deques.nim: Refactor the toDeque functionality (#15166)

This commit polishes the new proc introduced by d9ed816b10.

Changes:
- Rename to `toDeque` for more consistency with well-known procs like
  `toHashSet` and `toTable`.
- Rename the `openArray` parameter. The name `arr` was potentially less
  clear given that the proc can be used with a seq (or string).
- Add a `since` annotation.
- Reword the doc comment, and clarify that ordering is preserved.
- Add runnableExamples.
- Add "see also" cross linking between `initDeque` and `toDeque`.
- Remove duplicate `nextPowerOfTwo`. The `initImpl` template already
  includes it.
- Implement the proc using the `items` iterator, rather than indexing.
  This matches the implementation of `sets.toHashSet` and
  `tables.toTable`.
- Add a test within `when isMainModule`.
- Add a changelog entry.

changelog.md

@@ -145,6 +145,9 @@
 
 - Export `asyncdispatch.PDispatcher.handles` so that an external library can register them.
 
+- Added `deques.toDeque`, which creates a deque from an openArray. The usage is
+  similar to procs such as `sets.toHashSet` and `tables.toTable`. Previously,
+  it was necessary to create an empty deque and add items manually.
 
 ## Language changes
 - The `=destroy` hook no longer has to reset its target, as the compiler now automatically inserts

lib/pure/collections/deques.nim

@@ -82,13 +82,25 @@ proc initDeque*[T](initialSize: int = 4): Deque[T] =
   ## Optionally, the initial capacity can be reserved via `initialSize`
   ## as a performance optimization.
   ## The length of a newly created deque will still be 0.
+  ##
+  ## See also:
+  ## * `toDeque proc <#toDeque,openArray[T]>`_
   result.initImpl(initialSize)
 
-proc initDeque*[T](arr: openArray[T]): Deque[T] =
-  ## Create a new deque initialized with an open array
-  result.initImpl(nextPowerOfTwo(arr.len))
-  for i in 0 ..< arr.len:
-    result.addLast(arr[i])
+proc toDeque*[T](x: openArray[T]): Deque[T] {.since: (1, 3).} =
+  ## Creates a new deque that contains the elements of `x` (in the same order).
+  ##
+  ## See also:
+  ## * `initDeque proc <#initDeque,int>`_
+  runnableExamples:
+    var a = toDeque([7, 8, 9])
+    assert len(a) == 3
+    assert a.popFirst == 7
+    assert len(a) == 2
+
+  result.initImpl(x.len)
+  for item in items(x):
+    result.addLast(item)
 
 proc len*[T](deq: Deque[T]): int {.inline.} =
   ## Return the number of elements of `deq`.
@@ -534,6 +546,7 @@ when isMainModule:
   assert first == 123
   assert second == 9
   assert($deq == "[4, 56, 6, 789]")
+  assert deq == [4, 56, 6, 789].toDeque
 
   assert deq[0] == deq.peekFirst and deq.peekFirst == 4
   #assert deq[^1] == deq.peekLast and deq.peekLast == 789