CSS Grid algorithm 2/N: auto-placement

Summary:
Implements the CSS Grid auto placement algorithm ([§8.5](https://www.w3.org/TR/css-grid-1/#auto-placement-algo)).

## How?
- **AutoPlacement** — the four-step placement algorithm.
- **LinePlacement** — grid-line resolution ([§8.3](https://www.w3.org/TR/css-grid-1/#grid-placement-errors)), including invalid-input handling: `line 0` is treated as `auto`, and `span 0` / negative spans clamp to `1`.
- **OccupancyGrid** — overlap tracking for placed items.
- Unit tests (line resolution), algorithm tests, and regression tests for the invalid-input cases.

## Not yet implemented
These will be introduced later. Default `grid-auto-flow` is `row` so it won't be an issue.

- **Dense packing** (`grid-auto-flow: dense`)
- **Column flow** (`grid-auto-flow: column`)

## Notes
- Stacked on react/yoga#1923 (data structures); until that lands this diff also shows its `GridItem.h` / `GridTrack.h` — those belong to react/yoga#1923.

cc - rozele This a small chunk from react/yoga#1894 PR.

X-link: react/yoga#1994

Differential Revision: D111722622

Pulled By: rozele

Author: intergalacticspacehighway

packages/react-native/ReactCommon/yoga/yoga/CMakeLists.txt

@@ -20,7 +20,8 @@ include(${YOGA_ROOT}/cmake/project-defaults.cmake)
 
 file(GLOB SOURCES CONFIGURE_DEPENDS
     ${CMAKE_CURRENT_SOURCE_DIR}/*.cpp
-    ${CMAKE_CURRENT_SOURCE_DIR}/**/*.cpp)
+    ${CMAKE_CURRENT_SOURCE_DIR}/**/*.cpp
+    ${CMAKE_CURRENT_SOURCE_DIR}/algorithm/grid/*.cpp)
 
 add_library(yogacore STATIC ${SOURCES})
 

