refine SkPath::approximateBytesUsed()

Instead of looking at point/verb/weight counts, add an
SkPathRef::approximateBytesUsed() using their reserve counts.

This shows SkPathBuilder::snapshot() can return more memory-efficient
SkPaths than SkPathBuilder::detach(), at the cost of a copy.

Change-Id: I4e208c41643480d7682daba6ac674ffa63c74de2
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/326608
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Klein <mtklein@google.com>

include/private/SkPathRef.h

@@ -294,6 +294,8 @@ public:
     int countVerbs() const { return fVerbs.count(); }
     int countWeights() const { return fConicWeights.count(); }
 
+    size_t approximateBytesUsed() const;
+
     /**
      * Returns a pointer one beyond the first logical verb (last verb in memory order).
      */

src/core/SkPath.cpp

@@ -525,11 +525,8 @@ int SkPath::getVerbs(uint8_t dst[], int max) const {
 size_t SkPath::approximateBytesUsed() const {
     size_t size = sizeof (SkPath);
     if (fPathRef != nullptr) {
-        size += fPathRef->countPoints() * sizeof(SkPoint)
-              + fPathRef->countVerbs()
-              + fPathRef->countWeights() * sizeof(SkScalar);
+        size += fPathRef->approximateBytesUsed();
     }
-
     return size;
 }
 

src/core/SkPathRef.cpp

@@ -50,6 +50,13 @@ void SkPath::shrinkToFit() {
 
 //////////////////////////////////////////////////////////////////////////////
 
+size_t SkPathRef::approximateBytesUsed() const {
+    return sizeof(SkPathRef)
+         + fPoints      .reserved() * sizeof(fPoints      [0])
+         + fVerbs       .reserved() * sizeof(fVerbs       [0])
+         + fConicWeights.reserved() * sizeof(fConicWeights[0]);
+}
+
 SkPathRef::~SkPathRef() {
     // Deliberately don't validate() this path ref, otherwise there's no way
     // to read one that's not valid and then free its memory without asserting.

tests/PathBuilderTest.cpp

@@ -270,3 +270,23 @@ DEF_TEST(pathbuilder_addPolygon, reporter) {
         }
     }
 }
+
+DEF_TEST(pathbuilder_shrinkToFit, reporter) {
+    // SkPathBuilder::snapshot() creates copies of its arrays for perfectly sized paths,
+    // where SkPathBuilder::detach() moves its larger scratch arrays for speed.
+    bool any_smaller = false;
+    for (int pts = 0; pts < 10; pts++) {
+
+        SkPathBuilder b;
+        for (int i = 0; i < pts; i++) {
+            b.lineTo(i,i);
+        }
+        b.close();
+
+        SkPath s = b.snapshot(),
+               d = b.detach();
+        REPORTER_ASSERT(reporter, s.approximateBytesUsed() <= d.approximateBytesUsed());
+        any_smaller |=            s.approximateBytesUsed() <  d.approximateBytesUsed();
+    }
+    REPORTER_ASSERT(reporter, any_smaller);
+}