Fix Array#replace on shifted arrays

spec/std/array_spec.cr

@@ -960,11 +960,51 @@ describe "Array" do
     end
   end
 
-  it "does replace" do
-    a = [1, 2, 3]
-    b = [1]
-    b.replace a
-    b.should eq(a)
+  describe "#replace" do
+    it "replaces all elements" do
+      a = [1, 2, 3]
+      b = [4, 5, 6]
+      a.replace(b).should be(a)
+      a.should eq(b)
+    end
+
+    it "reuses the buffer if possible" do
+      a = [1, 2, 3, 4, 5]
+      a.shift
+      b = [6, 7, 8, 9, 10]
+      a.replace(b).should be(a)
+      a.should eq(b)
+      a.@capacity.should eq(5)
+      a.@offset_to_buffer.should eq(0)
+
+      a = [1, 2, 3, 4, 5]
+      a.shift(2)
+      b = [6, 7, 8, 9]
+      a.replace(b).should be(a)
+      a.should eq(b)
+      a.@capacity.should eq(5)
+      a.@offset_to_buffer.should eq(1)
+    end
+
+    it "resizes the buffer if capacity is not enough" do
+      a = [1, 2, 3, 4, 5]
+      b = [6, 7, 8, 9, 10, 11]
+      a.replace(b).should be(a)
+      a.should eq(b)
+      a.@capacity.should eq(10)
+      a.@offset_to_buffer.should eq(0)
+    end
+
+    it "clears unused elements if new size is smaller" do
+      a = [1, 2, 3, 4, 5]
+      b = [6, 7, 8]
+      a.replace(b).should be(a)
+      a.should eq(b)
+      a.@capacity.should eq(5)
+      a.@offset_to_buffer.should eq(0)
+      a.unsafe_fetch(3).should eq(0)
+      a.unsafe_fetch(4).should eq(0)
+    end
   end
 
   it "does reverse with an odd number of elements" do

src/array.cr

@@ -1392,9 +1392,17 @@ class Array(T)
   # a2 # => [1, 2, 3]
   # ```
   def replace(other : Array) : self
-    @size = other.size
-    resize_to_capacity(Math.pw2ceil(@size)) if @size > @capacity
+    if other.size > @capacity
+      reset_buffer_to_root_buffer
+      resize_to_capacity(calculate_new_capacity(other.size))
+    elsif other.size > remaining_capacity
+      shift_buffer_by(remaining_capacity - other.size)
+    elsif other.size < @size
+      (@buffer + other.size).clear(@size - other.size)
+    end
+
     @buffer.copy_from(other.to_unsafe, other.size)
+    @size = other.size
     self
   end
 
@@ -2051,6 +2059,7 @@ class Array(T)
     @capacity - @offset_to_buffer
   end
 
+  # behaves like `calculate_new_capacity(@capacity + 1)`
   private def calculate_new_capacity
     return INITIAL_CAPACITY if @capacity == 0
 
@@ -2061,6 +2070,18 @@ class Array(T)
     end
   end
 
+  private def calculate_new_capacity(new_size)
+    new_capacity = @capacity == 0 ? INITIAL_CAPACITY : @capacity
+    while new_capacity < new_size
+      if new_capacity < CAPACITY_THRESHOLD
+        new_capacity *= 2
+      else
+        new_capacity += (new_capacity + 3 * CAPACITY_THRESHOLD) // 4
+      end
+    end
+    new_capacity
+  end
+
   private def increase_capacity
     resize_to_capacity(calculate_new_capacity)
   end