Fixes cross staff issue with beam collision avoidance.

[lilypond.git] / lily / beam.cc

/*
  This file is part of LilyPond, the GNU music typesetter.

  Copyright (C) 1997--2011 Han-Wen Nienhuys <hanwen@xs4all.nl>
  Jan Nieuwenhuizen <janneke@gnu.org>

  LilyPond 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 3 of the License, or
  (at your option) any later version.

  LilyPond is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  GNU General Public License for more details.

  You should have received a copy of the GNU General Public License
  along with LilyPond.  If not, see <http://www.gnu.org/licenses/>.
*/

/*
  TODO:

  - Determine auto knees based on positions if it's set by the user.

  - the code is littered with * and / staff_space calls for
  #'positions. Consider moving to real-world coordinates?

  Problematic issue is user tweaks (user tweaks are in staff-coordinates.)

  Notes:

  - Stems run to the Y-center of the beam.

  - beam_translation is the offset between Y centers of the beam.
*/

#include "beam.hh"

#include "align-interface.hh"
#include "beam-scoring-problem.hh"
#include "beaming-pattern.hh"
#include "directional-element-interface.hh"
#include "grob-array.hh"
#include "international.hh"
#include "interval-set.hh"
#include "item.hh"
#include "least-squares.hh"
#include "lookup.hh"
#include "main.hh"
#include "misc.hh"
#include "note-head.hh"
#include "output-def.hh"
#include "pointer-group-interface.hh"
#include "rhythmic-head.hh"
#include "spanner.hh"
#include "staff-symbol-referencer.hh"
#include "stem.hh"
#include "warn.hh"

#if DEBUG_BEAM_SCORING
#include "text-interface.hh" // debug output.
#include "font-interface.hh" // debug output.
#endif

#include <map>


Beam_stem_segment::Beam_stem_segment ()
{
  max_connect_ = 1000;          // infinity
  stem_ = 0;
  width_ = 0.0;
  stem_x_ = 0.0;
  rank_ = 0;
  stem_index_ = 0;
  dir_ = CENTER;
}

bool
beam_segment_less (Beam_segment const& a, Beam_segment const& b)
{
  return a.horizontal_[LEFT] < b.horizontal_[LEFT];
}

Beam_segment::Beam_segment ()
{
  vertical_count_ = 0;
}

void
Beam::add_stem (Grob *me, Grob *s)
{
  if (Stem::get_beam (s))
    {
      programming_error ("Stem already has beam");
      return ;
    }

  Pointer_group_interface::add_grob (me, ly_symbol2scm ("stems"), s);
  s->set_object ("beam", me->self_scm ());
  add_bound_item (dynamic_cast<Spanner *> (me), dynamic_cast<Item *> (s));
}

Real
Beam::get_beam_thickness (Grob *me)
{
  return robust_scm2double (me->get_property ("beam-thickness"), 0)
    * Staff_symbol_referencer::staff_space (me);
}

/* Return the translation between 2 adjoining beams. */
Real
Beam::get_beam_translation (Grob *me)
{
  int beam_count = get_beam_count (me);
  Real staff_space = Staff_symbol_referencer::staff_space (me);
  Real line = Staff_symbol_referencer::line_thickness (me);
  Real beam_thickness = get_beam_thickness (me);
  Real fract = robust_scm2double (me->get_property ("length-fraction"), 1.0);

  Real beam_translation = beam_count < 4
    ? (2 * staff_space + line - beam_thickness) / 2.0
    : (3 * staff_space + line - beam_thickness) / 3.0;

  return fract * beam_translation;
}

/* Maximum beam_count. */
int
Beam::get_beam_count (Grob *me)
{
  int m = 0;

  extract_grob_set (me, "stems", stems);
  for (vsize i = 0; i < stems.size (); i++)
    {
      Grob *stem = stems[i];
      m = max (m, (Stem::beam_multiplicity (stem).length () + 1));
    }
  return m;
}

MAKE_SCHEME_CALLBACK (Beam, calc_normal_stems, 1);
SCM
Beam::calc_normal_stems (SCM smob)
{
  Grob *me = unsmob_grob (smob);

  extract_grob_set (me, "stems", stems);
  SCM val = Grob_array::make_array ();
  Grob_array *ga = unsmob_grob_array (val);
  for (vsize i = 0; i < stems.size ();  i++)
    if (Stem::is_normal_stem (stems[i]))
      ga->add (stems[i]);

  return val;
}

MAKE_SCHEME_CALLBACK (Beam, calc_direction, 1);
SCM
Beam::calc_direction (SCM smob)
{
  Grob *me = unsmob_grob (smob);

