DisplayComponent: switch to mode with closest refresh rate

Since we unfortunately use floats for refresh rate, it's hard to compare
them exactly. The switch command makes this more complicated because the
user enters the refresh rate as decimal.

Change how selection works: while other parameters must be matched
exactly, select the closest refresh rate otherwise.

Also, add switching to explicit modes. This can be done with e.g.:

    "host:switch mode=1"

This should always strictly switch to mode 1, without going through the
command's normal matching rules.

This should help with debugging, especially on the forums.

src/display/DisplayComponent.cpp

@@ -306,13 +306,14 @@ QString DisplayComponent::debugInformation()
 }
 
 ///////////////////////////////////////////////////////////////////////////////////////////////////
-static bool modeEqualsFuzzy(const DMVideoMode& m1, const DMVideoMode& m2, float tolerance)
+static float modeDistance(const DMVideoMode& m1, const DMVideoMode& m2)
 {
-  return m1.height == m2.height &&
-         m1.width == m2.width &&
-         fabs(m1.refreshRate - m2.refreshRate) < tolerance &&
-         m1.bitsPerPixel == m2.bitsPerPixel &&
-         m1.interlaced == m2.interlaced;
+  if (m1.height == m2.height &&
+      m1.width == m2.width &&
+      m1.bitsPerPixel == m2.bitsPerPixel &&
+      m1.interlaced == m2.interlaced)
+    return fabs(m1.refreshRate - m2.refreshRate);
+  return -1;
 }
 
 /////////////////////////////////////////////////////////////////////////////////////////
@@ -344,6 +345,7 @@ void DisplayComponent::switchCommand(QString command)
   }
   DMVideoMode current_mode = *m_displayManager->displays[currentDisplay]->videoModes[id];
   DMVideoMode mode = current_mode;
+  int best_mode = -1; // if -1, select it by using the mode variable above
 
   foreach (QString a, command.split(" "))
   {
@@ -364,6 +366,14 @@ void DisplayComponent::switchCommand(QString command)
       if (ok)
         mode.refreshRate = rate;
     }
+    else if (a.startsWith("mode="))
+    {
+      a = a.mid(5);
+      bool ok;
+      int new_id = a.toInt(&ok);
+      if (ok && m_displayManager->isValidDisplayMode(currentDisplay, new_id))
+        best_mode = new_id;
+    }
     else if (a.indexOf("x") >= 0)
     {
       QStringList sub = a.split("x");
@@ -382,29 +392,46 @@ void DisplayComponent::switchCommand(QString command)
   }
 
   QLOG_INFO() << "Current mode:" << current_mode.getPrettyName();
-  QLOG_INFO() << "Mode requested by command:" << mode.getPrettyName();
 
-  foreach (auto cur, m_displayManager->displays[currentDisplay]->videoModes)
+  if (best_mode < 0)
   {
-    // Require matching one digit before the decimal point.
-    // This doesn't work if there are several modes that would match. To do
-    // this without requiring the user to give the refresh rate with full
-    // precision, we'd have to find a closest match instead.
-    if (modeEqualsFuzzy(*cur, mode, 0.1))
+    QLOG_INFO() << "Mode requested by command:" << mode.getPrettyName();
+
+    foreach (auto cur, m_displayManager->displays[currentDisplay]->videoModes)
     {
-      QLOG_INFO() << "Found mode to switch to:" << cur->getPrettyName();
-      if (m_displayManager->setDisplayMode(currentDisplay, cur->id))
+      // "Score" according to what was requested
+      float d_cur = modeDistance(*cur, mode);
+      if (d_cur < 0)
+        continue;
+      if (best_mode < 0)
       {
-        m_lastDisplay = m_lastVideoMode = -1;
+        best_mode = cur->id;
       }
       else
       {
-        QLOG_INFO() << "Switching failed.";
+        // "Score" according to best mode
+        float d_best = modeDistance(*m_displayManager->displays[currentDisplay]->videoModes[best_mode],
+                                    mode);
+        if (d_cur < d_best)
+          best_mode = cur->id;
       }
-      return;
     }
   }
 
+  if (best_mode >= 0)
+  {
+    QLOG_INFO() << "Found mode to switch to:" << m_displayManager->displays[currentDisplay]->videoModes[best_mode]->getPrettyName();
+    if (m_displayManager->setDisplayMode(currentDisplay, best_mode))
+    {
+      m_lastDisplay = m_lastVideoMode = -1;
+    }
+    else
+    {
+      QLOG_INFO() << "Switching failed.";
+    }
+    return;
+  }
+
   QLOG_INFO() << "Requested mode not found.";
 }