[gtk/font-chooser-sample-text] fontchooser: Determine sample text intelligently

[Matthias Clasen <matthiasc src gnome org>]

commit 69d5dc7c9a3f704ba9d423035864382fea390d67
Author: Matthias Clasen <mclasen redhat com>
Date:   Tue Sep 8 15:14:17 2020 -0400

    fontchooser: Determine sample text intelligently
    
    Look at the languages supported by a font, and pick
    a suitable sample text from the pango list of sample
    texts. We can only implement this on platforms using
    fontconfig, since it relies on pangofc apis.
    
    This bumps the pango dependency to 1.47.1.

 gtk/gtkfontchooserwidget.c | 130 ++++++++++++++++++++++++++++++++++++++++++++-
 meson.build                |   2 +-
 2 files changed, 130 insertions(+), 2 deletions(-)
---
diff --git a/gtk/gtkfontchooserwidget.c b/gtk/gtkfontchooserwidget.c
index b3dfa9b348..9be3d33cd2 100644
--- a/gtk/gtkfontchooserwidget.c
+++ b/gtk/gtkfontchooserwidget.c
@@ -58,6 +58,9 @@
 #include "gtkmaplistmodel.h"
 
 #include <hb-ot.h>
+#if defined(HAVE_PANGOFT) && defined(HAVE_HARFBUZZ)
+#include <pango/pangofc-font.h>
+#endif
 
 #include "language-names.h"
 #include "script-names.h"
@@ -108,6 +111,7 @@ struct _GtkFontChooserWidget
   GtkWidget       *font_name_label;
   char            *preview_text;
   gboolean         show_preview_entry;
+  gboolean         preview_text_set;
 
   GtkWidget *size_label;
   GtkWidget *size_spin;
@@ -210,6 +214,7 @@ gtk_font_chooser_widget_set_property (GObject         *object,
       break;
     case GTK_FONT_CHOOSER_PROP_PREVIEW_TEXT:
       gtk_font_chooser_widget_set_preview_text (fontchooser, g_value_get_string (value));
+      fontchooser->preview_text_set = TRUE;
       break;
     case GTK_FONT_CHOOSER_PROP_SHOW_PREVIEW_ENTRY:
       gtk_font_chooser_widget_set_show_preview_entry (fontchooser, g_value_get_boolean (value));
@@ -432,6 +437,123 @@ resize_by_scroll_cb (GtkEventControllerScroll *controller,
                             gtk_adjustment_get_step_increment (adj) * dx);
 }
 
+static void
+maybe_update_preview_text (GtkFontChooserWidget *self,
+                           PangoFontFace        *face,
+                           PangoFontDescription *desc)
+{
+#if defined(HAVE_PANGOFT) && defined(HAVE_HARFBUZZ)
+  PangoContext *context;
+  PangoFont *font;
+  const char *sample;
+
+  /* If the user has typed text into the entry, we don't touch it */
+  if (self->preview_text_set)
+    return;
+
+  /* We do the work only once, and cache the result on the PangoFontFace */
+  sample = (const char *)g_object_get_data (G_OBJECT (face), "gtk-sample-text");
+  if (sample)
+    {
+      gtk_font_chooser_widget_set_preview_text (self, sample);
+      return;
+    }
+
+  context = gtk_widget_get_pango_context (GTK_WIDGET (self));
+  font = pango_context_load_font (context, desc);
+
+  if (PANGO_IS_FC_FONT (font))
+    {
+      PangoLanguage **languages;
+      GHashTable *langs = NULL;
+      PangoLanguage *default_lang;
+      PangoLanguage *alt_default = NULL;
+      PangoLanguage *lang = NULL;
+      int i;
+      const char *p;
+
+      default_lang = pango_language_get_default ();
+      p = pango_language_to_string (default_lang);
+
+      /* The default language tends to be of the form en-us.
+       * Since fontconfig languages just have the language part,
+       * and we want to use direct pointer comparisons, we need
+       * an PangoLanguage for the shortened default language.
+       */
+      if (strchr (p, '-'))
+        {
+          char q[10];
+          for (i = 0; p[i] != '-' && i < 9; i++)
+            q[i] = p[i];
+          q[i] = '\0';
+          alt_default = pango_language_from_string (q);
+        }
+
+      languages = pango_fc_font_get_languages (PANGO_FC_FONT (font));
+
+      /* If the font supports the default language, just use it. */
+      for (i = 0; languages[i]; i++)
+        {
+          if (languages[i] == default_lang || languages[i] == alt_default)
+            {
+              lang = default_lang;
+              goto found;
+            }
+        }
+
+      /* Otherwise, we make a list of representative languages */
+      langs = g_hash_table_new (NULL, NULL);
+
+      for (i = 0; languages[i]; i++)
+        {
+          const PangoScript *scripts;
+          int num, j;
+
+          scripts = pango_language_get_scripts (languages[i], &num);
+          for (j = 0; j < num; j++)
+            {
+              lang = pango_script_get_sample_language (scripts[j]);
+              if (lang)
+                g_hash_table_add (langs, lang);
+            }
+        }
+
+      /* ... and compare it to the users default and preferred languages */
+      if (g_hash_table_contains (langs, default_lang) ||
+          g_hash_table_contains (langs, alt_default))
+        {
+          lang = default_lang;
+        }
+      else
+        {
+          PangoLanguage **preferred;
+
+          preferred = pango_language_get_preferred ();
+          if (preferred)
+            {
+              for (i = 0; preferred[i]; i++)
+                {
+                  if (g_hash_table_contains (langs, preferred[i]))
+                    {
+                      lang = preferred[i];
+                      break;
+                    }
+                }
+            }
+        }
+      g_hash_table_unref (langs);
+
+found:
+      sample = pango_language_get_sample_string (lang);
+      gtk_font_chooser_widget_set_preview_text (self, sample);
+      g_object_set_data (G_OBJECT (face), "gtk-sample-text", (gpointer)sample);
+    }
+
+  g_object_unref (font);
+#endif
+}
+
+
 static void
 selection_changed_cb (GtkSingleSelection   *selection,
                       GParamSpec           *pspec,
@@ -452,8 +574,11 @@ selection_changed_cb (GtkSingleSelection   *selection,
       desc = pango_font_face_describe (face);
       pango_font_description_set_variations (self->font_desc, NULL);
       gtk_font_chooser_widget_merge_font_desc (self, desc);
-      pango_font_description_free (desc);
       g_simple_action_set_enabled (G_SIMPLE_ACTION (self->tweak_action), TRUE);
+
+      maybe_update_preview_text (self, face, desc);
+
+      pango_font_description_free (desc);
     }
   else
     {
@@ -1933,6 +2058,9 @@ static void
 gtk_font_chooser_widget_set_preview_text (GtkFontChooserWidget *fontchooser,
                                           const char           *text)
 {
+  if (fontchooser->preview_text == text)
+    return;
+
   g_free (fontchooser->preview_text);
   fontchooser->preview_text = g_strdup (text);
 
diff --git a/meson.build b/meson.build
index 2e9354f847..242d0e394a 100644
--- a/meson.build
+++ b/meson.build
@@ -23,7 +23,7 @@ else
 endif
 
 glib_req           = '>= @0@.@1@.@2@'.format(glib_major_req, glib_minor_req, glib_micro_req)
-pango_req          = '>= 1.45.5'
+pango_req          = '>= 1.47.0'
 fribidi_req        = '>= 0.19.7'
 cairo_req          = '>= 1.14.0'
 gdk_pixbuf_req     = '>= 2.30.0'