[C++] Handle only relevant slices of child arrays when hashing scalars from ListArrays

[felipecrv]

Describe the enhancement requested

Issue is explained in the Python code below:

import pyarrow as pa

a = pa.array([
    [{'a': 5}, {'a': 6}],
    [{'a': 7}, None]
])
b = pa.array([
    [{'a': 7}, None]
])

# a[1] and b[0] are represented as 2-element slices of a child array containing struct values
# they start on different offsets, but obviously compare as equal
assert a[1] == b[0]

# logically equal values should hash to the same value, so when hashing the hashing
# of the child array should start at the offset and not from 0 as it's done by default.
hash1 = hash(a[1])
hash2 = hash(b[0])
assert hash1 == hash2

#35814 fixes the bug for lists of structs, but the same bug might exist for other nested types:

• struct
• sparse union
• dense union
• run-end encoded
• more?

Component(s)

C++

[HyukjinKwon]

Let me work on this!