[core] Added CharArrayBuffer

Implements a custom std::streambuf to avoid creating temporary
std::string objects and thus optimizing image decode.

Suggested by @artemp in
https://github.com/mapbox/mapbox-gl-native/pull/5417#issuecomment-227700063.

2 files changed, 7 insertions, 5 deletions

diff --git a/platform/default/jpeg_reader.cpp b/platform/default/jpeg_reader.cpp
index bb99aab55c..9bcf3c6bc1 100644
--- a/platform/default/jpeg_reader.cpp
+++ b/platform/default/jpeg_reader.cpp
@@ -1,4 +1,5 @@
 #include <mbgl/util/image.hpp>
+#include <mbgl/util/char_array_buffer.hpp>
 
 #include <istream>
 #include <sstream>
@@ -21,7 +22,7 @@ struct jpeg_stream_wrapper {
 
 static void init_source(j_decompress_ptr cinfo) {
     jpeg_stream_wrapper* wrap = reinterpret_cast<jpeg_stream_wrapper*>(cinfo->src);
-    wrap->stream->seekg(0);
+    wrap->stream->seekg(0, std::ios_base::beg);
 }
 
 static boolean fill_input_buffer(j_decompress_ptr cinfo) {
@@ -89,8 +90,8 @@ struct jpeg_info_guard {
 };
 
 PremultipliedImage decodeJPEG(const uint8_t* data, size_t size) {
-    std::istringstream source(std::string(reinterpret_cast<const char*>(data), size));
-    std::istream stream(source.rdbuf());
+    util::CharArrayBuffer dataBuffer { reinterpret_cast<const char*>(data), size };
+    std::istream stream(&dataBuffer);
 
     jpeg_decompress_struct cinfo;
     jpeg_info_guard iguard(&cinfo);
diff --git a/platform/default/png_reader.cpp b/platform/default/png_reader.cpp
index e3f6fdef49..0461ec61a6 100644
--- a/platform/default/png_reader.cpp
+++ b/platform/default/png_reader.cpp
@@ -1,5 +1,6 @@
 #include <mbgl/util/image.hpp>
 #include <mbgl/util/premultiply.hpp>
+#include <mbgl/util/char_array_buffer.hpp>
 #include <mbgl/platform/log.hpp>
 
 #include <istream>
@@ -44,8 +45,8 @@ struct png_struct_guard {
 };
 
 PremultipliedImage decodePNG(const uint8_t* data, size_t size) {
-    std::istringstream source(std::string(reinterpret_cast<const char*>(data), size));
-    std::istream stream(source.rdbuf());
+    util::CharArrayBuffer dataBuffer { reinterpret_cast<const char*>(data), size };
+    std::istream stream(&dataBuffer);
 
     png_byte header[8] = { 0 };
     stream.read(reinterpret_cast<char*>(header), 8);