packages/react-native/ReactCommon/yoga/yoga/algorithm/grid/AutoPlacement.cpp

@@ -0,0 +1,331 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#include <yoga/algorithm/grid/AutoPlacement.h>
+
+#include <yoga/algorithm/grid/LinePlacement.h>
+#include <yoga/algorithm/grid/OccupancyGrid.h>
+#include <yoga/debug/AssertFatal.h>
+#include <yoga/node/Node.h>
+#include <yoga/style/GridLine.h>
+
+#include <algorithm>
+#include <array>
+#include <cstddef>
+#include <cstdint>
+#include <unordered_map>
+#include <unordered_set>
+#include <utility>
+#include <vector>
+
+namespace facebook::yoga {
+
+namespace {
+
+bool isPlaceable(const Node* child) {
+  return child->style().positionType() != PositionType::Absolute &&
+      child->style().display() != Display::None;
+}
+
+} // namespace
+
+// 8.5. Grid Item Placement Algorithm
+// https://www.w3.org/TR/css-grid-1/#auto-placement-algo
+AutoPlacement AutoPlacement::performAutoPlacement(Node* node) {
+  std::vector<AutoPlacementItem> items;
+  items.reserve(node->getChildCount());
+  std::unordered_set<Node*> placedItems;
+  placedItems.reserve(node->getChildCount());
+
+  int32_t minColumnStart = 0;
+  int32_t minRowStart = 0;
+  int32_t maxColumnEnd =
+      static_cast<int32_t>(node->style().gridTemplateColumns().size());
+  int32_t maxRowEnd =
+      static_cast<int32_t>(node->style().gridTemplateRows().size());
+  OccupancyGrid occupancy;
+  occupancy.occupied.reserve(node->getChildCount());
+
+  // Records a placed grid area and updates the running grid bounds.
+  const auto recordGridArea = [&](const AutoPlacementItem& area) {
+    yoga::assertFatal(
+        area.columnEnd > area.columnStart,
+        "Grid item column end must be greater than column start");
+    yoga::assertFatal(
+        area.rowEnd > area.rowStart,
+        "Grid item row end must be greater than row start");
+    items.push_back(area);
+    placedItems.insert(area.node);
+    occupancy.markOccupied(
+        area.rowStart, area.rowEnd, area.columnStart, area.columnEnd);
+    minColumnStart = std::min(minColumnStart, area.columnStart);
+    minRowStart = std::min(minRowStart, area.rowStart);
+    maxColumnEnd = std::max(maxColumnEnd, area.columnEnd);
+    maxRowEnd = std::max(maxRowEnd, area.rowEnd);
+  };
+
+  const auto explicitColumnLineCount =
+      static_cast<int32_t>(node->style().gridTemplateColumns().size() + 1);
+  const auto explicitRowLineCount =
+      static_cast<int32_t>(node->style().gridTemplateRows().size() + 1);
+
+  // Step 1: Position anything that is not auto-positioned. In level 1 a span is
+  // always definite (default 1), so an item is definitely positioned when both
+  // its row and column have at least one integer line.
+  for (Node* child : node->getLayoutChildren()) {
+    if (!isPlaceable(child)) {
+      continue;
+    }
+
+    const auto& gridItemColumnStart = child->style().gridColumnStart();
+    const auto& gridItemColumnEnd = child->style().gridColumnEnd();
+    const auto& gridItemRowStart = child->style().gridRowStart();
+    const auto& gridItemRowEnd = child->style().gridRowEnd();
+    const bool hasDefiniteColumn = isDefiniteLine(gridItemColumnStart) ||
+        isDefiniteLine(gridItemColumnEnd);
+    const bool hasDefiniteRow =
+        isDefiniteLine(gridItemRowStart) || isDefiniteLine(gridItemRowEnd);
+
+    if (hasDefiniteColumn && hasDefiniteRow) {
+      const auto columnPlacement = resolveLinePlacement(
+          gridItemColumnStart, gridItemColumnEnd, explicitColumnLineCount);
+      const auto rowPlacement = resolveLinePlacement(
+          gridItemRowStart, gridItemRowEnd, explicitRowLineCount);
+
+      recordGridArea(
+          AutoPlacementItem{
+              .columnStart = columnPlacement.start,
+              .columnEnd = columnPlacement.end,
+              .rowStart = rowPlacement.start,
+              .rowEnd = rowPlacement.end,
+              .node = child});
+    }
+  }
+
+  // Step 2: Process the items locked to a given row (definite row, no definite
+  // column), placing each into the first non-overlapping slot to its right.
+  std::unordered_map<int32_t, int32_t> rowStartToColumnStartCache;
+  for (Node* child : node->getLayoutChildren()) {
+    if (!isPlaceable(child)) {
+      continue;
+    }
+
