Fix for reductions in miniexpr (depends on reduc type)

Author: FrancescAlted

src/blosc2/blosc2_ext.pyx

@@ -1925,8 +1925,11 @@ cdef int aux_miniexpr(me_udata *udata, int64_t nchunk, int32_t nblock,
 
     cdef me_expr* miniexpr_handle = udata.miniexpr_handle
     cdef void* aux_reduc_ptr
-    # Calculate blocks per chunk using CEILING division (chunks are padded to fit whole blocks)
-    cdef int nblocks_per_chunk = (udata.array.chunknitems + udata.array.blocknitems - 1) // udata.array.blocknitems
+    # For reduction operations, we need to track which block we're processing
+    # The linear_block_index should be based on the INPUT array structure, not the output array
+    # Get the first input array's chunk and block structure
+    cdef b2nd_array_t* first_input = udata.inputs[0]
+    cdef int nblocks_per_chunk = (first_input.chunknitems + first_input.blocknitems - 1) // first_input.blocknitems
     # Calculate the global linear block index: nchunk * blocks_per_chunk + nblock
     # This works because blocks never span chunks (chunks are padded to block boundaries)
     cdef int64_t linear_block_index = nchunk * nblocks_per_chunk + nblock

src/blosc2/lazyexpr.py

@@ -2031,8 +2031,25 @@ def reduce_slices(  # noqa: C901
         res_eval = blosc2.uninit(shape, dtype, chunks=chunks, blocks=blocks, cparams=cparams, **kwargs)
         # Compute the number of blocks in the result
         nblocks = res_eval.nbytes // res_eval.blocksize
-        # Initialize to zeros since some blocks may be padding and won't be written
-        aux_reduc = np.zeros(nblocks, dtype=dtype)
+        # Initialize aux_reduc based on the reduction operation
+        # Padding blocks won't be written, so initial values matter for the final reduction
+        if reduce_op == ReduceOp.SUM or reduce_op == ReduceOp.ANY:
+            aux_reduc = np.zeros(nblocks, dtype=dtype)
+        elif reduce_op == ReduceOp.PROD or reduce_op == ReduceOp.ALL:
+            aux_reduc = np.ones(nblocks, dtype=dtype)
+        elif reduce_op == ReduceOp.MIN:
+            if np.issubdtype(dtype, np.integer):
+                aux_reduc = np.full(nblocks, np.iinfo(dtype).max, dtype=dtype)
+            else:
+                aux_reduc = np.full(nblocks, np.inf, dtype=dtype)
+        elif reduce_op == ReduceOp.MAX:
+            if np.issubdtype(dtype, np.integer):
+                aux_reduc = np.full(nblocks, np.iinfo(dtype).min, dtype=dtype)
+            else:
+                aux_reduc = np.full(nblocks, -np.inf, dtype=dtype)
+        else:
+            # For other operations, zeros should be safe
+            aux_reduc = np.zeros(nblocks, dtype=dtype)
         try:
             print("expr->miniexpr:", expression, reduce_op)
             expression = f"{reduce_op_str}({expression})"