Add draw_utils::GetBottomGapBetweenRects().

Currently draw_utils::GetBottomFillRect() returns the gap on the
bottom side of a page created by insetting the page. However,
in two-up view, the bottom gap for a page has height equal to the
difference in height between the current page and its adjacent one.

draw_utils::GetBottomGapBetweenRects() takes the bottom component of
a page and a rectangle, returning the portion of the rectangle that
is below the page. This will be used in PDFiumEngine to compute the
bottom empty space for a page in two-up view as mentioned above.

Bug: 51472
Change-Id: I436600264106340ca6699799359149f29c797844
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1721318Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Jeremy Chinsen <chinsenj@google.com>
Cr-Commit-Position: refs/heads/master@{#682097}

pdf/draw_utils/coordinates.cc

@@ -17,6 +17,15 @@ void ExpandDocumentSize(const pp::Size& rect_size, pp::Size* doc_size) {
   doc_size->Enlarge(width_diff, rect_size.height());
 }
 
+pp::Rect GetBottomGapBetweenRects(int page_rect_bottom,
+                                  const pp::Rect& bottom_rect) {
+  if (page_rect_bottom >= bottom_rect.bottom())
+    return pp::Rect(0, 0, 0, 0);
+
+  return pp::Rect(bottom_rect.x(), page_rect_bottom, bottom_rect.width(),
+                  bottom_rect.bottom() - page_rect_bottom);
+}
+
 PageInsetSizes GetPageInsetsForTwoUpView(
     size_t page_index,
     size_t num_of_pages,

pdf/draw_utils/coordinates.h

@@ -31,6 +31,13 @@ struct PageInsetSizes {
 // |doc_size|'s height.
 void ExpandDocumentSize(const pp::Size& rect_size, pp::Size* doc_size);
 
+// Given |page_rect_bottom| and |bottom_rect| in the same coordinate space,
+// return a pp::Rect object representing the portion of |bottom_rect| that is
+// below |page_rect_bottom|. Returns an empty rectangle if |page_rect_bottom|
+// is greater than or equal to |bottom_rect.bottom()|.
+pp::Rect GetBottomGapBetweenRects(int page_rect_bottom,
+                                  const pp::Rect& bottom_rect);
+
 // Given |page_index|, and |num_of_pages|, return the configuration of
 // |single_view_insets| and |horizontal_separator| for the current page in
 // two-up view.
@@ -40,6 +47,7 @@ PageInsetSizes GetPageInsetsForTwoUpView(
     const PageInsetSizes& single_view_insets,
     int horizontal_separator);
 
+// TODO (chinsenj): move Get*FillRect() functions to bottom of coordinates.h.
 // Given |page_rect| in document coordinates, |inset_sizes|, and
 // |bottom_separator|, return a pp::Rect object representing the gap on the
 // left side of the page created by insetting the page. I.e. the difference,

pdf/draw_utils/coordinates_unittest.cc

@@ -61,6 +61,21 @@ TEST(CoordinateTest, ExpandDocumentSize) {
   CompareSize({250, 1450}, doc_size);
 }
 
+TEST(CoordinateTest, GetBottomGapBetweenRects) {
+  CompareRect({95, 600, 350, 50},
+              GetBottomGapBetweenRects(600, {95, 200, 350, 450}));
+
+  CompareRect({200, 500, 350, 10},
+              GetBottomGapBetweenRects(500, {200, 0, 350, 510}));
+
+  // Test rectangle with a negative bottom value.
+  CompareRect({150, -100, 400, 150},
+              GetBottomGapBetweenRects(-100, {150, 0, 400, 50}));
+
+  // Test case where |page_rect_bottom| >= |dirty_rect.bottom()|.
+  CompareRect({0, 0, 0, 0}, GetBottomGapBetweenRects(1400, {0, 10, 300, 500}));
+}
+
 TEST(CoordinateTest, GetPageInsetsForTwoUpView) {
   // Page is on the left side and isn't the last page in the document.
   CompareInsetSizes(kLeftInsets,