String Manipulation in Numba Cuda: Clip first k characters from a string array, k comes from another array

We have two arrays arr1 (which has string elements) and arr2 (which has integers).
I want to clip first arr2[i] characters from arr[i]. These arrays are very large and so I want to implement this in Numba cuda. Pythonic implementation is as follows:

arr1 = ['abc', 'def', 'xyz']
arr2 = [1,2,3]

def python_clipper(arr1,arr2):
    for i in range(len(arr1)):
        arr1[i] = arr1[i][arr2[i]:]
    return arr1

print(python_clipper(arr1,arr2)) # ['bc', 'f', '']

The above implementation works fine. But when I create a cuda function out of this python function like so:

@cuda.jit()
def cuda_clipper(arr1,arr2):
    i = cuda.grid(1)
    arr1[i] = arr1[i][arr2[i]:]

blockspergrid, threadsperblock = len(arr1),1
cuda_clipper[blockspergrid, threadsperblock](arr1,arr2) # ['bc', 'f', '']
print(arr1)

I get the following error:

numba.core.errors.TypingError: Failed in cuda mode pipeline (step: nopython frontend)
Failed in nopython mode pipeline (step: nopython frontend)
No implementation of function Function(<function _empty_string at 0x7f0456884d30>) found for signature:
 
 >>> _empty_string(int64, int64, bool)
 
There are 2 candidate implementations:
      - Of which 2 did not match due to:
      Overload in function 'register_jitable.<locals>.wrap.<locals>.ov_wrap': File: numba/core/extending.py: Line 159.
        With argument(s): '(int64, int64, bool)':
       Rejected as the implementation raised a specific error:
         NumbaRuntimeError: Failed in nopython mode pipeline (step: native lowering)
       NRT required but not enabled
       During: lowering "s = call $10load_global.3(kind, char_width, length, is_ascii, func=$10load_global.3, args=[Var(kind, unicode.py:276), Var(char_width, unicode.py:276), Var(length, unicode.py:276), Var(is_ascii, unicode.py:276)], kws=(), vararg=None, varkwarg=None, target=None)" at /mnt/local-raid10/workspace/user/anaconda3/envs/condaenv/lib/python3.9/site-packages/numba/cpython/unicode.py (277)
  raised from /mnt/local-raid10/workspace/user/anaconda3/envs/condaenv/lib/python3.9/site-packages/numba/core/runtime/context.py:19

During: resolving callee type: Function(<function _empty_string at 0x7f0456884d30>)
During: typing of call at /mnt/local-raid10/workspace/user/anaconda3/envs/condaenv/lib/python3.9/site-packages/numba/cpython/unicode.py (1700)


File "../../anaconda3/envs/condaenv/lib/python3.9/site-packages/numba/cpython/unicode.py", line 1700:
            def getitem_slice(s, idx):
                <source elided>
                    # It's heterogeneous in kind OR stride != 1
                    ret = _empty_string(kind, span, is_ascii)
                    ^

During: typing of intrinsic-call at /mnt/local-raid10/workspace/user/trim/trim_new_implementation/string_numba.py (143)

File "string_numba.py", line 143:
def cuda_clipper(arr1,arr2):
    <source elided>
    i = cuda.grid(1)
    arr1[i] = arr1[i][arr2[i]:]
    ^

I am under the impression that slicing the string is the problem as a similar implementation works fine with an array. I have tried to make the arr1 into an array of array, but that preprocess itself takes some time rendering cuda useless to improve the performance. How can I directly work with str within numba rather than thinking of circumventing the problem.

> We have two arrays arr1 (which has string elements) and arr2 (which has integers)

You don't have arrays. You have lists. As you can see from the documentation, there is no python string or list support on the GPU.

Therefore what you are trying to do is currently not supported in Numba CUDA.