[gnome-calculator] Fix #22, solve on [=] key if the input is not a variable name.

[Robert Roth <robertroth src gnome org>]

commit 2123989d5fcfc06d47ca71148893616e101d448d
Author: Alberto González Palomo <bugs sentido-labs com>
Date:   Wed Nov 13 22:57:28 2019 +0000

    Fix #22, solve on [=] key if the input is not a variable name.
    
    If the input is a variable name, this would be an assignment
    so the "=" sign is inserted.
    Otherwise, the equation is solved which makes usage much more
    comfortable in certain keyboards like mine.

 src/math-display.vala | 11 +++++++++++
 1 file changed, 11 insertions(+)
---
diff --git a/src/math-display.vala b/src/math-display.vala
index a54bb778..11cb5ba4 100644
--- a/src/math-display.vala
+++ b/src/math-display.vala
@@ -24,6 +24,8 @@ public class MathDisplay : Gtk.Viewport
     /* Spinner widget that shows if we're calculating a response */
     Gtk.Spinner spinner;
 
+    Regex only_variable_name = /^_*\p{L}+(_|\p{L})*$/;
+
     public MathDisplay (MathEquation equation)
     {
         _equation = equation;
@@ -210,6 +212,15 @@ public class MathDisplay : Gtk.Viewport
 
         var c = Gdk.keyval_to_unicode (event.keyval);
 
+        /* Solve on [=] if the input is not a variable name */
+        if (event.keyval == Gdk.Key.equal || event.keyval == Gdk.Key.KP_Equal)
+        {
+            if (!only_variable_name.match((string) equation.equation))
+            {
+                event.keyval = Gdk.Key.KP_Enter;
+            }
+        }
+
         /* Solve on enter */
         if (event.keyval == Gdk.Key.Return || event.keyval == Gdk.Key.KP_Enter)
         {