Add array initializer

Generate an array with initialized values based on
a function of the index.

Change-Id: I1d7d83ba9cacde47b8c736f26dedc15294929937
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/289489
Reviewed-by: Mike Klein <mtklein@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Herb Derby <herb@google.com>

src/core/SkArenaAlloc.h

@@ -126,6 +126,19 @@ public:
         return array;
     }
 
+    template <typename T, typename Initializer>
+    T* makeInitializedArray(size_t count, Initializer initializer) {
+        AssertRelease(SkTFitsIn<uint32_t>(count));
+        uint32_t safeCount = ToU32(count);
+        T* array = (T*)this->commonArrayAlloc<T>(safeCount);
+
+        // Initialize array elements.
+        for (size_t i = 0; i < safeCount; i++) {
+            new (&array[i]) T(initializer(i));
+        }
+        return array;
+    }
+
     // Only use makeBytesAlignedTo if none of the typed variants are impractical to use.
     void* makeBytesAlignedTo(size_t size, size_t align) {
         AssertRelease(SkTFitsIn<uint32_t>(size));

tests/ArenaAllocTest.cpp

@@ -169,6 +169,14 @@ DEF_TEST(ArenaAlloc, r) {
         start.start = current;
     }
 
+    {
+        SkSTArenaAlloc<64> arena;
+        auto a = arena.makeInitializedArray<int>(8, [](size_t i ) { return i; });
+        for (size_t i = 0; i < 8; i++) {
+            REPORTER_ASSERT(r, a[i] == (int)i);
+        }
+    }
+
     REPORTER_ASSERT(r, created == 128);
     REPORTER_ASSERT(r, destroyed == 128);