Implement relative utilities

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

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "core/layout/ng/ng_relative_utils.h"
+
+#include "core/layout/ng/geometry/ng_logical_offset.h"
+#include "core/layout/ng/geometry/ng_physical_size.h"
+#include "core/style/ComputedStyle.h"
+#include "platform/LengthFunctions.h"
+#include "wtf/Optional.h"
+
+namespace blink {
+
+// Returns child's relative position wrt containing fragment.

[ikilpatrick, 2017/03/21 17:51:17]

"Returns the child's relative position wrt the containing fragment."

[atotic, 2017/03/21 19:13:54]

done. 

+NGLogicalOffset ComputeRelativeOffset(const ComputedStyle& child_style,
+                                      NGWritingMode container_writing_mode,
+                                      TextDirection container_direction,
+                                      NGLogicalSize container_logical_size) {
+  NGLogicalOffset offset;
+  NGPhysicalSize container_size =
+      container_logical_size.ConvertToPhysical(container_writing_mode);
+
+  Optional<LayoutUnit> left, right, top, bottom;
+
+  if (!child_style.left().isAuto())
+    left = valueForLength(child_style.left(), container_size.width);
+  if (!child_style.right().isAuto())
+    right = valueForLength(child_style.right(), container_size.width);
+  if (!child_style.top().isAuto())
+    top = valueForLength(child_style.top(), container_size.height);
+  if (!child_style.bottom().isAuto())
+    bottom = valueForLength(child_style.bottom(), container_size.height);
+
+  // Implements confict resolution rules from spec:
+  // https://www.w3.org/TR/css-position-3/#rel-pos
+  if (!left && !right) {
+    left = LayoutUnit();
+    right = LayoutUnit();
+  }
+  if (!left)
+    left = -right.value();
+  if (!right)
+    right = -left.value();
+  if (!top && !bottom) {
+    top = LayoutUnit();
+    bottom = LayoutUnit();
+  }
+  if (!top)
+    top = -bottom.value();
+  if (!bottom)
+    bottom = -top.value();
+
+  bool is_ltr = container_direction == TextDirection::kLtr;
+
+  switch (container_writing_mode) {
+    case kHorizontalTopBottom:
+      offset.inline_offset = is_ltr ? left.value() : right.value();
+      offset.block_offset = top.value();
+      break;
+    case kVerticalRightLeft:
+    case kSidewaysRightLeft:
+      offset.inline_offset = is_ltr ? top.value() : bottom.value();
+      offset.block_offset = right.value();
+      break;
+    case kVerticalLeftRight:
+      offset.inline_offset = is_ltr ? top.value() : bottom.value();
+      offset.block_offset = left.value();
+      break;
+    case kSidewaysLeftRight:
+      offset.inline_offset = is_ltr ? bottom.value() : top.value();
+      offset.block_offset = left.value();
+      break;
+  }
+  return offset;
+}
+
+}  // namespace blink