libshell: Adjusted autocompletion to insert common prefix first

If all the possible completions have the same prefix, it will be
appended as the first completion.

doomsday/libdeng2/include/de/data/string.h

@@ -203,14 +203,15 @@ class DENG2_PUBLIC String : public QString
     dint compareWithoutCase(String const &str, int n) const;
 
     /**
-     * Compares two strings (case sensitive) to see how many characters they
-     * have in common starting from the left.
+     * Compares two strings to see how many characters they have in common
+     * starting from the left.
      *
-     * @param str  String.
+     * @param str          String.
+     * @param sensitivity  Case sensitivity.
      *
      * @return  Number of characters the two strings have in common from the left.
      */
-    int commonPrefixLength(String const &str) const;
+    int commonPrefixLength(String const &str, Qt::CaseSensitivity sensitivity = Qt::CaseSensitive) const;
 
     /**
      * Converts the string to UTF-8 and returns it as a byte block.

doomsday/libdeng2/src/data/string.cpp

@@ -354,13 +354,20 @@ dint String::compareWithoutCase(const String &str, int n) const
     return left(n).compare(str.left(n), Qt::CaseInsensitive);
 }
 
-int String::commonPrefixLength(String const &str) const
+int String::commonPrefixLength(String const &str, Qt::CaseSensitivity sensitivity) const
 {
     int count = 0;
     int len = qMin(str.size(), size());
     for(int i = 0; i < len; ++i, ++count)
     {
-        if(at(i) != str.at(i)) break;
+        if(sensitivity == Qt::CaseSensitive)
+        {
+            if(at(i) != str.at(i)) break;
+        }
+        else
+        {
+            if(at(i).toLower() != str.at(i).toLower()) break;
+        }
     }
     return count;
 }

doomsday/libshell/src/lineeditwidget.cpp

@@ -214,14 +214,27 @@ DENG2_PIMPL(LineEditWidget)
         return word;
     }
 
-    QList<String> completionsForBase(String base) const
+    QList<String> completionsForBase(String base, String &commonPrefix) const
     {
+        bool first = true;
         QList<String> suggestions;
         foreach(String term, lexicon.terms())
         {
             if(term.startsWith(base, Qt::CaseInsensitive) && term.size() > base.size())
             {
                 suggestions.append(term);
+
+                // Determine if all the suggestions have a common prefix.
+                if(first)
+                {
+                    commonPrefix = term;
+                    first = false;
+                }
+                else if(!commonPrefix.isEmpty())
+                {
+                    int len = commonPrefix.commonPrefixLength(term, Qt::CaseInsensitive);
+                    commonPrefix = commonPrefix.left(len);
+                }
             }
         }
         qSort(suggestions);
@@ -236,7 +249,18 @@ DENG2_PIMPL(LineEditWidget)
             if(!base.isEmpty())
             {
                 // Find all the possible completions and apply the first one.
-                suggestions = completionsForBase(base);
+                String commonPrefix;
+                suggestions = completionsForBase(base, commonPrefix);
+                if(!commonPrefix.isEmpty() && commonPrefix != base)
+                {
+                    completion.ordinal = -1;
+                    commonPrefix.remove(0, base.size());
+                    completion.pos = cursor;
+                    completion.size = commonPrefix.size();
+                    text.insert(cursor, commonPrefix);
+                    cursor += completion.size;
+                    return true;
+                }
                 if(!suggestions.isEmpty())
                 {
                     completion.ordinal = (forwardCycle? 0 : suggestions.size() - 1);
@@ -256,9 +280,23 @@ DENG2_PIMPL(LineEditWidget)
             cursor = completion.pos;
             String const base = wordBehindCursor();
 
-            // Go to next suggestion.
-            completion.ordinal = de::wrap(completion.ordinal + (forwardCycle? 1 : -1),
-                                          0, suggestions.size());
+            if(completion.ordinal < 0)
+            {
+                // This occurs after a common prefix is inserted rather than
+                // a full suggestion.
+                completion.ordinal = (forwardCycle? 0 : suggestions.size() - 1);
+
+                if(base + text.mid(completion.pos, completion.size) == suggestions[completion.ordinal])
+                {
+                    // We already had this one, skip it.
+                    cycleCompletion(forwardCycle);
+                }
+            }
+            else
+            {
+                cycleCompletion(forwardCycle);
+            }
+
             String comp = suggestions[completion.ordinal];
             comp.remove(0, base.size());
 
@@ -271,6 +309,12 @@ DENG2_PIMPL(LineEditWidget)
         return false;
     }
 
+    void cycleCompletion(bool forwardCycle)
+    {
+        completion.ordinal = de::wrap(completion.ordinal + (forwardCycle? 1 : -1),
+                                      0, suggestions.size());
+    }
+
     void acceptCompletion()
     {
         completion.reset();