Fix Array(T)#[]=(Int, Int, Array(T)) on shifted arrays

spec/std/array_spec.cr

@@ -445,6 +445,24 @@ describe "Array" do
       a[3..] = [4, 5, 6]
       a.should eq([1, 2, 3, 4, 5, 6])
     end
+
+    it "reuses the buffer if possible" do
+      a = [1, 2, 3, 4, 5]
+      a.pop
+      a[4, 0] = [6]
+      a.should eq([1, 2, 3, 4, 6])
+      a.@capacity.should eq(5)
+      a.@offset_to_buffer.should eq(0)
+    end
+
+    it "resizes the buffer if capacity is not enough" do
+      a = [1, 2, 3, 4, 5]
+      a.shift
+      a[4, 0] = [6, 7, 8, 9]
+      a.should eq([2, 3, 4, 5, 6, 7, 8, 9])
+      a.@capacity.should eq(10)
+      a.@offset_to_buffer.should eq(1)
+    end
   end
 
   describe "values_at" do

src/array.cr

@@ -531,7 +531,7 @@ class Array(T)
       @size -= diff
     else
       # Need to grow
-      resize_to_capacity(Math.pw2ceil(@size + diff))
+      resize_if_cant_insert(diff)
       (@buffer + start + values.size).move_from(@buffer + start + count, size - start - count)
       (@buffer + start).copy_from(values.to_unsafe, values.size)
       @size += diff
@@ -2071,6 +2071,10 @@ class Array(T)
   end
 
   private def calculate_new_capacity(new_size)
+    # Resizing is done via `Pointer#realloc` on the root buffer, so the space
+    # between the root and real buffers remains untouched
+    new_size += @offset_to_buffer
+
     new_capacity = @capacity == 0 ? INITIAL_CAPACITY : @capacity
     while new_capacity < new_size
       if new_capacity < CAPACITY_THRESHOLD
@@ -2118,13 +2122,11 @@ class Array(T)
   end
 
   private def resize_if_cant_insert(insert_size)
-    # Resize if we exceed the remaining capacity.
-    # `remaining_capacity - @size` is the actual number of slots we have
-    # to push new elements.
-    if insert_size > remaining_capacity - @size
-      # The new capacity that we need is what we already have occupied
-      # because of shift (`@offset_to_buffer`) plus my size plus the insert size.
-      resize_to_capacity(Math.pw2ceil(@offset_to_buffer + @size + insert_size))
+    # Resize if we exceed the remaining capacity. This is less than `@capacity`
+    # if the array has been shifted and `@offset_to_buffer` is nonzero
+    new_size = @size + insert_size
+    if new_size > remaining_capacity
+      resize_to_capacity(calculate_new_capacity(new_size))
     end
   end