item: Add a char offset

Add a char_offset field to PangoItem, compute it as part of
itemization and update it when splitting items. Keeping this
number around cuts down on the amount of list and utf8 walking
we need to do later.

We have to do some extra shenanigans to preserve abi in the
face of pango's open-coded structs, so we introduce a
PangoItemPrivate type that is used internally. On 64bit,
PangoItem has a 4 byte whole, so we can keep the size of
PangoItemPrivate the same. No such luck on 32bit.

1 files changed, 73 insertions, 0 deletions

diff --git a/pango/pango-item-private.h b/pango/pango-item-private.h
new file mode 100644
index 00000000..8bb7e1cd
--- /dev/null
+++ b/pango/pango-item-private.h
@@ -0,0 +1,73 @@
+/* Pango
+ *
+ * Copyright (C) 2021 Matthias Clasen
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Library General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Library General Public License for more details.
+ *
+ * You should have received a copy of the GNU Library General Public
+ * License along with this library; if not, write to the
+ * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+ * Boston, MA 02111-1307, USA.
+ */
+
+#ifndef __PANGO_ITEM_PRIVATE_H__
+#define __PANGO_ITEM_PRIVATE_H__
+
+#include <pango/pango-item.h>
+
+G_BEGIN_DECLS
+
+/**
+ * We have to do some extra work for adding the char_offset field
+ * to PangoItem to preserve ABI in the face of pango's open-coded
+ * structs.
+ *
+ * Internally, pango uses the PangoItemPrivate type, and we use
+ * a bit in the PangoAnalysis flags to indicate whether we are
+ * dealing with a PangoItemPrivate struct or not.
+ */
+
+#define PANGO_ANALYSIS_FLAG_HAS_CHAR_OFFSET (1 << 7)
+
+typedef struct _PangoItemPrivate PangoItemPrivate;
+
+#ifdef __x86_64__
+
+struct _PangoItemPrivate
+{
+  int offset;
+  int length;
+  int num_chars;
+  int char_offset;
+  PangoAnalysis analysis;
+};
+
+#else
+
+struct _PangoItemPrivate
+{
+  int offset;
+  int length;
+  int num_chars;
+  PangoAnalysis analysis;
+  int char_offset;
+}
+
+#endif
+
+G_STATIC_ASSERT (offsetof (PangoItem, offset) == offsetof (PangoItemPrivate, offset));
+G_STATIC_ASSERT (offsetof (PangoItem, length) == offsetof (PangoItemPrivate, length));
+G_STATIC_ASSERT (offsetof (PangoItem, num_chars) == offsetof (PangoItemPrivate, num_chars));
+G_STATIC_ASSERT (offsetof (PangoItem, analysis) == offsetof (PangoItemPrivate, analysis));
+
+G_END_DECLS
+
+#endif /* __PANGO_ITEM_PRIVATE_H__ */