[GitHub] [arrow] rok commented on pull request #34797: GH-34796: [C++] Add FromTensor, ToTensor and strides methods to FixedShapeTensorArray

["rok (via GitHub)" <gi...@apache.org>]

rok commented on PR #34797:
URL: https://github.com/apache/arrow/pull/34797#issuecomment-1499479421

   A change introduced late in #8510 (and then removed) was that `ComputeRowMajorStrides` is replaced with a general `ComputeStrides` to calculate strides of permuted tensors which should help matching pytorch behavior:
   
   ```python
   import torch
   
   def print_strides(shape, permutation):
       permutation2 = [p - 1 for p in permutation[1:]]
       shape2 = shape[1:]
       x = torch.randn(shape).permute(permutation)
       y = torch.randn(shape2).permute(permutation2)
       strides = [z * 8 for z in x.stride()]
       strides2 = [z * 8 for z in y.stride()]
       
       print("[full tensor] shape:", shape, "permutation:", permutation, "strides:", strides)
       print("[cell tensor] shape:", shape2, "permutation:", permutation2, "strides:", strides2)
   
   shapes = [
       (3, 3, 4),
       (3, 3, 4),
       (3, 4, 3),
       (3, 4, 3),
   ]
   permutations = [
       (0, 1, 2),
       (0, 2, 1),
       (0, 1, 2),
       (0, 2, 1),
   ]
   
   for shape, permutation in zip(shapes, permutations):
       print_strides(shape, permutation)
   ```
   
   ```
   [full tensor] shape: (3, 3, 4) permutation: (0, 1, 2) strides: [96, 32, 8]
   [cell tensor] shape: (3, 4) permutation: [0, 1] strides: [32, 8]
   [full tensor] shape: (3, 3, 4) permutation: (0, 2, 1) strides: [96, 8, 32]
   [cell tensor] shape: (3, 4) permutation: [1, 0] strides: [8, 32]
   [full tensor] shape: (3, 4, 3) permutation: (0, 1, 2) strides: [96, 24, 8]
   [cell tensor] shape: (4, 3) permutation: [0, 1] strides: [24, 8]
   [full tensor] shape: (3, 4, 3) permutation: (0, 2, 1) strides: [96, 8, 24]
   [cell tensor] shape: (4, 3) permutation: [1, 0] strides: [8, 24]
   ```


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