+    const auto& gridItemColumnStart = child->style().gridColumnStart();
+    const auto& gridItemColumnEnd = child->style().gridColumnEnd();
+    const auto& gridItemRowStart = child->style().gridRowStart();
+    const auto& gridItemRowEnd = child->style().gridRowEnd();
+    const bool hasDefiniteColumn = isDefiniteLine(gridItemColumnStart) ||
+        isDefiniteLine(gridItemColumnEnd);
+    const bool hasDefiniteRow =
+        isDefiniteLine(gridItemRowStart) || isDefiniteLine(gridItemRowEnd);
+
+    if (hasDefiniteRow && !hasDefiniteColumn) {
+      const auto rowPlacement = resolveLinePlacement(
+          gridItemRowStart, gridItemRowEnd, explicitRowLineCount);
+      const auto rowStart = rowPlacement.start;
+      const auto rowEnd = rowPlacement.end;
+
+      const auto columnSpan =
+          resolveLinePlacement(
+              gridItemColumnStart, gridItemColumnEnd, explicitColumnLineCount)
+              .span;
+
+      auto columnStart = rowStartToColumnStartCache.contains(rowStart)
+          ? rowStartToColumnStartCache[rowStart]
+          : minColumnStart;
+      auto columnEnd = columnStart + columnSpan;
+
+      while (const auto* overlap = occupancy.hasOverlap(
+                 rowStart, rowEnd, columnStart, columnEnd)) {
+        columnStart = overlap->columnEnd;
+        columnEnd = columnStart + columnSpan;
+      }
+
+      recordGridArea(
+          AutoPlacementItem{
+              .columnStart = columnStart,
+              .columnEnd = columnEnd,
+              .rowStart = rowStart,
+              .rowEnd = rowEnd,
+              .node = child});
+      rowStartToColumnStartCache[rowStart] = columnEnd;
+    }
+  }
+
+  // Step 3: Determine the columns of the implicit grid, accounting for definite
+  // columns that extend the grid and for the largest auto-placed column span.
+  auto largestColumnSpan = 1;
+  for (Node* child : node->getLayoutChildren()) {
+    if (!isPlaceable(child)) {
+      continue;
+    }
+
+    const auto& gridItemColumnStart = child->style().gridColumnStart();
+    const auto& gridItemColumnEnd = child->style().gridColumnEnd();
+    const bool hasDefiniteColumn = isDefiniteLine(gridItemColumnStart) ||
+        isDefiniteLine(gridItemColumnEnd);
+
+    const auto columnPlacement = resolveLinePlacement(
+        gridItemColumnStart, gridItemColumnEnd, explicitColumnLineCount);
+    if (hasDefiniteColumn) {
+      minColumnStart = std::min(minColumnStart, columnPlacement.start);
+      maxColumnEnd = std::max(maxColumnEnd, columnPlacement.end);
+    } else {
+      largestColumnSpan = std::max(largestColumnSpan, columnPlacement.span);
+    }
+  }
+
+  // If the largest span is wider than the current grid, extend the end.
+  const auto currentGridWidth = maxColumnEnd - minColumnStart;
+  if (largestColumnSpan > currentGridWidth) {
+    maxColumnEnd = minColumnStart + largestColumnSpan;
+  }
+
+  // Step 4: Position the remaining grid items using the auto-placement cursor.
+  std::array<int32_t, 2> autoPlacementCursor = {minColumnStart, minRowStart};
+  for (Node* child : node->getLayoutChildren()) {
+    if (!isPlaceable(child) || placedItems.contains(child)) {
+      continue;
+    }
+
+    const auto& gridItemColumnStart = child->style().gridColumnStart();
+    const auto& gridItemColumnEnd = child->style().gridColumnEnd();
+    const auto& gridItemRowStart = child->style().gridRowStart();
+    const auto& gridItemRowEnd = child->style().gridRowEnd();
+    const bool hasDefiniteColumn = isDefiniteLine(gridItemColumnStart) ||
+        isDefiniteLine(gridItemColumnEnd);
+    const bool hasDefiniteRow =
+        isDefiniteLine(gridItemRowStart) || isDefiniteLine(gridItemRowEnd);
+
+    const auto columnPlacement = resolveLinePlacement(
+        gridItemColumnStart, gridItemColumnEnd, explicitColumnLineCount);
+    const auto rowPlacement = resolveLinePlacement(
+        gridItemRowStart, gridItemRowEnd, explicitRowLineCount);
+
+    // If the item has a definite column position, keep it and search downward
+    // for a row where it does not overlap an occupied cell.
+    if (hasDefiniteColumn) {
+      const auto columnStart = columnPlacement.start;
+      const auto columnEnd = columnPlacement.end;
+
+      // Set the cursor to the item's column-start line; if that moves the
+      // cursor backwards, advance to the next row.
