From 7a43d00a6468d33548751bbf22b08cd51205ffc1 Mon Sep 17 00:00:00 2001
From: ringabout <43030857+ringabout@users.noreply.github.com>
Date: Fri, 20 Jan 2023 18:43:45 +0800
Subject: [PATCH] fixes #21278; `deques.shrink` off by one bug (#21284)

fixes #21278; deques.shrink off ny one bug

(cherry picked from commit b82b5d44afbd860075f806f71a62d595f2376d6e)
---
 lib/pure/collections/deques.nim | 2 +-
 tests/stdlib/tdeques.nim        | 6 ++++++
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/lib/pure/collections/deques.nim b/lib/pure/collections/deques.nim
index c4eb5331ec..dc5ef2a0ad 100644
--- a/lib/pure/collections/deques.nim
+++ b/lib/pure/collections/deques.nim
@@ -441,7 +441,7 @@ proc shrink*[T](deq: var Deque[T], fromFirst = 0, fromLast = 0) =
     deq.head = (deq.head + 1) and deq.mask
 
   for i in 0 ..< fromLast:
-    destroy(deq.data[deq.tail])
+    destroy(deq.data[(deq.tail - 1) and deq.mask])
     deq.tail = (deq.tail - 1) and deq.mask
 
   dec deq.count, fromFirst + fromLast
diff --git a/tests/stdlib/tdeques.nim b/tests/stdlib/tdeques.nim
index 03e951fce4..7b2518cda1 100644
--- a/tests/stdlib/tdeques.nim
+++ b/tests/stdlib/tdeques.nim
@@ -183,6 +183,12 @@ proc main() =
     clear(a)
     doAssert len(a) == 0
 
+  block: # bug #21278
+    var a = [10, 20, 30, 40].toDeque
+
+    a.shrink(fromFirst = 0, fromLast = 1)
+    doAssert $a == "[10, 20, 30]"
+
 
 static: main()
 main()