From 9dfc2d924d440560adb2db13c758b2c5b3b7dd47 Mon Sep 17 00:00:00 2001
From: John Firebaugh <john.firebaugh@gmail.com>
Date: Wed, 31 May 2017 13:53:48 -0700
Subject: [core] Collection-level immutability

Introduce a second level of immutability, over each of the collections held by a style: sources, images, and layers. Tracking immutability at this level allows us to short-circuit significant portions of the RenderStyle update logic via a simple equality check, greatly improving performance.
---
 include/mbgl/style/image.hpp    |  2 +-
 include/mbgl/util/immutable.hpp | 15 +++++++++++++++
 2 files changed, 16 insertions(+), 1 deletion(-)

(limited to 'include')

diff --git a/include/mbgl/style/image.hpp b/include/mbgl/style/image.hpp
index 55760ca503..ff3bfedf46 100644
--- a/include/mbgl/style/image.hpp
+++ b/include/mbgl/style/image.hpp
@@ -24,7 +24,7 @@ public:
     bool isSdf() const;
 
     class Impl;
-    Immutable<Impl> impl;
+    Immutable<Impl> baseImpl;
 };
 
 } // namespace style
diff --git a/include/mbgl/util/immutable.hpp b/include/mbgl/util/immutable.hpp
index 2cdce8772d..eb26c0d282 100644
--- a/include/mbgl/util/immutable.hpp
+++ b/include/mbgl/util/immutable.hpp
@@ -115,4 +115,19 @@ Immutable<S> staticImmutableCast(const Immutable<U>& u) {
     return Immutable<S>(std::static_pointer_cast<const S>(u.ptr));
 }
 
+/**
+ * Constrained mutation of an immutable reference. Makes a temporarily-mutable copy of the
+ * input Immutable using the inner type's copy constructor, runs the given callable on the
+ * mutable copy, and then freezes the copy and reassigns it to the input reference.
+ *
+ * Note that other Immutables referring to the same inner instance are not affected; they
+ * continue to referencing the original immutable instance.
+ */
+template <class T, class Fn>
+void mutate(Immutable<T>& immutable, Fn&& fn) {
+    Mutable<T> mut = makeMutable<T>(*immutable);
+    std::forward<Fn>(fn)(*mut);
+    immutable = std::move(mut);
+}
+
 } // namespace mbgl
-- 
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