Another piece of random useless code in QGIS. Happy April 1st!

Type "bored" into the coordinates box (best results with a map loaded in canvas)

Click on tiles to move them to the empty space.
Click on the empty tile to toggle tile numbers.

src/app/CMakeLists.txt

@@ -57,6 +57,7 @@ SET(QGIS_APP_SRCS
   qgsmapcanvasdockwidget.cpp
   qgsmaplayerstyleguiutils.cpp
   qgsmapsavedialog.cpp
+  qgspuzzlewidget.cpp
   qgsversionmigration.cpp
   qgsrulebasedlabelingwidget.cpp
   qgssavestyletodbdialog.cpp
@@ -269,6 +270,7 @@ SET (QGIS_APP_MOC_HDRS
   qgsmapcanvasdockwidget.h
   qgsmaplayerstyleguiutils.h
   qgsmapsavedialog.h
+  qgspuzzlewidget.h
   qgsrulebasedlabelingwidget.h
   qgssavestyletodbdialog.h
   qgssnappinglayertreemodel.h

src/app/qgisapp.cpp

@@ -242,6 +242,7 @@ Q_GUI_EXPORT extern int qt_defaultDpiX();
 #include "qgspluginmanager.h"
 #include "qgspluginregistry.h"
 #include "qgspointxy.h"
+#include "qgspuzzlewidget.h"
 #include "qgsruntimeprofiler.h"
 #include "qgshandlebadlayers.h"
 #include "qgsprintlayout.h"
@@ -1335,6 +1336,19 @@ QgisApp::QgisApp( QSplashScreen *splash, bool restorePlugins, bool skipVersionCh
   connect( QgsProject::instance(), &QgsProject::isDirtyChanged, mActionRevertProject, toggleRevert );
   connect( QgsProject::instance(), &QgsProject::fileNameChanged, mActionRevertProject, toggleRevert );
 
+  // the most important part of the initialization: make sure that people can play puzzle if they need
+  QgsPuzzleWidget *puzzleWidget = new QgsPuzzleWidget( mMapCanvas );
+  mCentralContainer->insertWidget( 2, puzzleWidget );
+  connect( mCoordsEdit, &QgsStatusBarCoordinatesWidget::weAreBored, this, [ this, puzzleWidget ]
+  {
+    puzzleWidget->letsGetThePartyStarted();
+    mCentralContainer->setCurrentIndex( 2 );
+  } );
+  connect( puzzleWidget, &QgsPuzzleWidget::done, this, [ this ]
+  {
+    mCentralContainer->setCurrentIndex( 0 );
+  } );
+
 } // QgisApp ctor
 
 QgisApp::QgisApp()

