gutf8: Add a g_utf8_truncate_middle() function

Adds a helper to truncate UTF8 strings in the middle, allowing to make
them fit certain size constraints.

This function is modeled after similar functionality that has existed
since 2008 in nautilus and in eel before that.

docs/reference/glib/glib-sections.txt.in

@@ -3131,6 +3131,7 @@ g_utf8_strchr
 g_utf8_strrchr
 g_utf8_strreverse
 g_utf8_substring
+g_utf8_truncate_middle
 g_utf8_validate
 g_utf8_validate_len
 g_utf8_make_valid

glib/gunicode.h

@@ -833,6 +833,10 @@ gchar   *g_utf8_strncpy           (gchar       *dest,
                                    const gchar *src,
                                    gsize        n);
 
+GLIB_AVAILABLE_IN_2_78
+gchar   *g_utf8_truncate_middle   (const gchar *string,
+                                   gsize        truncate_length);
+
 /* Find the UTF-8 character corresponding to ch, in string p. These
    functions are equivalants to strchr and strrchr */
 GLIB_AVAILABLE_IN_ALL

glib/gutf8.c

@@ -460,6 +460,72 @@ g_utf8_strncpy (gchar       *dest,
   return dest;
 }
 
+/**
+ * g_utf8_truncate_middle:
+ * @string: (transfer none): a nul-terminated UTF-8 encoded string
+ * @truncate_length: the new size of @string, in characters, including the ellipsis character
+ *
+ * Cuts off the middle of the string, preserving half of @truncate_length
+ * characters at the beginning and half at the end.
+ * 
+ * If @string is already short enough, this returns a copy of @string.
+ * If @truncate_length is `0`, an empty string is returned.
+ *
+ * Returns: (transfer full): a newly-allocated copy of @string ellipsized in the middle
+ *
+ * Since: 2.78
+ */
+gchar *
+g_utf8_truncate_middle (const gchar *string,
+                        gsize        truncate_length)
+{
+  const gchar *ellipsis = "…";
+  const gsize ellipsis_bytes = strlen (ellipsis);
+
+  gsize length;
+  gsize left_substring_length;
+  gchar *left_substring_end;
+  gchar *right_substring_begin;
+  gchar *right_substring_end;
+  gsize left_bytes;
+  gsize right_bytes;
+  gchar *result;
+
+  g_return_val_if_fail (string != NULL, NULL);
+
+  length = g_utf8_strlen (string, -1);
+  /* Current string already smaller than requested length */
+  if (length <= truncate_length)
+    return g_strdup (string);
+  if (truncate_length == 0)
+    return g_strdup ("");
+
+  /* Find substrings to keep, ignore ellipsis character for that */
+  truncate_length -= 1;
+
+  left_substring_length = truncate_length / 2;
+
+  left_substring_end = g_utf8_offset_to_pointer (string, left_substring_length);
+  right_substring_begin = g_utf8_offset_to_pointer (left_substring_end,
+                                                    length - truncate_length);
+  right_substring_end = g_utf8_offset_to_pointer (right_substring_begin,
+                                                  truncate_length - left_substring_length);
+
+  g_assert (*right_substring_end == '\0');
+
+  left_bytes = left_substring_end - string;
+  right_bytes = right_substring_end - right_substring_begin;
+
+  result = g_malloc (left_bytes + ellipsis_bytes + right_bytes + 1);
+
+  strncpy (result, string, left_bytes);
+  memcpy (result + left_bytes, ellipsis, ellipsis_bytes);
+  strncpy (result + left_bytes + ellipsis_bytes, right_substring_begin, right_bytes);
+  result[left_bytes + ellipsis_bytes + right_bytes] = '\0';
+
+  return result;
+}
+
 /* unicode_strchr */
 
 /**

glib/tests/utf8-misc.c

@@ -162,6 +162,58 @@ test_utf8_make_valid (void)
   g_free (r);
 }
 
+static void
+truncate_middle_helper (const char *in_str,
+                        gsize       truncate_len,
+                        const char *out_str)
+{
+  gchar *string = g_utf8_truncate_middle (in_str, truncate_len);
+  g_assert_cmpstr (string, ==, out_str);
+  g_free (string);
+}
+
+static void
+test_utf8_truncate_middle (void)
+{
+  truncate_middle_helper ("foo", 0, "");
+  truncate_middle_helper ("foo", 1, "…");
+  truncate_middle_helper ("foo", 2, "…o");
+  truncate_middle_helper ("foo", 3, "foo");
+  truncate_middle_helper ("foo", 4, "foo");
+  truncate_middle_helper ("foo", 5, "foo");
+  truncate_middle_helper ("foo", 6, "foo");
+  truncate_middle_helper ("foo", 7, "foo");
+
+  truncate_middle_helper ("a_much_longer_foo", 0, "");
+  truncate_middle_helper ("a_much_longer_foo", 1, "…");
+  truncate_middle_helper ("a_much_longer_foo", 2, "…o");
+  truncate_middle_helper ("a_much_longer_foo", 3, "a…o");
+  truncate_middle_helper ("a_much_longer_foo", 4, "a…oo");
+  truncate_middle_helper ("a_much_longer_foo", 5, "a_…oo");
+  truncate_middle_helper ("a_much_longer_foo", 6, "a_…foo");
+  truncate_middle_helper ("a_much_longer_foo", 7, "a_m…foo");
+  truncate_middle_helper ("a_much_longer_foo", 8, "a_m…_foo");
+  truncate_middle_helper ("a_much_longer_foo", 9, "a_mu…_foo");
+
+  truncate_middle_helper ("something_even", 8, "som…even");
+  truncate_middle_helper ("something_odd", 8, "som…_odd");
+  truncate_middle_helper ("something_even", 9, "some…even");
+  truncate_middle_helper ("something_odd", 9, "some…_odd");
+  truncate_middle_helper ("something_even", 10, "some…_even");
+  truncate_middle_helper ("something_odd", 10, "some…g_odd");
+  truncate_middle_helper ("something_even", 11, "somet…_even");
+  truncate_middle_helper ("something_odd", 11, "somet…g_odd");
+  truncate_middle_helper ("something_even", 12, "somet…g_even");
+  truncate_middle_helper ("something_odd", 12, "somet…ng_odd");
+  truncate_middle_helper ("something_even", 13, "someth…g_even");
+  truncate_middle_helper ("something_odd", 13, "something_odd");
+  truncate_middle_helper ("something_even", 14, "something_even");
+  truncate_middle_helper ("something_odd", 13, "something_odd");
+
+  truncate_middle_helper ("ääääääääää", 5, "ää…ää");
+  truncate_middle_helper ("あぃいぅうぇえぉ", 7, "あぃい…ぇえぉ");
+}
+
 int
 main (int   argc,
       char *argv[])
@@ -174,6 +226,7 @@ main (int   argc,
   g_test_add_func ("/utf8/reverse", test_utf8_reverse);
   g_test_add_func ("/utf8/substring", test_utf8_substring);
   g_test_add_func ("/utf8/make-valid", test_utf8_make_valid);
+  g_test_add_func ("/utf8/truncate-middle", test_utf8_truncate_middle);
 
   return g_test_run();
 }