Add g_markup_parse_context_get_offset

This is a small, helpful api that helps for finding error
locations in xml files, along the same lines as
g_markup_parse_context_get_position.

glib/gmarkup.c

@@ -78,6 +78,7 @@ struct _GMarkupParseContext
 
   gint line_number;
   gint char_number;
+  gsize offset;
 
   GMarkupParseState state;
 
@@ -182,6 +183,7 @@ g_markup_parse_context_new (const GMarkupParser *parser,
 
   context->line_number = 1;
   context->char_number = 1;
+  context->offset = 0;
 
   context->partial_chunk = NULL;
   context->spare_chunks = NULL;
@@ -746,6 +748,7 @@ advance_char (GMarkupParseContext *context)
 {
   context->iter++;
   context->char_number++;
+  context->offset++;
 
   if (G_UNLIKELY (context->iter == context->current_text_end))
       return FALSE;
@@ -1921,6 +1924,29 @@ g_markup_parse_context_get_position (GMarkupParseContext *context,
     *char_number = context->char_number;
 }
 
+/**
+ * g_markup_parse_context_get_offset:
+ * @context: a #GMarkupParseContext
+ *
+ * Retrieves the current offset from the beginning of the document,
+ * in bytes.
+ *
+ * The information is meant to accompany the values returned by
+ * [method@GLib.MarkupParseContext.get_position], and comes with the
+ * same accuracy guarantees.
+ *
+ * Returns: the offset
+ *
+ * Since: 2.88
+ */
+gsize
+g_markup_parse_context_get_offset (GMarkupParseContext *context)
+{
+  g_return_val_if_fail (context != NULL, 0);
+
+  return context->offset;
+}
+
 /**
  * g_markup_parse_context_get_user_data:
  * @context: a #GMarkupParseContext

glib/gmarkup.h

@@ -227,6 +227,8 @@ GLIB_AVAILABLE_IN_ALL
 void                 g_markup_parse_context_get_position (GMarkupParseContext *context,
                                                           gint                *line_number,
                                                           gint                *char_number);
+GLIB_AVAILABLE_IN_2_88
+gsize                g_markup_parse_context_get_offset   (GMarkupParseContext *context);
 GLIB_AVAILABLE_IN_ALL
 gpointer             g_markup_parse_context_get_user_data (GMarkupParseContext *context);
 

glib/tests/markup-parse.c

@@ -158,6 +158,7 @@ test_file (const gchar       *filename,
   GError *local_error = NULL;
   GMarkupParseContext *context;
   gint line, col;
+  gsize offset;
   guint n_failures = 0;
   guint n_tests = 0;
   const gsize chunk_sizes_bytes[] = { 1, 2, 5, 12, 1024 };
@@ -178,6 +179,8 @@ test_file (const gchar       *filename,
   g_markup_parse_context_get_position (context, &line, &col);
   g_assert_cmpint (line, ==, 1);
   g_assert_cmpint (col, ==, 1);
+  offset = g_markup_parse_context_get_offset (context);
+  g_assert_cmpint (offset, ==, 0);
 
   if (!g_markup_parse_context_parse (context, contents, -1, NULL) ||
       !g_markup_parse_context_end_parse (context, NULL))