src/app/qgspuzzlewidget.cpp

@@ -0,0 +1,185 @@
+/***************************************************************************
+  qgspuzzlewidget.cpp
+  --------------------------------------
+  Date                 : 1st of April 2018
+  Copyright            : (C) 2018 by Martin Dobias
+  Email                : wonder dot sk at gmail dot com
+ ***************************************************************************
+ *                                                                         *
+ *   This program is free software; you can redistribute it and/or modify  *
+ *   it under the terms of the GNU General Public License as published by  *
+ *   the Free Software Foundation; either version 2 of the License, or     *
+ *   (at your option) any later version.                                   *
+ *                                                                         *
+ ***************************************************************************/
+
+#include "qgspuzzlewidget.h"
+
+#include "qgsmapcanvas.h"
+
+#include <QGraphicsPixmapItem>
+#include <QMessageBox>
+
+
+static bool testSolved( QVector<int> &positions )
+{
+  for ( int i = 0; i < positions.count() - 1; ++i )
+  {
+    if ( positions[i] != i )
+      return false;
+  }
+  return true;
+}
+
+static int findEmpty( QVector<int> &positions )
+{
+  for ( int i = 0; i < positions.count(); ++i )
+  {
+    if ( positions[i] == -1 )
+      return i;
+  }
+  Q_ASSERT( false );
+  return -1;
+}
+
+// half of the possible permutations lead to an unsolvable puzzle, so we initialize the puzzle by
+// doing valid moves
+// https://en.wikipedia.org/wiki/15_puzzle
+static void shuffle( QVector<int> &positions, int count )
+{
+  int size = sqrt( positions.count() );
+  int idxEmpty = findEmpty( positions );
+  int cEmpty = idxEmpty % size, rEmpty = idxEmpty / size;
+  int moveX[] = { 0, 0, 1, -1 };
+  int moveY[] = { 1, -1, 0, 0 };
+  int cOther, rOther;
+  for ( int i = 0; i < count; ++i )
+  {
+    do
+    {
+      int move = qrand() % 4;
+      cOther = cEmpty + moveX[move];
+      rOther = rEmpty + moveY[move];
+    }
+    while ( cOther < 0 || cOther >= size || rOther < 0 || rOther >= size );
+
+    int idxOther = rOther * size + cOther;
+    std::swap( positions[idxEmpty], positions[idxOther] );
+    idxEmpty = idxOther;
+    cEmpty = idxEmpty % size;
+    rEmpty = idxEmpty / size;
+  }
+}
+
+QgsPuzzleWidget::QgsPuzzleWidget( QgsMapCanvas *canvas )
+  : mCanvas( canvas )
+{
+  setInteractive( false );
+  setBackgroundBrush( QBrush( Qt::lightGray ) );
+}
+
+void QgsPuzzleWidget::mousePressEvent( QMouseEvent *event )
+{
+  int idxEmpty = findEmpty( mPositions );
+  int rEmpty = idxEmpty / mSize;
+  int cEmpty = idxEmpty % mSize;
+  int cMouse = event->pos().x() / mTileWidth;
+  int rMouse = event->pos().y() / mTileHeight;
+  int idxMouse = rMouse * mSize + cMouse;
+  int dx = cMouse - cEmpty;
+  int dy = rMouse - rEmpty;
+
+  if ( ( dx == 0 && qAbs( dy ) == 1 ) || ( dy == 0 && qAbs( dx ) == 1 ) )
+  {
+    std::swap( mPositions[idxEmpty], mPositions[idxMouse] );
+    updateTilePositions();
+    if ( testSolved( mPositions ) )
+    {
+      QMessageBox::information( nullptr, tr( "QGIS" ), tr( "Well done!\n\nNow let's get back to work, shall we?" ) );
+      emit done();
+    }
+  }
+  else if ( dx == 0 && dy == 0 )
+  {
+    // toggle text help when clicked on empty tile
+    for ( int i = 0; i < mTextItems.count(); ++i )
+      mTextItems[i]->setVisible( !mTextItems[i]->isVisible() );
+  }
+}
+
+void QgsPuzzleWidget::letsGetThePartyStarted()
+{
+  mPositions.clear();
+  delete mScene;
+  mItems.clear();
+  mTextItems.clear();
+
+  mScene = new QGraphicsScene;
+
+  QTemporaryFile f;
+  f.open();
+  f.close();
+
+  QString filename( f.fileName() );
+  mCanvas->saveAsImage( filename );
+
+  QPixmap pixmap;
+  pixmap.load( filename );
+
+  int tileWidth = pixmap.width() / mSize;
+  int tileHeight = pixmap.height() / mSize;
+  mTileWidth = tileWidth;
+  mTileHeight = tileHeight;
+
+  for ( int row = 0; row < mSize; ++row )
+    for ( int col = 0; col < mSize; ++col )
+    {
+      QGraphicsPixmapItem *item = new QGraphicsPixmapItem;
+      item->setPixmap( pixmap.copy( col * tileWidth, row * tileHeight, tileWidth, tileHeight ) );
+      mScene->addItem( item ); // takes ownership
+      mItems.append( item );
+
+      QGraphicsSimpleTextItem *textItem = new QGraphicsSimpleTextItem( QString::number( row * mSize + col + 1 ), item );
+      QRectF textRect = textItem->boundingRect();
+      textItem->setPos( ( tileWidth - textRect.width() ) / 2, ( tileHeight - textRect.height() ) / 2 );
+      textItem->setVisible( false );
+      mTextItems.append( textItem );
+    }
+
+  // extra lines to show tile split points
+  for ( int i = 1; i < mSize; ++i )
+  {
+    QGraphicsLineItem *lineHorz = new QGraphicsLineItem( 0, i * tileHeight, tileWidth * mSize, i * tileHeight );
+    QGraphicsLineItem *lineVert = new QGraphicsLineItem( i * tileWidth, 0, i * tileWidth, tileHeight * mSize );
+    lineHorz->setZValue( 10 );
+    lineVert->setZValue( 10 );
+    mScene->addItem( lineHorz );
+    mScene->addItem( lineVert );
+  }
+
+  // initialize positions
+  for ( int i = 0; i < mSize * mSize; ++i )
+    mPositions << i;
+  mPositions[mSize * mSize - 1] = -1;
+  shuffle( mPositions, 1000 );
+
+  mItems[mSize * mSize - 1]->setVisible( false ); // hide item for the missing piece
+
+  updateTilePositions();
+
+  setScene( mScene );
+}
+
+void QgsPuzzleWidget::updateTilePositions()
+{
+  for ( int i = 0; i < mSize * mSize; ++i )
+  {
+    int itemIndex = mPositions[i];
+    if ( itemIndex == -1 )
+      continue;  // empty tile
+
+    int r = i / mSize, c = i % mSize;
+    int x = c * mTileWidth, y = r * mTileHeight;
+    mItems[itemIndex]->setPos( x, y );
+  }
+}

