From 442a17b5726f3fc766617413000077d59466ecd6 Mon Sep 17 00:00:00 2001
From: dualfroz <me@dualfroz.com>
Date: Fri, 4 Sep 2026 09:18:56 +0200
Subject: [PATCH] fix(arrays): stop fill from growing the array on reversed
 spans

fill rebuilt the array as array[:start] + fill + array[end:]. When start
came after end those two slices overlapped, duplicating the shared
elements and growing the array while writing no value. Normalize start and
end into [0, length] and fill in place with an index loop so an empty span
is a no-op and the length stays fixed.
---
 src/pydash/arrays.py | 17 +++++++++++------
 tests/test_arrays.py |  8 ++++++++
 2 files changed, 19 insertions(+), 6 deletions(-)

diff --git a/src/pydash/arrays.py b/src/pydash/arrays.py
index 15fda4b..f87e38b 100644
--- a/src/pydash/arrays.py
+++ b/src/pydash/arrays.py
@@ -531,13 +531,18 @@ def fill(
 
     .. versionadded:: 3.1.0
     """
-    if end is None:
-        end = len(array)
-    else:
-        end = min(end, len(array))
+    length = len(array)
 
-    # Use this style of assignment so that `array` is mutated.
-    array[:] = array[:start] + [value] * len(array[start:end]) + array[end:]  # type: ignore
+    if end is None:
+        end = length
+
+    # Normalize to ``[0, length]``, negatives as slicing does, and fill in
+    # place so the length stays fixed.
+    start = max(length + start, 0) if start < 0 else min(start, length)
+    end = max(length + end, 0) if end < 0 else min(end, length)
+
+    for index in range(start, end):
+        array[index] = value  # type: ignore
     return array  # type: ignore
 
 
diff --git a/tests/test_arrays.py b/tests/test_arrays.py
index ff4d668..dedd309 100644
--- a/tests/test_arrays.py
+++ b/tests/test_arrays.py
@@ -158,6 +158,14 @@ def test_duplicates(case, expected):
         (([1, 2, 3, 4, 5], 0, 0, 5), [0, 0, 0, 0, 0]),
         (([1, 2, 3, 4, 5], 0, 0, 8), [0, 0, 0, 0, 0]),
         (([1, 2, 3, 4, 5], 0, 0, -1), [0, 0, 0, 0, 5]),
+        # start after end must be a no-op and must not grow the array.
+        (([1, 2, 3, 4], "x", 4, 1), [1, 2, 3, 4]),
+        (([1, 2, 3, 4, 5], 0, 3, 2), [1, 2, 3, 4, 5]),
+        # negative start supported the same way slicing does.
+        (([1, 2, 3, 4, 5], 0, -2), [1, 2, 3, 0, 0]),
+        (([1, 2, 3, 4, 5], 0, -2, -1), [1, 2, 3, 0, 5]),
+        # reversed negative span is a no-op, not an array-growing overlap.
+        (([1, 2, 3], "x", -1, -2), [1, 2, 3]),
     ],
 )
 def test_fill(case, expected):