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Write a Python function `get_pairs_count(arr, target_sum)` that takes a list of integers `arr` and an integer `target_sum` as input. The function should return the number of unique pairs `(a, b)` in the list such that `a + b == target_sum`. Each pair should be counted only once, and the order of elements in the pair does not matter. #### Example Usage ```python [main.nopy] get_pairs_count([1, 1, 1, 1], 2) # Output: 6 get_pairs_count([1, 5, 7, -1, 5], 6) # Output: 3 get_pairs_count([1, -2, 3], 1) # Output: 1 get_pairs_count([-1, -2, 3], -3) # Output: 1 ``` #### Constraints - The input list can contain both positive and negative integers. - The input list may contain duplicate values. - The function should handle an empty list gracefully by returning `0`.
def get_pairs_count(arr, target_sum):
"""
Count the number of unique pairs in the list whose sum equals the target_sum.
Args:
arr (list): List of integers.
target_sum (int): The target sum for the pairs.
Returns:
int: The number of pairs whose sum equals target_sum.
"""
# Initialize a counter for the pairs
count = 0
# Placeholder for the solution
return count