Nullify a concern

fastiv-io.c

@@ -317,6 +317,8 @@ load_wuffs_frame(struct load_wuffs_frame_context *ctx, GError **error)
 		(void *) (intptr_t) (wuffs_base__frame_config__duration(&fc) /
 			WUFFS_BASE__FLICKS_PER_MILLISECOND), NULL);
 
+	cairo_surface_set_user_data(surface, &fastiv_io_key_frame_previous,
+		ctx->result_tail, NULL);
 	if (ctx->result_tail)
 		cairo_surface_set_user_data(ctx->result_tail, &fastiv_io_key_frame_next,
 			surface, (cairo_destroy_func_t) cairo_surface_destroy);
@@ -466,6 +468,11 @@ open_wuffs(
 	while (load_wuffs_frame(&ctx, error))
 		;
 
+	// Wrap the chain around, since our caller receives only one pointer.
+	if (ctx.result)
+		cairo_surface_set_user_data(ctx.result, &fastiv_io_key_frame_previous,
+			ctx.result_tail, NULL);
+
 fail:
 	free(ctx.workbuf.ptr);
 	g_clear_pointer(&ctx.meta_exif, g_bytes_unref);
@@ -490,7 +497,7 @@ open_wuffs_using(wuffs_base__image_decoder *(*allocate)(),
 }
 
 static void
-trivial_cmyk_to_bgra(unsigned char *p, int len)
+trivial_cmyk_to_host_byte_order_argb(unsigned char *p, int len)
 {
 	// Inspired by gdk-pixbuf's io-jpeg.c:
 	//
@@ -498,10 +505,17 @@ trivial_cmyk_to_bgra(unsigned char *p, int len)
 	// does, see https://bugzilla.gnome.org/show_bug.cgi?id=618096
 	while (len--) {
 		int c = p[0], m = p[1], y = p[2], k = p[3];
+#if G_BYTE_ORDER == G_LITTLE_ENDIAN
 		p[0] = k * y / 255;
 		p[1] = k * m / 255;
 		p[2] = k * c / 255;
 		p[3] = 255;
+#else
+		p[3] = k * y / 255;
+		p[2] = k * m / 255;
+		p[1] = k * c / 255;
+		p[0] = 255;
+#endif
 		p += 4;
 	}
 }
@@ -637,11 +651,10 @@ open_libjpeg_turbo(const gchar *data, gsize len, GError **error)
 		return NULL;
 	}
 
-	// TODO(p): Support big-endian machines, too, those need ARGB ordering.
 	if (pixel_format == TJPF_CMYK) {
 		// CAIRO_STRIDE_ALIGNMENT is 4 bytes, so there will be no padding with
 		// ARGB/BGR/XRGB/BGRX.
-		trivial_cmyk_to_bgra(
+		trivial_cmyk_to_host_byte_order_argb(
 			cairo_image_surface_get_data(surface), width * height);
 	}
 
@@ -962,7 +975,7 @@ open_xcursor(const gchar *data, gsize len, GError **error)
 			last_nominal = image->size;
 		}
 
-		// TODO(p): Byte-swap if on big-endian. Wuffs doesn't even build there.
+		// The library automatically byte swaps in _XcursorReadImage().
 		cairo_surface_t *source = cairo_image_surface_create_for_data(
 			(unsigned char *) image->pixels, CAIRO_FORMAT_ARGB32,
 			image->width, image->height, image->width * sizeof *image->pixels);
@@ -1024,6 +1037,7 @@ cairo_user_data_key_t fastiv_io_key_orientation;
 cairo_user_data_key_t fastiv_io_key_icc;
 
 cairo_user_data_key_t fastiv_io_key_frame_next;
+cairo_user_data_key_t fastiv_io_key_frame_previous;
 cairo_user_data_key_t fastiv_io_key_frame_duration;
 cairo_user_data_key_t fastiv_io_key_loops;
 

fastiv-io.h

@@ -37,6 +37,10 @@ extern cairo_user_data_key_t fastiv_io_key_icc;
 /// The next frame in a sequence, as a surface, in a chain, pre-composited.
 /// There is no wrap-around.
 extern cairo_user_data_key_t fastiv_io_key_frame_next;
+/// The previous frame in a sequence, as a surface, in a chain, pre-composited.
+/// This is a weak pointer that wraps around, and needn't be present
+/// for static images.
+extern cairo_user_data_key_t fastiv_io_key_frame_previous;
 /// Frame duration in milliseconds as an intptr_t.
 extern cairo_user_data_key_t fastiv_io_key_frame_duration;
 /// How many times to repeat the animation, or zero for +inf, as a uintptr_t.

fastiv-view.c

@@ -452,9 +452,15 @@ fastiv_view_key_press_event(GtkWidget *widget, GdkEventKey *event)
 		gtk_widget_queue_resize(widget);
 		return TRUE;
 
+	case GDK_KEY_bracketleft:
+		if (!(self->frame = cairo_surface_get_user_data(
+				self->frame, &fastiv_io_key_frame_previous)))
+			self->frame = self->surface;
+		gtk_widget_queue_draw(widget);
+		return TRUE;
 	case GDK_KEY_bracketright:
 		if (!(self->frame = cairo_surface_get_user_data(
-				  self->frame, &fastiv_io_key_frame_next)))
+				self->frame, &fastiv_io_key_frame_next)))
 			self->frame = self->surface;
 		gtk_widget_queue_draw(widget);
 		return TRUE;