Eeschema: Make auto-decrementing net labels stop at zero

CHANGED: If you set the auto-increment value to a negative number,
created a net label and hit the insert key, when the name got below zero,
the net labels tried to go negative but created bad labels instead.
Now the code caps decrementing at zero,  warns users when it has
happened, and explains why.

Fixes https://gitlab.com/kicad/code/kicad/issues/6537

eeschema/sch_text.cpp

@@ -51,15 +51,18 @@
 using KIGFX::SCH_RENDER_SETTINGS;
 
 
-void IncrementLabelMember( wxString& name, int aIncrement )
+bool IncrementLabelMember( wxString& name, int aIncrement )
 {
     int  ii, nn;
     long number = 0;
 
-    ii = name.Len() - 1; nn = 0;
+    ii = name.Len() - 1;
+    nn = 0;
+
+    // No number found, but simply repeating the same label is valid
 
     if( !wxIsdigit( name.GetChar( ii ) ) )
-        return;
+        return true;
 
     while( ii >= 0 && wxIsdigit( name.GetChar( ii ) ) )
     {
@@ -67,14 +70,23 @@ void IncrementLabelMember( wxString& name, int aIncrement )
         nn++;
     }
 
-    ii++;   /* digits are starting at ii position */
+    ii++; /* digits are starting at ii position */
     wxString litt_number = name.Right( nn );
 
     if( litt_number.ToLong( &number ) )
     {
         number += aIncrement;
-        name.Remove( ii ); name << number;
+
+        // Don't let result go below zero
+
+        if( number > -1 )
+        {
+            name.Remove( ii );
+            name << number;
+            return true;
+        }
     }
+    return false;
 }
 
 
@@ -149,11 +161,15 @@ EDA_ITEM* SCH_TEXT::Clone() const
 }
 
 
-void SCH_TEXT::IncrementLabel( int aIncrement )
+bool SCH_TEXT::IncrementLabel( int aIncrement )
 {
     wxString text = GetText();
-    IncrementLabelMember( text, aIncrement );
-    SetText(text );
+    bool ReturnVal = IncrementLabelMember( text, aIncrement );
+    
+    if( ReturnVal )
+        SetText( text );
+
+    return ReturnVal;
 }
 
 

eeschema/sch_text.h

@@ -221,7 +221,7 @@ public:
      *
      * @param aIncrement = the increment value to add to the number ending the text.
      */
-    void IncrementLabel( int aIncrement );
+    bool IncrementLabel( int aIncrement );
 
     /**
      * Set a spin or rotation angle, along with specific horizontal and vertical justification

eeschema/tools/lib_pin_tool.cpp

@@ -80,7 +80,7 @@ static int GetLastPinNumSize()
 }
 
 
-extern void IncrementLabelMember( wxString& name, int aIncrement );
+extern bool IncrementLabelMember( wxString& name, int aIncrement );
 
 
 LIB_PIN_TOOL::LIB_PIN_TOOL() :

eeschema/tools/sch_edit_tool.cpp

@@ -904,7 +904,11 @@ int SCH_EDIT_TOOL::RepeatDrawItem( const TOOL_EVENT& aEvent )
             if( dynamic_cast<SCH_TEXT*>( newItem ) )
             {
                 SCH_TEXT* text = static_cast<SCH_TEXT*>( newItem );
-                text->IncrementLabel( cfg->m_Drawing.repeat_label_increment );
+
+                // If incrementing tries to go below zero, tell user why the value is repeated
+
+                if( !text->IncrementLabel( cfg->m_Drawing.repeat_label_increment ) )
+                    m_frame->ShowInfoBarMsg( _( "Label value cannot go below zero" ) );
             }
 
             newItem->Move( wxPoint( Mils2iu( cfg->m_Drawing.default_repeat_offset_x ),