  /* Beams with less than 2 two stems don't make much sense, but could happen
     when you do

     r8[ c8 r8]

  */

  Direction dir = CENTER;

  int count = normal_stem_count (me);
  if (count < 2)
    {
      extract_grob_set (me, "stems", stems);
      if (stems.size () == 0)
        {
          me->warning (_ ("removing beam with no stems"));
          me->suicide ();

          return SCM_UNSPECIFIED;
        }
      else
        {
          Grob *stem = first_normal_stem (me);

          /*
            This happens for chord tremolos.
          */
          if (!stem)
            stem = stems[0];

          if (is_direction (stem->get_property_data ("direction")))
            dir = to_dir (stem->get_property_data ("direction"));
          else
            dir = to_dir (stem->get_property ("default-direction"));
        }
    }

  if (count >= 1)
    {
      if (!dir)
        dir = get_default_dir (me);

      consider_auto_knees (me);
    }

  if (dir)
    {
      set_stem_directions (me, dir);
    }

  return scm_from_int (dir);
}



/* We want a maximal number of shared beams, but if there is choice, we
 * take the one that is closest to the end of the stem. This is for
 * situations like
 *
 *        x
 *       |
 *       |
 *   |===|
 *   |=
 *   |
 *  x
 */
int
position_with_maximal_common_beams (SCM left_beaming, SCM right_beaming,
                                    Direction left_dir,
                                    Direction right_dir)
{
  Slice lslice = int_list_to_slice (scm_cdr (left_beaming));

  int best_count = 0;
  int best_start = 0;
  for (int i = lslice[-left_dir];
       (i - lslice[left_dir]) * left_dir <= 0; i += left_dir)
    {
      int count = 0;
      for (SCM s = scm_car (right_beaming); scm_is_pair (s); s = scm_cdr (s))
        {
          int k = -right_dir * scm_to_int (scm_car (s)) + i;
          if (scm_c_memq (scm_from_int (k), left_beaming) != SCM_BOOL_F)
            count++;
        }

      if (count >= best_count)
        {
          best_count = count;
          best_start = i;
        }
    }

  return best_start;
}

MAKE_SCHEME_CALLBACK (Beam, calc_beaming, 1)
SCM
Beam::calc_beaming (SCM smob)
{
  Grob *me = unsmob_grob (smob);

  extract_grob_set (me, "stems", stems);

  Slice last_int;
  last_int.set_empty ();

  SCM last_beaming = scm_cons (SCM_EOL, scm_list_1 (scm_from_int (0)));
  Direction last_dir = CENTER;
  for (vsize i = 0; i < stems.size (); i++)
    {
      Grob *this_stem = stems[i];
      SCM this_beaming = this_stem->get_property ("beaming");

      Direction this_dir = get_grob_direction (this_stem);
      if (scm_is_pair (last_beaming) && scm_is_pair (this_beaming))
        {
          int start_point = position_with_maximal_common_beams
            (last_beaming, this_beaming,
             last_dir ? last_dir : this_dir,
             this_dir);

          Direction d = LEFT;
          Slice new_slice;
          do
            {
              new_slice.set_empty ();
              SCM s = index_get_cell (this_beaming, d);
              for (; scm_is_pair (s); s = scm_cdr (s))
                {
                  int new_beam_pos
                    = start_point - this_dir * scm_to_int (scm_car (s));

                  new_slice.add_point (new_beam_pos);
                  scm_set_car_x (s, scm_from_int (new_beam_pos));
                }
            }
          while (flip (&d) != LEFT);

          if (!new_slice.is_empty ())
            last_int = new_slice;
        }
      else
        {
          /*
            FIXME: what's this for?
           */
          SCM s = scm_cdr (this_beaming);
          for (; scm_is_pair (s); s = scm_cdr (s))
            {
              int np = -this_dir * scm_to_int (scm_car (s));
              scm_set_car_x (s, scm_from_int (np));
              last_int.add_point (np);
            }
        }

      if (scm_ilength (scm_cdr (this_beaming)) > 0)
        {
          last_beaming = this_beaming;
          last_dir = this_dir;
        }
    }

  return SCM_EOL;
}

bool
operator <(Beam_stem_segment const &a,
           Beam_stem_segment const &b)
{
  return a.rank_ < b.rank_;
}

typedef map<int, vector<Beam_stem_segment> >  Position_stem_segments_map;

