add image_reader abstraction layer to hanfle multiple image formats

(work-in-progress)

platform/default/image_reader.cpp

@@ -0,0 +1,67 @@
+#include "mbgl/util/image_reader.hpp"
+#include <boost/optional.hpp>
+
+namespace mbgl { namespace util {
+
+inline boost::optional<std::string> type_from_bytes(char const* data, size_t size)
+{
+    using result_type = boost::optional<std::string>;
+    if (size >= 4)
+    {
+        unsigned int magic = (data[0] << 24) | (data[1] << 16) | (data[2] << 8) | data[3];
+        if (magic == 0x89504E47U)
+        {
+            return result_type("png");
+        }
+        else if (magic == 0x49492A00 || magic == 0x4D4D002A)
+        {
+            return result_type("tiff");
+        }
+    }
+    if (size>=2)
+    {
+        unsigned int magic = ((data[0] << 8) | data[1]) & 0xffff;
+        if (magic == 0xffd8)
+        {
+            return result_type("jpeg");
+        }
+    }
+
+    if (size>=12)
+    {
+        if (data[0] == 'R' && data[1] == 'I' && data[2] == 'F' && data[3] == 'F' &&
+            data[8] == 'W' && data[9] == 'E' && data[10] == 'B' && data[11] == 'P')
+        {
+            return result_type("webp");
+        }
+    }
+    return result_type();
+}
+
+image_reader* get_image_reader(char const* data, size_t size)
+{
+    boost::optional<std::string> type = type_from_bytes(data,size);
+    if (type)
+    {
+        return factory<image_reader,std::string,char const*,size_t>::instance().create_object(*type, data,size);
+    }
+    else
+        throw image_reader_exception("image_reader: can't determine type from input data");
+}
+
+//image_reader* get_image_reader(std::string const& filename,std::string const& type)
+//{
+//    return factory<image_reader,std::string,std::string const&>::instance().create_object(type,filename);
+//}
+
+//image_reader* get_image_reader(std::string const& filename)
+//{
+    //boost::optional<std::string> type = type_from_filename(filename);
+    //if (type)
+    //{
+    //    return factory<image_reader,std::string,std::string const&>::instance().create_object(*type,filename);
+    //}
+//    return 0;
+//}
+
+}}