Initial implementation of Collapsing Margins computational logic for LayoutNG

third_party/WebKit/Source/core/layout/ng/ng_block_layout_algorithm.cc

Index: third_party/WebKit/Source/core/layout/ng/ng_block_layout_algorithm.cc
diff --git a/third_party/WebKit/Source/core/layout/ng/ng_block_layout_algorithm.cc b/third_party/WebKit/Source/core/layout/ng/ng_block_layout_algorithm.cc
index 9082202d9d96b05d55c48895f491046eab5438f6..ae4722e5639624aede739e87e0b97c50998941fa 100644
--- a/third_party/WebKit/Source/core/layout/ng/ng_block_layout_algorithm.cc
+++ b/third_party/WebKit/Source/core/layout/ng/ng_block_layout_algorithm.cc
@@ -14,6 +14,18 @@
 #include "platform/LengthFunctions.h"
 
 namespace blink {
+namespace {
+
+LayoutUnit ComputeCollapsedMarginBlockStart(
+    const NGMarginStrut& prev_margin_strut,
+    const NGMarginStrut& curr_margin_strut) {
+  return std::max(prev_margin_strut.margin_block_end,
+                  curr_margin_strut.margin_block_start) -
+         std::max(prev_margin_strut.negative_margin_block_end.abs(),
+                  curr_margin_strut.negative_margin_block_start.abs());
+}
+
+}  // namespace
 
 NGBlockLayoutAlgorithm::NGBlockLayoutAlgorithm(
     PassRefPtr<const ComputedStyle> style,
@@ -50,13 +62,16 @@ bool NGBlockLayoutAlgorithm::Layout(const NGConstraintSpace* constraint_space,
           return false;
         NGBoxStrut child_margins = computeMargins(
             *constraint_space_for_children_, *current_child_->Style());
+
+        LayoutUnit margin_block_start =
+            CollapseMargins(child_margins, fragment->MarginStrut());
+
         // TODO(layout-ng): Support auto margins
-        builder_->AddChild(
-            fragment,
-            NGLogicalOffset(child_margins.inline_start,
-                            content_size_ + child_margins.block_start));
+        builder_->AddChild(fragment,
+                           NGLogicalOffset(child_margins.inline_start,
+                                           content_size_ + margin_block_start));
 
-        content_size_ += fragment->BlockSize() + child_margins.BlockSum();
+        content_size_ += fragment->BlockSize() + margin_block_start;
         max_inline_size_ =
             std::max(max_inline_size_,
                      fragment->InlineSize() + child_margins.InlineSum());
@@ -64,7 +79,6 @@ bool NGBlockLayoutAlgorithm::Layout(const NGConstraintSpace* constraint_space,
         if (current_child_)
           return false;
       }
-
       state_ = kStateFinalize;
       return false;
     }
@@ -86,4 +100,31 @@ bool NGBlockLayoutAlgorithm::Layout(const NGConstraintSpace* constraint_space,
   return true;
 }
 
+LayoutUnit NGBlockLayoutAlgorithm::CollapseMargins(
+    const NGBoxStrut& margins,
+    const NGMarginStrut& children_margin_strut) {
+  // Calculate margin strut for the current child.
+  NGMarginStrut curr_margin_strut = children_margin_strut;
+  curr_margin_strut.AppendMarginBlockStart(margins.block_start);
+  curr_margin_strut.AppendMarginBlockEnd(margins.block_end);
+
+  // Set the margin strut for the resultant fragment if this is the first or
+  // last child fragment.
+  if (current_child_ == first_child_)
+    builder_->SetMarginStrutBlockStart(curr_margin_strut);
+  if (!current_child_->NextSibling())
+    builder_->SetMarginStrutBlockEnd(curr_margin_strut);
+
+  // Compute the margin block start for adjoining blocks.
+  LayoutUnit margin_block_start;
+  if (current_child_ != first_child_)
+    margin_block_start = ComputeCollapsedMarginBlockStart(
+        prev_child_margin_strut_, curr_margin_strut);
+
+  prev_child_margin_strut_ = curr_margin_strut;
+  // TODO(layout-ng): support other Margin Collapsing use cases,
+  // i.e. support 0 height elements etc.
+  return margin_block_start;
+}
+
 }  // namespace blink