+      const auto previousColumnPosition = autoPlacementCursor[0];
+      autoPlacementCursor[0] = columnStart;
+      if (autoPlacementCursor[0] < previousColumnPosition) {
+        autoPlacementCursor[1]++;
+      }
+
+      const auto rowSpan = rowPlacement.span;
+      while (const auto* overlap = occupancy.hasOverlap(
+                 autoPlacementCursor[1],
+                 autoPlacementCursor[1] + rowSpan,
+                 columnStart,
+                 columnEnd)) {
+        autoPlacementCursor[1] = overlap->rowEnd;
+      }
+
+      recordGridArea(
+          AutoPlacementItem{
+              .columnStart = columnStart,
+              .columnEnd = columnEnd,
+              .rowStart = autoPlacementCursor[1],
+              .rowEnd = autoPlacementCursor[1] + rowSpan,
+              .node = child});
+    }
+    // If the item has an automatic position in both axes, sweep the cursor
+    // left-to-right then top-to-bottom until it fits.
+    else if (!hasDefiniteRow) {
+      const auto itemColumnSpan = columnPlacement.span;
+      const auto itemRowSpan = rowPlacement.span;
+
+      bool foundPosition = false;
+      while (!foundPosition) {
+        while (autoPlacementCursor[0] + itemColumnSpan <= maxColumnEnd) {
+          const auto columnStart = autoPlacementCursor[0];
+          const auto columnEnd = columnStart + itemColumnSpan;
+          const auto rowStart = autoPlacementCursor[1];
+          const auto rowEnd = rowStart + itemRowSpan;
+
+          if (const auto* overlap = occupancy.hasOverlap(
+                  rowStart, rowEnd, columnStart, columnEnd)) {
+            autoPlacementCursor[0] = overlap->columnEnd;
+          } else {
+            recordGridArea(
+                AutoPlacementItem{
+                    .columnStart = columnStart,
+                    .columnEnd = columnEnd,
+                    .rowStart = rowStart,
+                    .rowEnd = rowEnd,
+                    .node = child});
+            foundPosition = true;
+            break;
+          }
+        }
+
+        if (!foundPosition) {
+          // Cursor overflowed the grid width; move to the next row.
+          autoPlacementCursor[1]++;
+          autoPlacementCursor[0] = minColumnStart;
+        }
+      }
+    }
+  }
+
+  return AutoPlacement{
+      .gridItems = std::move(items),
+      .minColumnStart = minColumnStart,
+      .minRowStart = minRowStart,
+      .maxColumnEnd = maxColumnEnd,
+      .maxRowEnd = maxRowEnd};
+}
+
+// 1. Runs the grid placement algorithm and normalizes the output into the
+// 0-based coordinate space.
+// 2. Builds the baseline sharing groups for each row.
+ResolvedAutoPlacement ResolvedAutoPlacement::resolveGridItemPlacements(
+    Node* node) {
+  const AutoPlacement autoPlacement = AutoPlacement::performAutoPlacement(node);
+
+  const int32_t minColumnStart = autoPlacement.minColumnStart;
+  const int32_t minRowStart = autoPlacement.minRowStart;
+
+  ResolvedAutoPlacement resolved;
+  resolved.minColumnStart = minColumnStart;
+  resolved.minRowStart = minRowStart;
+  resolved.maxColumnEnd = autoPlacement.maxColumnEnd;
+  resolved.maxRowEnd = autoPlacement.maxRowEnd;
+
+  resolved.gridItems.reserve(autoPlacement.gridItems.size());
+
+  const Align alignItems = node->style().alignItems();
+
+  for (const auto& placement : autoPlacement.gridItems) {
+    resolved.gridItems.emplace_back(
+        static_cast<size_t>(placement.columnStart - minColumnStart),
+        static_cast<size_t>(placement.columnEnd - minColumnStart),
+        static_cast<size_t>(placement.rowStart - minRowStart),
+        static_cast<size_t>(placement.rowEnd - minRowStart),
+        placement.node);
+
+    GridItem& item = resolved.gridItems.back();
+    Align alignSelf = item.node->style().alignSelf();
+    if (alignSelf == Align::Auto) {
+      alignSelf = alignItems;
+    }
+    // https://www.w3.org/TR/css-grid-1/#algo-baseline-shims - only items that
+    // span a single row participate in a row's baseline sharing group.
+    const bool spansOneRow = (item.rowEnd - item.rowStart) == 1;
+    if (alignSelf == Align::Baseline && spansOneRow) {
+      resolved.baselineItemGroups[item.rowStart].push_back(&item);
+    }
+  }
+
+  return resolved;
+}
+
+} // namespace facebook::yoga