Bug 613449. Extend GetLargestRectangle to support considering only rectangles that contain a given rectangle. r=robarnold

gfx/src/nsRegion.cpp

@@ -1339,12 +1339,22 @@ nsIntRegion nsRegion::ToOutsidePixels (nscoord aAppUnitsPerPixel) const
   return result;
 }
 
-// This algorithm works in three phases:
+// A cell's "value" is a pair consisting of
+// a) the area of the subrectangle it corresponds to, if it's in
+// aContainingRect and in the region, 0 otherwise
+// b) the area of the subrectangle it corresponds to, if it's in the region,
+// 0 otherwise
+// Addition, subtraction and identity are defined on these values in the
+// obvious way. Partial order is lexicographic.
+// A "large negative value" is defined with large negative numbers for both
+// fields of the pair. This negative value has the property that adding any
+// number of non-negative values to it always results in a negative value.
+//
+// The GetLargestRectangle algorithm works in three phases:
 //  1) Convert the region into a grid by adding vertical/horizontal lines for
 //     each edge of each rectangle in the region.
 //  2) For each rectangle in the region, for each cell it contains, set that
-//     cells's value to the area of the subrectangle it corresponds to. Cells
-//     that are not contained by any rectangle have the value 0.
+//     cells's value as described above.
 //  3) Calculate the submatrix with the largest sum such that none of its cells
 //     contain any 0s (empty regions). The rectangle represented by the
 //     submatrix is the largest rectangle in the region.
@@ -1370,7 +1380,7 @@ nsIntRegion nsRegion::ToOutsidePixels (nscoord aAppUnitsPerPixel) const
 //   S = array(m+1,n+1)
 //   S[0][i] = 0 for i in [0,n]
 //   S[j][0] = 0 for j in [0,m]
-//   S[j][i] = (if A[j-1][i-1] = 0 then some large negative number else A[j-1][i-1])
+//   S[j][i] = (if A[j-1][i-1] = 0 then some large negative value else A[j-1][i-1])
 //           + S[j-1][n] + S[j][i-1] - S[j-1][i-1]
 //
 //   // top, bottom, left, right, area
@@ -1452,13 +1462,52 @@ namespace {
 
   const PRInt64 kVeryLargeNegativeNumber = 0xffff000000000000ll;
 
+  struct SizePair {
+    PRInt64 mSizeContainingRect;
+    PRInt64 mSize;
+
+    SizePair() : mSizeContainingRect(0), mSize(0) {}
+
+    static SizePair VeryLargeNegative() {
+      SizePair result;
+      result.mSize = result.mSizeContainingRect = kVeryLargeNegativeNumber;
+      return result;
+    }
+    SizePair& operator=(const SizePair& aOther) {
+      mSizeContainingRect = aOther.mSizeContainingRect;
+      mSize = aOther.mSize;
+      return *this;
+    }
+    PRBool operator<(const SizePair& aOther) const {
+      if (mSizeContainingRect < aOther.mSizeContainingRect)
+        return PR_TRUE;
+      if (mSizeContainingRect > aOther.mSizeContainingRect)
+        return PR_FALSE;
+      return mSize < aOther.mSize;
+    }
+    PRBool operator>(const SizePair& aOther) const {
+      return aOther.operator<(*this);
+    }
+    SizePair operator+(const SizePair& aOther) const {
+      SizePair result = *this;
+      result.mSizeContainingRect += aOther.mSizeContainingRect;
+      result.mSize += aOther.mSize;
+      return result;
+    }
+    SizePair operator-(const SizePair& aOther) const {
+      SizePair result = *this;
+      result.mSizeContainingRect -= aOther.mSizeContainingRect;
+      result.mSize -= aOther.mSize;
+      return result;
+    }
+  };
+
   // Returns the sum and indices of the subarray with the maximum sum of the
   // given array (A,n), assuming the array is already in prefix sum form.
-  PRInt64 MaxSum1D(const nsTArray<PRInt64> &A, PRInt32 n,
-                   PRInt32 *minIdx, PRInt32 *maxIdx) {
+  SizePair MaxSum1D(const nsTArray<SizePair> &A, PRInt32 n,
+                    PRInt32 *minIdx, PRInt32 *maxIdx) {
     // The min/max indicies of the largest subarray found so far
-    PRInt64 min = 0,
-            max = 0;
+    SizePair min, max;
     PRInt32 currentMinIdx = 0;
 