src/app/qgspuzzlewidget.h

@@ -0,0 +1,52 @@
+/***************************************************************************
+  qgspuzzlewidget.h
+  --------------------------------------
+  Date                 : 1st of April 2018
+  Copyright            : (C) 2018 by Martin Dobias
+  Email                : wonder dot sk at gmail dot com
+ ***************************************************************************
+ *                                                                         *
+ *   This program is free software; you can redistribute it and/or modify  *
+ *   it under the terms of the GNU General Public License as published by  *
+ *   the Free Software Foundation; either version 2 of the License, or     *
+ *   (at your option) any later version.                                   *
+ *                                                                         *
+ ***************************************************************************/
+
+#ifndef QGSPUZZLEWIDGET_H
+#define QGSPUZZLEWIDGET_H
+
+#include <QGraphicsView>
+
+class QgsMapCanvas;
+
+
+class QgsPuzzleWidget : public QGraphicsView
+{
+    Q_OBJECT
+  public:
+    explicit QgsPuzzleWidget( QgsMapCanvas *canvas = nullptr );
+
+  protected:
+    void mousePressEvent( QMouseEvent *event ) override;
+
+  signals:
+    void done();
+
+  public slots:
+    void letsGetThePartyStarted();
+
+  private:
+    void updateTilePositions();
+
+  private:
+    QgsMapCanvas *mCanvas = nullptr;
+    QGraphicsScene *mScene = nullptr;
+    int mSize = 4;  //!< Number of items in one row/column
+    int mTileWidth = 0, mTileHeight = 0;
+    QVector<QGraphicsItem *> mItems;
+    QVector<QGraphicsItem *> mTextItems;
+    QVector<int> mPositions;  //!< Indices of items (-1 where the piece is missing)
+};
+
+#endif // QGSPUZZLEWIDGET_H

src/app/qgsstatusbarcoordinateswidget.cpp

@@ -138,6 +138,11 @@ void QgsStatusBarCoordinatesWidget::validateCoordinates()
     refreshMapCanvas();
     return;
   }
+  else if ( mLineEdit->text() == QStringLiteral( "bored" ) )
+  {
+    // it's friday afternoon and too late to start another piece of work...
+    emit weAreBored();
+  }
 
   bool xOk = false;
   bool  yOk = false;

src/app/qgsstatusbarcoordinateswidget.h

@@ -52,6 +52,7 @@ class APP_EXPORT QgsStatusBarCoordinatesWidget : public QWidget
 
   signals:
     void coordinatesChanged();
+    void weAreBored();
 
   private slots:
     void showMouseCoordinates( const QgsPointXY &p );