra unbounded copies. Benchmark conversion overhead and an optional Numba primitive only after correctness is fixed. No source modified.