Drawable performance fixes (2) (#1610)

Co-authored-by: Tim Sylvester <tim@wetdogweather.com>
Co-authored-by: Bart Louwers <bart.louwers@gmail.com>

test/util/string_indexer.test.cpp

@@ -0,0 +1,49 @@
+#include <mbgl/test/util.hpp>
+
+#include <mbgl/util/string_indexer.hpp>
+
+#include <cstdint>
+
+using namespace mbgl;
+
+TEST(StringIndexer, AddStrings) {
+    StringIndexer::clear();
+    EXPECT_EQ(StringIndexer::size(), 0);
+
+    const auto id1 = StringIndexer::get("test string1");
+    EXPECT_EQ(id1, 0);
+    EXPECT_EQ(StringIndexer::size(), 1);
+
+    const auto id2 = StringIndexer::get("test string2");
+    EXPECT_EQ(id2, 1);
+    EXPECT_EQ(StringIndexer::size(), 2);
+}
+
+TEST(StringIndexer, GetString) {
+    StringIndexer::clear();
+    EXPECT_EQ(StringIndexer::size(), 0);
+
+    constexpr auto str = "test string1";
+
+    const auto id1 = StringIndexer::get(str);
+    EXPECT_EQ(id1, 0);
+    EXPECT_EQ(StringIndexer::size(), 1);
+
+    const std::string str1 = StringIndexer::get(id1);
+    EXPECT_EQ(str, str1);
+
+    const auto str2 = str;
+    const auto id2 = StringIndexer::get(str2);
+    EXPECT_EQ(id1, id2);
+}
+
+TEST(StringIndexer, GetOOBIdentity) {
+    StringIndexer::clear();
+    EXPECT_EQ(StringIndexer::size(), 0);
+
+    std::string str;
+    EXPECT_DEATH_IF_SUPPORTED(str = StringIndexer::get(42), "id < identityToString.size()");
+    EXPECT_TRUE(str.empty());
+    EXPECT_DEATH_IF_SUPPORTED(str = StringIndexer::get(-1), "id < identityToString.size()");
+    EXPECT_TRUE(str.empty());
+}