     *minIdx = 0;
@@ -1467,7 +1516,7 @@ namespace {
     // Because we're given the array in prefix sum form, we know the first
     // element is 0
     for(PRInt32 i = 1; i < n; i++) {
-      PRInt64 cand = A[i] - min;
+      SizePair cand = A[i] - min;
       if (cand > max) {
         max = cand;
         *minIdx = currentMinIdx;
@@ -1483,7 +1532,7 @@ namespace {
   }
 }
 
-nsRect nsRegion::GetLargestRectangle () const {
+nsRect nsRegion::GetLargestRectangle (const nsRect& aContainingRect) const {
   nsRect bestRect;
 
   if (mRectCount <= 1) {
@@ -1502,6 +1551,12 @@ nsRect nsRegion::GetLargestRectangle () const {
     yaxis.InsertCoord(currentRect->y);
     yaxis.InsertCoord(currentRect->YMost());
   }
+  if (!aContainingRect.IsEmpty()) {
+    xaxis.InsertCoord(aContainingRect.x);
+    xaxis.InsertCoord(aContainingRect.XMost());
+    yaxis.InsertCoord(aContainingRect.y);
+    yaxis.InsertCoord(aContainingRect.YMost());
+  }
 
   // Step 2: Fill out the grid with the areas
   // Note: due to the ordering of rectangles in the region, it is not always
@@ -1509,9 +1564,8 @@ nsRect nsRegion::GetLargestRectangle () const {
   PRInt32 matrixHeight = yaxis.GetNumStops() - 1;
   PRInt32 matrixWidth = xaxis.GetNumStops() - 1;
   PRInt32 matrixSize = matrixHeight * matrixWidth;
-  nsTArray<PRInt64> areas(matrixSize);
+  nsTArray<SizePair> areas(matrixSize);
   areas.SetLength(matrixSize);
-  memset(areas.Elements(), 0, matrixSize * sizeof(PRInt64));
 
   iter.Reset();
   while ((currentRect = iter.Next())) {
@@ -1524,7 +1578,11 @@ nsRect nsRegion::GetLargestRectangle () const {
       nscoord height = yaxis.StopSize(y);
       for (PRInt32 x = xstart; x < xend; x++) {
         nscoord width = xaxis.StopSize(x);
-        areas[y*matrixWidth+x] = width*PRInt64(height);
+        PRInt64 size = width*PRInt64(height);
+        if (currentRect->Intersects(aContainingRect)) {
+          areas[y*matrixWidth+x].mSizeContainingRect = size;
+        }
+        areas[y*matrixWidth+x].mSize = size;
       }
     }
   }
@@ -1534,21 +1592,17 @@ nsRect nsRegion::GetLargestRectangle () const {
     // First get the prefix sum array
     PRInt32 m = matrixHeight + 1;
     PRInt32 n = matrixWidth + 1;
-    nsTArray<PRInt64> pareas(m*n);
+    nsTArray<SizePair> pareas(m*n);
     pareas.SetLength(m*n);
-    // Zero out the first row
-    for (PRInt32 x = 0; x < n; x++)
-      pareas[x] = 0;
     for (PRInt32 y = 1; y < m; y++) {
-      // Zero out the left column
-      pareas[y*n] = 0;
       for (PRInt32 x = 1; x < n; x++) {
-        PRInt64 area = areas[(y-1)*matrixWidth+x-1];
-        if (!area)
-          area = kVeryLargeNegativeNumber;
-        area += pareas[    y*n+x-1]
-              + pareas[(y-1)*n+x  ]
-              - pareas[(y-1)*n+x-1];
+        SizePair area = areas[(y-1)*matrixWidth+x-1];
+        if (!area.mSize) {
+          area = SizePair::VeryLargeNegative();
+        }
+        area = area + pareas[    y*n+x-1]
+                    + pareas[(y-1)*n+x  ]
+                    - pareas[(y-1)*n+x-1];
         pareas[y*n+x] = area;
       }
     }
@@ -1556,18 +1610,19 @@ nsRect nsRegion::GetLargestRectangle () const {
     // No longer need the grid
     areas.SetLength(0);
 
