Alter layout engine to conform closer to W3C spec

The primary goals of this change are:
  - Better conformance to the W3C flexbox standard (https://www.w3.org/TR/css-flexbox-1/)
    and a clear articulation of the areas where it deviates from the spec.
  - Support for flex-shrink.
  - Conformance with layout effects of "overflow: hidden".

Specifically, here are the limitations of this implementation as compared to the W3C
flexbox standard (this is also documented in Layout.js):
  - Display property is always assumed to be 'flex' except for Text nodes, which
    are assumed to be 'inline-flex'.
  - The 'zIndex' property (or any form of z ordering) is not supported. Nodes are
    stacked in document order.
  - The 'order' property is not supported. The order of flex items is always defined
    by document order.
  - The 'visibility' property is always assumed to be 'visible'. Values of 'collapse'
    and 'hidden' are not supported.
  - The 'wrap' property supports only 'nowrap' (which is the default) or 'wrap'. The
    rarely-used 'wrap-reverse' is not supported.
  - Rather than allowing arbitrary combinations of flexGrow, flexShrink and
    flexBasis, this algorithm supports only the three most common combinations:
      - flex: 0 is equiavlent to flex: 0 0 auto
      - flex: n (where n is a positive value) is equivalent to flex: n 0 0
      - flex: -1 (or any negative value) is equivalent to flex: 0 1 auto
  - Margins cannot be specified as 'auto'. They must be specified in terms of pixel
    values, and the default value is 0.
  - The 'baseline' value is not supported for alignItems and alignSelf properties.
  - Values of width, maxWidth, minWidth, height, maxHeight and minHeight must be
    specified as pixel values, not as percentages.
  - There is no support for calculation of dimensions based on intrinsic aspect ratios
    (e.g. images).
  - There is no support for forced breaks.
  - It does not support vertical inline directions (top-to-bottom or bottom-to-top text).

And here is how the implementation deviates from the standard (this is also documented in
Layout.js):
  - Section 4.5 of the spec indicates that all flex items have a default minimum
    main size. For text blocks, for example, this is the width of the widest word.
    Calculating the minimum width is expensive, so we forego it and assume a default
    minimum main size of 0.
  - Min/Max sizes in the main axis are not honored when resolving flexible lengths.
  - The spec indicates that the default value for 'flexDirection' is 'row', but
    the algorithm below assumes a default of 'column'.

src/java/src/com/facebook/csslayout/CSSCachedMeasurement.java

@@ -0,0 +1,19 @@
+/**
+ * Copyright (c) 2014, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+package com.facebook.csslayout;
+
+public class CSSCachedMeasurement {
+  public float availableWidth;
+  public float availableHeight;
+  public CSSMeasureMode widthMeasureMode = null;
+  public CSSMeasureMode heightMeasureMode = null;
+  
+  public float computedWidth;
+  public float computedHeight;
+}

