Overview

What's up! 😃 Currently, there're two functions to deal with permuting and reversing arrays(permuted_axes and reversed_axes).
As mentioned from related issue, they're not in-place, so I added permute_axes and reverse_axes. The new implementation for permute_axes uses a bitmask-based approach to efficiently track and process cycles in axis permutations!

Related issue

• Reversing and permuting axes do not require consuming an array #1443

Changes

1. permute_axes (in-place):

   • A new function with a cycle detection algorithm based on bitmasking.
   • try to maintain the zero-allocation, in-place modification approach

2. reverse_axes (in-place):

   • simply use &mut self

Details on permute_axes

How it works

The new implementation uses a bitmask to track visited axes during permutation cycles:

1. Cycle Detection: When permuting axes, we identify cycles of positions that need to be updated.
2. Bitmask Tracking: A single usize is used as a bitmask to efficiently track which axes have been processed.
3. In-place Updates: Each cycle is processed in-place by following the permutation pattern.

Example

Consider permuting a 2×2 array from [[1, 2], [3, 4]] with axes permutation [1, 0] (transpose):

// Original array
let mut arr = array![[1, 2], [3, 4]];
// Original: dims = [2, 2], strides = [2, 1]

// Permute axes [1, 0]
arr.permute_axes([1, 0]);
// Result: dims = [2, 2], strides = [1, 2]
// Array is now [[1, 3], [2, 4]]

Here's how the algorithm processes this:

1. Start with axis 1 (new_axis=0):

   • Store initial values: dim[1]=2, stride[1]=1
   • Follow cycle: 1 → 0 → 1
   • Update dim[1] = dim[0] = 2
   • Update stride[1] = stride[0] = 2
   • Mark axes 0 and 1 as visited

2. Process axis 0 (new_axis=1):

   • Already visited in the previous cycle, skip

The bitmask approach ensures each axis is processed exactly once, even in complex permutations with multiple cycles. The key operations are:

• visited |= 1 << axis: Mark axis as visited
• (visited & (1 << axis)) != 0: Check if axis is visited

@akern40 @nilgoyette added test cases similar to the original functions(premuted_axes and reversed_axes), but there might be missing cases. please take a look when you guys have some time! 🚀