From 0ed883ef11dcf76e980aba4d15add106df3cfb39 Mon Sep 17 00:00:00 2001
From: cui fliter <imcusg@gmail.com>
Date: Wed, 9 Sep 2026 16:57:29 +0800
Subject: [PATCH] algorithm: handle empty inputs in rotateLeft (#26191)

`rotateLeft` and `rotatedLeft` raised `DivByZeroDefect` for empty
containers because their whole-container overloads computed `dist mod
arg.len` before checking for an empty input.

Handle empty inputs explicitly:

- `rotateLeft` returns `0` and leaves the container unchanged.
- `rotatedLeft` returns an empty sequence.

Add a regression test covering both overloads.

The slice overloads are intentionally left unchanged because zero-length
slice semantics need separate consideration.

Signed-off-by: cuishuang <imcusg@gmail.com>
---
 lib/pure/algorithm.nim      | 4 ++++
 tests/stdlib/talgorithm.nim | 6 ++++++
 2 files changed, 10 insertions(+)

diff --git a/lib/pure/algorithm.nim b/lib/pure/algorithm.nim
index c38b4233c5..f86260328f 100644
--- a/lib/pure/algorithm.nim
+++ b/lib/pure/algorithm.nim
@@ -865,6 +865,8 @@ proc rotateLeft*[T](arg: var openArray[T]; dist: int): int {.discardable.} =
     a.rotateLeft(-6)
     assert a == [1, 2, 3, 4, 5]
   let argLen = arg.len
+  if argLen == 0:
+    return 0
   let distLeft = ((dist mod argLen) + argLen) mod argLen
   arg.rotateInternal(0, distLeft, argLen)
 
@@ -914,5 +916,7 @@ proc rotatedLeft*[T](arg: openArray[T]; dist: int): seq[T] =
     a = rotatedLeft(a, -6)
     assert a == @[1, 2, 3, 4, 5]
   let argLen = arg.len
+  if argLen == 0:
+    return newSeq[T]()
   let distLeft = ((dist mod argLen) + argLen) mod argLen
   arg.rotatedInternal(0, distLeft, argLen)
diff --git a/tests/stdlib/talgorithm.nim b/tests/stdlib/talgorithm.nim
index 589111b482..b04d43201f 100644
--- a/tests/stdlib/talgorithm.nim
+++ b/tests/stdlib/talgorithm.nim
@@ -74,6 +74,12 @@ block:
   doAssert s5.rotateLeft(3 ..< 10, 11) == 6
   doAssert s5 == "xxxefgabcdxxx"
 
+  block:
+    var x: seq[int] = @[]
+    doAssert x.rotateLeft(1) == 0
+    doAssert x == @[]
+    doAssert x.rotatedLeft(1) == @[]
+
   block product:
     doAssert product(newSeq[seq[int]]()) == newSeq[seq[int]](), "empty input"
     doAssert product(@[newSeq[int](), @[], @[]]) == newSeq[seq[int]](), "bit more empty input"