src/java/src/com/facebook/csslayout/CSSLayout.java

@@ -14,6 +14,10 @@ import java.util.Arrays;
  * Where the output of {@link LayoutEngine#layoutNode(CSSNode, float)} will go in the CSSNode.
  */
 public class CSSLayout {
+  // This value was chosen based on empiracle data. Even the most complicated
+  // layouts should not require more than 16 entries to fit within the cache.
+  public static final int MAX_CACHED_RESULT_COUNT = 16;
+  
   public static final int POSITION_LEFT = 0;
   public static final int POSITION_TOP = 1;
   public static final int POSITION_RIGHT = 2;
@@ -25,24 +29,38 @@ public class CSSLayout {
   public float[] position = new float[4];
   public float[] dimensions = new float[2];
   public CSSDirection direction = CSSDirection.LTR;
-
-  /**
-   * This should always get called before calling {@link LayoutEngine#layoutNode(CSSNode, float)}
-   */
+  
+  public float flexBasis;
+  
+  public int generationCount;
+  public CSSDirection lastParentDirection;
+  
+  public int nextCachedMeasurementsIndex;
+  public CSSCachedMeasurement[] cachedMeasurements = new CSSCachedMeasurement[MAX_CACHED_RESULT_COUNT];
+  public float[] measuredDimensions = new float[2];
+  
+  public CSSCachedMeasurement cachedLayout = new CSSCachedMeasurement();
+  
+  CSSLayout() {
+    resetResult();
+  }
+  
   public void resetResult() {
     Arrays.fill(position, 0);
     Arrays.fill(dimensions, CSSConstants.UNDEFINED);
     direction = CSSDirection.LTR;
-  }
-
-  public void copy(CSSLayout layout) {
-    position[POSITION_LEFT] = layout.position[POSITION_LEFT];
-    position[POSITION_TOP] = layout.position[POSITION_TOP];
-    position[POSITION_RIGHT] = layout.position[POSITION_RIGHT];
-    position[POSITION_BOTTOM] = layout.position[POSITION_BOTTOM];
-    dimensions[DIMENSION_WIDTH] = layout.dimensions[DIMENSION_WIDTH];
-    dimensions[DIMENSION_HEIGHT] = layout.dimensions[DIMENSION_HEIGHT];
-    direction = layout.direction;
+    
+    flexBasis = 0;
+    
+    generationCount = 0;
+    lastParentDirection = null;
+    
+    nextCachedMeasurementsIndex = 0;
+    measuredDimensions[DIMENSION_WIDTH] = CSSConstants.UNDEFINED;
+    measuredDimensions[DIMENSION_HEIGHT] = CSSConstants.UNDEFINED;
+    
+    cachedLayout.widthMeasureMode = null;
+    cachedLayout.heightMeasureMode = null;
   }
 
   @Override

src/java/src/com/facebook/csslayout/CSSLayoutContext.java

@@ -17,4 +17,5 @@ package com.facebook.csslayout;
  */
 public class CSSLayoutContext {
   /*package*/ final MeasureOutput measureOutput = new MeasureOutput();
+  int currentGenerationCount;
 }

src/java/src/com/facebook/csslayout/CSSNode.java

@@ -63,9 +63,8 @@ public class CSSNode {
 
   public int lineIndex = 0;
 
-  /*package*/ CSSNode nextAbsoluteChild;
-  /*package*/ CSSNode nextFlexChild;
-
+  /*package*/ CSSNode nextChild;
+  
   private @Nullable ArrayList<CSSNode> mChildren;
   private @Nullable CSSNode mParent;
   private @Nullable MeasureFunction mMeasureFunction = null;
@@ -139,7 +138,6 @@ public class CSSNode {
    * Performs the actual layout and saves the results in {@link #layout}
    */
   public void calculateLayout(CSSLayoutContext layoutContext) {
-    layout.resetResult();
     LayoutEngine.layoutNode(layoutContext, this, CSSConstants.UNDEFINED, CSSConstants.UNDEFINED, null);
   }
 

src/java/src/com/facebook/csslayout/CSSOverflow.java

@@ -0,0 +1,14 @@
+/**
+ * Copyright (c) 2014, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+package com.facebook.csslayout;
+
+public enum CSSOverflow {
+  VISIBLE,
+  HIDDEN,
+}

src/java/src/com/facebook/csslayout/CSSStyle.java

@@ -23,6 +23,7 @@ public class CSSStyle {
   public CSSAlign alignSelf;
   public CSSPositionType positionType;
   public CSSWrap flexWrap;
+  public CSSOverflow overflow;
   public float flex;
 
   public Spacing margin = new Spacing();
@@ -51,6 +52,7 @@ public class CSSStyle {
     alignSelf = CSSAlign.AUTO;
     positionType = CSSPositionType.RELATIVE;
     flexWrap = CSSWrap.NOWRAP;
+    overflow = CSSOverflow.VISIBLE;
     flex = 0f;
 
     margin.reset();;

src/java/tests/com/facebook/csslayout/LayoutCachingTest.java

@@ -43,7 +43,7 @@ public class LayoutCachingTest {
     root.addChildAt(c0, 0);
     root.addChildAt(c1, 1);
     c0.addChildAt(c0c0, 0);
-
+    
     root.calculateLayout(layoutContext);
     assertTreeHasNewLayout(true, root);
     markLayoutAppliedForTree(root);