1 files changed, 89 insertions, 0 deletions
diff --git a/layout/base/Baseline.cpp b/layout/base/Baseline.cpp
new file mode 100644
index 0000000000..00166be9ef
--- /dev/null
+++ b/layout/base/Baseline.cpp
@@ -0,0 +1,89 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "Baseline.h"
+#include "nsIFrame.h"
+
+namespace mozilla {
+
+nscoord Baseline::SynthesizeBOffsetFromMarginBox(const nsIFrame* aFrame,
+                                                 WritingMode aWM,
+                                                 BaselineSharingGroup aGroup) {
+  MOZ_ASSERT(!aWM.IsOrthogonalTo(aFrame->GetWritingMode()));
+  auto margin = aFrame->GetLogicalUsedMargin(aWM);
+  if (aGroup == BaselineSharingGroup::First) {
+    if (aWM.IsAlphabeticalBaseline()) {
+      // First baseline for inverted-line content is the block-start margin
+      // edge, as the frame is in effect "flipped" for alignment purposes.
+      return MOZ_UNLIKELY(aWM.IsLineInverted())
+                 ? -margin.BStart(aWM)
+                 : aFrame->BSize(aWM) + margin.BEnd(aWM);
+    }
+    nscoord marginBoxCenter = (aFrame->BSize(aWM) + margin.BStartEnd(aWM)) / 2;
+    return marginBoxCenter - margin.BStart(aWM);
+  }
+  MOZ_ASSERT(aGroup == BaselineSharingGroup::Last);
+  if (aWM.IsAlphabeticalBaseline()) {
+    // Last baseline for inverted-line content is the block-start margin edge,
+    // as the frame is in effect "flipped" for alignment purposes.
+    return MOZ_UNLIKELY(aWM.IsLineInverted())
+               ? aFrame->BSize(aWM) + margin.BStart(aWM)
+               : -margin.BEnd(aWM);
+  }
+  // Round up for central baseline offset, to be consistent with ::First.
+  nscoord marginBoxSize = aFrame->BSize(aWM) + margin.BStartEnd(aWM);
+  nscoord marginBoxCenter = (marginBoxSize / 2) + (marginBoxSize % 2);
+  return marginBoxCenter - margin.BEnd(aWM);
+}
+
+nscoord Baseline::SynthesizeBOffsetFromBorderBox(const nsIFrame* aFrame,
+                                                 WritingMode aWM,
+                                                 BaselineSharingGroup aGroup) {
+  nscoord borderBoxSize =
+      MOZ_UNLIKELY(aWM.IsOrthogonalTo(aFrame->GetWritingMode()))
+          ? aFrame->ISize(aWM)
+          : aFrame->BSize(aWM);
+  if (aGroup == BaselineSharingGroup::First) {
+    return MOZ_LIKELY(aWM.IsAlphabeticalBaseline()) ? borderBoxSize
+                                                    : borderBoxSize / 2;
+  }
+  MOZ_ASSERT(aGroup == BaselineSharingGroup::Last);
+  // Round up for central baseline offset, to be consistent with ::First.
+  auto borderBoxCenter = (borderBoxSize / 2) + (borderBoxSize % 2);
+  return MOZ_LIKELY(aWM.IsAlphabeticalBaseline()) ? 0 : borderBoxCenter;
+}
+
+nscoord Baseline::SynthesizeBOffsetFromContentBox(const nsIFrame* aFrame,
+                                                  WritingMode aWM,
+                                                  BaselineSharingGroup aGroup) {
+  WritingMode wm = aFrame->GetWritingMode();
+  MOZ_ASSERT(!aWM.IsOrthogonalTo(wm));
+  const auto bp = aFrame->GetLogicalUsedBorderAndPadding(wm)
+                      .ApplySkipSides(aFrame->GetLogicalSkipSides())
+                      .ConvertTo(aWM, wm);
+
+  if (MOZ_UNLIKELY(aWM.IsCentralBaseline())) {
+    nscoord contentBoxBSize = aFrame->BSize(aWM) - bp.BStartEnd(aWM);
+    if (aGroup == BaselineSharingGroup::First) {
+      return contentBoxBSize / 2 + bp.BStart(aWM);
+    }
+    // Return the same center position as for ::First, but as offset from end:
+    nscoord halfContentBoxBSize = (contentBoxBSize / 2) + (contentBoxBSize % 2);
+    return halfContentBoxBSize + bp.BEnd(aWM);
+  }
+  if (aGroup == BaselineSharingGroup::First) {
+    // First baseline for inverted-line content is the block-start content
+    // edge, as the frame is in effect "flipped" for alignment purposes.
+    return MOZ_UNLIKELY(aWM.IsLineInverted())
+               ? bp.BStart(aWM)
+               : aFrame->BSize(aWM) - bp.BEnd(aWM);
+  }
+  // Last baseline for inverted-line content is the block-start content edge,
+  // as the frame is in effect "flipped" for alignment purposes.
+  return MOZ_UNLIKELY(aWM.IsLineInverted())
+             ? aFrame->BSize(aWM) - bp.BStart(aWM)
+             : bp.BEnd(aWM);
+}
+
+}  // namespace mozilla