Fix ZeroDivisionError on zero operands in least_common_multiple (#14845)

* Fix ZeroDivisionError on zero operands in least_common_multiple

least_common_multiple_slow raised ZeroDivisionError for a zero operand (0 % n in the loop guard), e.g. least_common_multiple_slow(0, 5), and least_common_multiple_fast raised it for (0, 0) via floor-division by gcd(0, 0) == 0. The least common multiple is 0 whenever an operand is 0 (consistent with math.lcm), so both functions return 0 in that case. Adds doctests covering the zero inputs.

* Add zero-operand cases to least_common_multiple unit test

* Apply suggestion from @cclauss

---------

Co-authored-by: Christian Clauss <cclauss@me.com>

Author: CharlesCNorton

maths/least_common_multiple.py

@@ -14,7 +14,15 @@ def least_common_multiple_slow(first_num: int, second_num: int) -> int:
     10
     >>> least_common_multiple_slow(12, 76)
     228
+    >>> least_common_multiple_slow(0, 5)
+    0
+    >>> least_common_multiple_slow(5, 0)
+    0
+    >>> least_common_multiple_slow(0, 0)
+    0
     """
+    if first_num == 0 or second_num == 0:
+        return 0
     max_num = first_num if first_num >= second_num else second_num
     common_mult = max_num
     while (common_mult % first_num > 0) or (common_mult % second_num > 0):
@@ -30,7 +38,15 @@ def least_common_multiple_fast(first_num: int, second_num: int) -> int:
     10
     >>> least_common_multiple_fast(12,76)
     228
+    >>> least_common_multiple_fast(0, 5)
+    0
+    >>> least_common_multiple_fast(5, 0)
+    0
+    >>> least_common_multiple_fast(0, 0)
+    0
     """
+    if 0 in {first_num, second_num}:
+        return 0
     return first_num // greatest_common_divisor(first_num, second_num) * second_num
 
 
@@ -59,8 +75,11 @@ class TestLeastCommonMultiple(unittest.TestCase):
         (12, 25),
         (10, 25),
         (6, 9),
+        (0, 5),
+        (5, 0),
+        (0, 0),
     )
-    expected_results = (20, 195, 124, 210, 1462, 60, 300, 50, 18)
+    expected_results = (20, 195, 124, 210, 1462, 60, 300, 50, 18, 0, 0, 0)
 
     def test_lcm_function(self) -> None:
         for i, (first_num, second_num) in enumerate(self.test_inputs):