Auto-initialize deques

lib/pure/collections/deques.nim

@@ -62,6 +62,12 @@ type
     data: seq[T]
     head, tail, count, mask: int
 
+proc initImpl[T](deq: var Deque[T], initialSize: int = 4) =
+  if deq.mask == 0:
+    assert isPowerOfTwo(initialSize)
+    deq.mask = initialSize-1
+    newSeq(deq.data, initialSize)
+
 proc initDeque*[T](initialSize: int = 4): Deque[T] =
   ## Create a new empty deque.
   ##
@@ -73,15 +79,14 @@ proc initDeque*[T](initialSize: int = 4): Deque[T] =
   ## If you need to accept runtime values for this you could use the
   ## `nextPowerOfTwo proc<math.html#nextPowerOfTwo,int>`_ from the
   ## `math module<math.html>`_.
-  assert isPowerOfTwo(initialSize)
-  result.mask = initialSize-1
-  newSeq(result.data, initialSize)
+  result.initImpl(initialSize)
 
 proc len*[T](deq: Deque[T]): int {.inline.} =
   ## Return the number of elements of `deq`.
   result = deq.count
 
 template emptyCheck(deq) =
+  initImpl(deq)
   # Bounds check for the regular deque access.
   when compileOption("boundChecks"):
     if unlikely(deq.count < 1):
@@ -121,6 +126,7 @@ proc `[]`*[T](deq: var Deque[T], i: Natural): var T {.inline.} =
     assert a[3] == 40
     doAssertRaises(IndexError, echo a[8])
 
+  initImpl(deq)
   xBoundsCheck(deq, i)
   return deq.data[(deq.head + i) and deq.mask]
 
@@ -134,6 +140,7 @@ proc `[]=`*[T](deq: var Deque[T], i: Natural, val: T) {.inline.} =
     a[3] = 66
     assert $a == "[99, 20, 30, 66, 50]"
 
+  initImpl(deq)
   xBoundsCheck(deq, i)
   deq.data[(deq.head + i) and deq.mask] = val
 
@@ -164,6 +171,7 @@ proc `[]`*[T](deq: var Deque[T], i: BackwardsIndex): var T {.inline.} =
     assert a[^4] == 20
     doAssertRaises(IndexError, echo a[^9])
 
+  initImpl(deq)
   xBoundsCheck(deq, deq.len - int(i))
   return deq[deq.len - int(i)]
 
@@ -179,6 +187,7 @@ proc `[]=`*[T](deq: var Deque[T], i: BackwardsIndex, x: T) {.inline.} =
     a[^3] = 77
     assert $a == "[10, 20, 77, 40, 99]"
 
+  initImpl(deq)
   xBoundsCheck(deq, deq.len - int(i))
   deq[deq.len - int(i)] = x
 
@@ -256,6 +265,7 @@ proc contains*[T](deq: Deque[T], item: T): bool {.inline.} =
   return false
 
 proc expandIfNeeded[T](deq: var Deque[T]) =
+  initImpl(deq)
   var cap = deq.mask + 1
   if unlikely(deq.count >= cap):
     var n = newSeq[T](cap * 2)