Compute (un-)premultiply_table at compile time

Change-Id: I34e624fcd5d11d02c26e775f5acdddec1fca9d87
Reviewed-on: https://gerrit.libreoffice.org/64428
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

1 files changed, 35 insertions, 24 deletions

diff --git a/vcl/source/bitmap/BitmapTools.cxx b/vcl/source/bitmap/BitmapTools.cxx
index 4bac9745795e..f9095a86f541 100644
--- a/vcl/source/bitmap/BitmapTools.cxx
+++ b/vcl/source/bitmap/BitmapTools.cxx
@@ -8,6 +8,11 @@
  *
  */
 
+#include <sal/config.h>
+
+#include <array>
+#include <utility>
+
 #include <vcl/BitmapTools.hxx>
 
 #include <sal/log.hxx>
@@ -268,7 +273,7 @@ BitmapEx* CreateFromCairoSurface(Size aSize, cairo_surface_t * pSurface)
     cairo_surface_flush(pPixels);
     unsigned char *pSrc = cairo_image_surface_get_data( pPixels );
     unsigned int nStride = cairo_image_surface_get_stride( pPixels );
-    vcl::bitmap::lookup_table unpremultiply_table = vcl::bitmap::get_unpremultiply_table();
+    vcl::bitmap::lookup_table const & unpremultiply_table = vcl::bitmap::get_unpremultiply_table();
     for( unsigned long y = 0; y < static_cast<unsigned long>(aSize.Height()); y++ )
     {
         sal_uInt32 *pPix = reinterpret_cast<sal_uInt32 *>(pSrc + nStride * y);
@@ -727,7 +732,7 @@ void CanvasCairoExtractBitmapData( BitmapEx const & aBmpEx, Bitmap & aBitmap, un
     ::Color aColor;
     unsigned int nAlpha = 255;
 
-    vcl::bitmap::lookup_table premultiply_table = vcl::bitmap::get_premultiply_table();
+    vcl::bitmap::lookup_table const & premultiply_table = vcl::bitmap::get_premultiply_table();
     for( nY = 0; nY < nHeight; nY++ )
     {
         ::Scanline pReadScan;
@@ -1046,45 +1051,51 @@ void CanvasCairoExtractBitmapData( BitmapEx const & aBmpEx, Bitmap & aBitmap, un
         return bRet;
     }
 
-    static sal_uInt8 unpremultiply(sal_uInt8 c, sal_uInt8 a)
+    static constexpr sal_uInt8 unpremultiply(sal_uInt8 c, sal_uInt8 a)
     {
         return (a == 0) ? 0 : (c * 255 + a / 2) / a;
     }
 
-    static sal_uInt8 premultiply(sal_uInt8 c, sal_uInt8 a)
+    static constexpr sal_uInt8 premultiply(sal_uInt8 c, sal_uInt8 a)
     {
         return (c * a + 127) / 255;
     }
 
-    lookup_table get_unpremultiply_table()
+    template<int... Is> static constexpr std::array<sal_uInt8, 256> make_unpremultiply_table_row_(
+        int a, std::integer_sequence<int, Is...>)
     {
-        static bool inited;
-        static sal_uInt8 unpremultiply_table[256][256];
+        return {unpremultiply(Is, a)...};
+    }
 
-        if (!inited)
-        {
-            for (int a = 0; a < 256; ++a)
-                for (int c = 0; c < 256; ++c)
-                    unpremultiply_table[a][c] = unpremultiply(c, a);
-            inited = true;
-        }
+    template<int... Is> static constexpr lookup_table make_unpremultiply_table_(
+        std::integer_sequence<int, Is...>)
+    {
+        return {make_unpremultiply_table_row_(Is, std::make_integer_sequence<int, 256>{})...};
+    }
 
+    lookup_table const & get_unpremultiply_table()
+    {
+        static constexpr auto unpremultiply_table = make_unpremultiply_table_(
+            std::make_integer_sequence<int, 256>{});
         return unpremultiply_table;
     }
 
-    lookup_table get_premultiply_table()
+    template<int... Is> static constexpr std::array<sal_uInt8, 256> make_premultiply_table_row_(
+        int a, std::integer_sequence<int, Is...>)
     {
-        static bool inited;
-        static sal_uInt8 premultiply_table[256][256];
+        return {premultiply(Is, a)...};
+    }
 
-        if (!inited)
-        {
-            for (int a = 0; a < 256; ++a)
-                for (int c = 0; c < 256; ++c)
-                    premultiply_table[a][c] = premultiply(c, a);
-            inited = true;
-        }
+    template<int... Is> static constexpr lookup_table make_premultiply_table_(
+        std::integer_sequence<int, Is...>)
+    {
+        return {make_premultiply_table_row_(Is, std::make_integer_sequence<int, 256>{})...};
+    }
 
+    lookup_table const & get_premultiply_table()
+    {
+        static constexpr auto premultiply_table = make_premultiply_table_(
+            std::make_integer_sequence<int, 256>{});
         return premultiply_table;
     }