[graf] Additional options for TPad::PlaceBox() method

This is needed so that the new RooBrowser doesn't need to access the
private collision grid of the TPad. These additional options allow the
placement of the box with choice of priority over directions as well as
option to place within the margins of the pad.

Wanted for 6.28 release too!

core/base/inc/TVirtualPad.h

@@ -255,7 +255,7 @@ class TVirtualPad : public TObject, public TAttLine, public TAttFill,
    virtual Int_t    IncrementPaletteColor(Int_t i, TString opt) = 0;
    virtual Int_t    NextPaletteColor() = 0;
 
-   virtual Bool_t   PlaceBox(TObject *o, Double_t w, Double_t h, Double_t &xl, Double_t &yb) = 0;
+   virtual Bool_t   PlaceBox(TObject *o, Double_t w, Double_t h, Double_t &xl, Double_t &yb, Option_t* opt = "lb") = 0;
 
    virtual TObject *CreateToolTip(const TBox *b, const char *text, Long_t delayms) = 0;
    virtual void     DeleteToolTip(TObject *tip) = 0;

graf2d/gpad/inc/TPad.h

@@ -386,7 +386,7 @@ friend class TWebCanvas;
    Int_t             NextPaletteColor() override;
 
    void              DrawCollideGrid();
-   Bool_t            PlaceBox(TObject *o, Double_t w, Double_t h, Double_t &xl, Double_t &yb) override;
+   Bool_t            PlaceBox(TObject *o, Double_t w, Double_t h, Double_t &xl, Double_t &yb, Option_t* option = "lb") override;
 
    virtual void      x3d(Option_t *type=""); // Depreciated
 

graf2d/gpad/src/TPad.cxx

@@ -12,6 +12,7 @@
 #include <cstring>
 #include <cstdlib>
 #include <iostream>
+#include <locale>
 #include <memory>
 
 #include "TROOT.h"
@@ -3067,33 +3068,77 @@ Bool_t TPad::Collide(Int_t i, Int_t j, Int_t w, Int_t h)
 ///
 /// \return `true` if the box could be placed, `false` if not.
 ///
-/// \param[in]  o        pointer to the box to be placed
-/// \param[in]  w        box width to be placed
-/// \param[in]  h        box height to be placed
-/// \param[out] xl       x position of the bottom left corner of the placed box
-/// \param[out] yb       y position of the bottom left corner of the placed box
+/// \param[in]  o               pointer to the box to be placed
+/// \param[in]  w               box width to be placed
+/// \param[in]  h               box height to be placed
+/// \param[out] xl              x position of the bottom left corner of the placed box
+/// \param[out] yb              y position of the bottom left corner of the placed box
+/// \param[in]  option          l=left, r=right, t=top, b=bottom, w=within margins. Order determines
+///                             priority for placement. Default is "lb" (prioritises horizontal over vertical)
 
-Bool_t TPad::PlaceBox(TObject *o, Double_t w, Double_t h, Double_t &xl, Double_t &yb)
+Bool_t TPad::PlaceBox(TObject *o, Double_t w, Double_t h, Double_t &xl, Double_t &yb, Option_t* option)
 {
    FillCollideGrid(o);
 
    Int_t iw = (int)(fCGnx*w);
    Int_t ih = (int)(fCGny*h);
 
-   Int_t nxmax = fCGnx-iw-1;
-   Int_t nymax = fCGny-ih-1;
+   Int_t nxbeg = 0;
+   Int_t nybeg = 0;
+   Int_t nxend = fCGnx-iw-1;
+   Int_t nyend = fCGny-ih-1;
+   Int_t dx = 1;
+   Int_t dy = 1;
+
+   bool isFirstVertical = false;
+   bool isFirstHorizontal = false;
+
+   for (std::size_t i = 0; option[i] != '\0'; ++i) {
+      char letter = std::tolower(option[i]);
+      if (letter == 'w') {
+         nxbeg += fCGnx*GetLeftMargin();
+         nybeg += fCGny*GetBottomMargin();
+         nxend -= fCGnx*GetRightMargin();
+         nyend -= fCGny*GetTopMargin();
+      } else if (letter == 't' || letter == 'b') {
+         isFirstVertical = !isFirstHorizontal;
+         // go from top to bottom instead of bottom to top
+         dy = letter == 't' ? -1 : 1;
+      } else if (letter == 'l' || letter == 'r') {
+         isFirstHorizontal = !isFirstVertical;
+         // go from right to left instead of left to right
+         dx = letter == 'r' ? -1 : 1;
+      }
+   }
+
+   if(dx < 0) std::swap(nxbeg, nxend);
+   if(dy < 0) std::swap(nybeg, nyend);
+
+   auto attemptPlacement = [&](Int_t i, Int_t j) {
+      if (Collide(i, j, iw, ih)) {
+         return false;
+      } else {
+         xl = (Double_t)(i) / (Double_t)(fCGnx);
+         yb = (Double_t)(j) / (Double_t)(fCGny);
+         return true;
+      }
+   };
 
-   for (Int_t i = 0; i<nxmax; i++) {
-      for (Int_t j = 0; j<=nymax; j++) {
-         if (Collide(i,j,iw,ih)) {
-            continue;
-         } else {
-            xl = (Double_t)(i)/(Double_t)(fCGnx);
-            yb = (Double_t)(j)/(Double_t)(fCGny);
-            return kTRUE;
+   if(!isFirstVertical) {
+      for (Int_t i = nxbeg; i != nxend; i += dx) {
+         for (Int_t j = nybeg; j != nyend; j += dy) {
+            if (attemptPlacement(i, j)) return true;
+         }
+      }
+   } else {
+      // prioritizing vertical over horizontal
+      for (Int_t j = nybeg; j != nyend; j += dy) {
+         for (Int_t i = nxbeg; i != nxend; i += dx) {
+            if (attemptPlacement(i, j)) return true;
          }
       }
    }
+
    return kFALSE;
 }