Fix incorrect output in Binary Search when duplicates exist(Fixes #13840)

searches/binary_search.py

@@ -207,7 +207,10 @@
         midpoint = left + (right - left) // 2
         current_item = sorted_collection[midpoint]
         if current_item == item:
-            return midpoint
+            if midpoint > 0 and sorted_collection[midpoint - 1] == item:
+                right = midpoint - 1  # Keep searching left
+            else:
+                return midpoint 
         elif item < current_item:
             right = midpoint - 1
         else:

tests/test_binary_search_issue.py

@@ -0,0 +1,38 @@
+import sys
+import os
+from hypothesis import given, strategies as st
+
+# Add root directory to python path
+sys.path.append(os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
+
+from searches.binary_search import binary_search
+
+# Strategy to generate a sorted list of integers
+sorted_list_strategy = st.lists(st.integers(), min_size=0).map(sorted)
+
+@given(
+    arr=sorted_list_strategy,
+    # Pick a random integer as a potential target
+    target=st.integers() 
+)
+def test_binary_search_property_based(arr, target):
+    """
+    Property-based test:
+    1. If target is in arr, binary_search MUST return an index 'i' 
+       such that arr[i] == target.
+    2. If target is in arr, it MUST be the FIRST occurrence.
+    3. If target is not in arr, binary_search MUST return -1.
+    """
+
+    result = binary_search(arr, target)
+
+    if target in arr:
+        assert result != -1, f"Target {target} was in {arr} but not found."
+        assert arr[result] == target, f"Index {result} pointed to {arr[result]}, not {target}."
+
+        # Property: Verify it is the FIRST occurrence
+        # Only check if the target is actually at the result index
+        for i in range(result):
+            assert arr[i] != target, f"Found target {target} at index {result}, but earlier occurrence at {i}."
+    else:
+        assert result == -1, f"Target {target} was not in {arr}, but found at index {result}."