-    PRInt64 bestArea = 0;
+    SizePair bestArea;
     struct {
       PRInt32 left, top, right, bottom;
     } bestRectIndices = { 0, 0, 0, 0 };
     for (PRInt32 m1 = 0; m1 < m; m1++) {
       for (PRInt32 m2 = m1+1; m2 < m; m2++) {
-        nsTArray<PRInt64> B;
+        nsTArray<SizePair> B;
         B.SetLength(n);
-        for (PRInt32 i = 0; i < n; i++)
+        for (PRInt32 i = 0; i < n; i++) {
           B[i] = pareas[m2*n+i] - pareas[m1*n+i];
+        }
         PRInt32 minIdx, maxIdx;
-        PRInt64 area = MaxSum1D(B, n, &minIdx, &maxIdx);
+        SizePair area = MaxSum1D(B, n, &minIdx, &maxIdx);
         if (area > bestArea) {
           bestRectIndices.left = minIdx;
           bestRectIndices.top = m1;

gfx/src/nsRegion.h

@@ -184,7 +184,13 @@ public:
   nsRegion ConvertAppUnitsRoundOut (PRInt32 aFromAPP, PRInt32 aToAPP) const;
   nsRegion ConvertAppUnitsRoundIn (PRInt32 aFromAPP, PRInt32 aToAPP) const;
   nsIntRegion ToOutsidePixels (nscoord aAppUnitsPerPixel) const;
-  nsRect GetLargestRectangle () const;
+  /**
+   * Gets the largest rectangle contained in the region.
+   * @param aContainingRect if non-empty, we choose a rectangle that
+   * maximizes the area intersecting with aContainingRect (and break ties by
+   * then choosing the largest rectangle overall)
+   */
+  nsRect GetLargestRectangle (const nsRect& aContainingRect = nsRect()) const;
 
   /**
    * Make sure the region has at most aMaxRects by adding area to it
@@ -421,7 +427,10 @@ public:
   PRUint32 GetNumRects () const { return mImpl.GetNumRects (); }
   nsIntRect GetBounds () const { return FromRect (mImpl.GetBounds ()); }
   nsRegion ToAppUnits (nscoord aAppUnitsPerPixel) const;
-  nsIntRect GetLargestRectangle () const { return FromRect (mImpl.GetLargestRectangle()); }
+  nsIntRect GetLargestRectangle (const nsIntRect& aContainingRect = nsIntRect()) const
+  {
+    return FromRect (mImpl.GetLargestRectangle( ToRect(aContainingRect) ));
+  }
 
   /**
    * Make sure the region has at most aMaxRects by adding area to it

gfx/tests/TestRegion.cpp

@@ -131,6 +131,24 @@ class TestLargestRegion {
     }
     return success;
   }
+  static PRBool TestContainsSpecifiedRect() {
+    nsRegion r(nsRect(0, 0, 100, 100));
+    r.Or(r, nsRect(0, 300, 50, 50));
+    if (r.GetLargestRectangle(nsRect(0, 300, 10, 10)) != nsRect(0, 300, 50, 50)) {
+      fail("Chose wrong rectangle");
+      return PR_FALSE;
+    }
+    return PR_TRUE;
+  }
+  static PRBool TestContainsSpecifiedOverflowingRect() {
+    nsRegion r(nsRect(0, 0, 100, 100));
+    r.Or(r, nsRect(0, 300, 50, 50));
+    if (r.GetLargestRectangle(nsRect(0, 290, 10, 20)) != nsRect(0, 300, 50, 50)) {
+      fail("Chose wrong rectangle");
+      return PR_FALSE;
+    }
+    return PR_TRUE;
+  }
 public:
   static PRBool Test() {
     if (!TestSingleRect(nsRect(0, 52, 720, 480)) ||
@@ -143,6 +161,10 @@ public:
       return PR_FALSE;
     if (!TwoRectTest())
       return PR_FALSE;
+    if (!TestContainsSpecifiedRect())
+      return PR_FALSE;
+    if (!TestContainsSpecifiedOverflowingRect())
+      return PR_FALSE;
     passed("TestLargestRegion");
     return PR_TRUE;
   }