// TODO - should store result in a property?
vector<Beam_segment>
Beam::get_beam_segments (Grob *me_grob, Grob **common)
{
  /* ugh, this has a side-effect that we need to ensure that
     Stem #'beaming is correct */
  (void) me_grob->get_property ("beaming");

  Spanner *me = dynamic_cast<Spanner*> (me_grob);

  extract_grob_set (me, "stems", stems);
  Grob *commonx = common_refpoint_of_array (stems, me, X_AXIS);

  commonx = me->get_bound (LEFT)->common_refpoint (commonx, X_AXIS);
  commonx = me->get_bound (RIGHT)->common_refpoint (commonx, X_AXIS);

  *common = commonx;

  int gap_count = robust_scm2int (me->get_property ("gap-count"), 0);
  Real gap_length = robust_scm2double (me->get_property ("gap"), 0.0);

  Position_stem_segments_map stem_segments;
  Real lt = me->layout ()->get_dimension (ly_symbol2scm ("line-thickness"));

  /* There are two concepts of "rank" that are used in the following code.
     The beam_rank is the vertical position of the beam (larger numbers are
     closer to the noteheads). Beam_stem_segment.rank_, on the other hand,
     is the horizontal position of the segment (this is incremented by two
     for each stem; the beam segment on the right side of the stem has
     a higher rank (by one) than its neighbour to the left). */
  Slice ranks;
  for (vsize i = 0; i < stems.size (); i++)
    {
      Grob *stem = stems[i];
      Real stem_width = robust_scm2double (stem->get_property ("thickness"), 1.0) * lt;
      Real stem_x = stem->relative_coordinate (commonx, X_AXIS);
      SCM beaming = stem->get_property ("beaming");
      Direction d = LEFT;
      do
        {
          // Find the maximum and minimum beam ranks.
          // Given that RANKS is never reset to empty, the interval will always be
          // smallest for the left beamlet of the first stem, and then it might grow.
          // Do we really want this? (It only affects the tremolo gaps) --jneem
          for (SCM s = index_get_cell (beaming, d);
               scm_is_pair (s); s = scm_cdr (s))
            {
              if (!scm_is_integer (scm_car (s)))
                continue;

              int beam_rank = scm_to_int (scm_car (s));
              ranks.add_point (beam_rank);
            }

          for (SCM s = index_get_cell (beaming, d);
               scm_is_pair (s); s = scm_cdr (s))
            {
              if (!scm_is_integer (scm_car (s)))
                continue;

              int beam_rank = scm_to_int (scm_car (s));
              Beam_stem_segment seg;
              seg.stem_ = stem;
              seg.stem_x_ = stem_x;
              seg.rank_ = 2 * i + (d+1)/2;
              seg.width_ = stem_width;
              seg.stem_index_ = i;
              seg.dir_ = d;
              seg.max_connect_ = robust_scm2int (stem->get_property ("max-beam-connect"), 1000);

              Direction stem_dir = get_grob_direction (stem);

              seg.gapped_
                = (stem_dir * beam_rank < (stem_dir * ranks[-stem_dir] + gap_count));
              stem_segments[beam_rank].push_back (seg);
            }
        }
      while (flip (&d) != LEFT);
    }

  Drul_array<Real> break_overshoot
    = robust_scm2drul (me->get_property ("break-overshoot"),
                       Drul_array<Real> (-0.5, 0.0));

  vector<Beam_segment> segments;
  for (Position_stem_segments_map::const_iterator i (stem_segments.begin ());
       i != stem_segments.end (); i++)
    {
      vector<Beam_stem_segment> segs = (*i).second;
      vector_sort (segs, less<Beam_stem_segment> ());

      Beam_segment current;

      // Iterate over all of the segments of the current beam rank,
      // merging the adjacent Beam_stem_segments into one Beam_segment
      // when appropriate.
      int vertical_count =  (*i).first;
      for (vsize j = 0; j < segs.size (); j++)
        {
          // Keeping track of the different directions here is a little tricky.
          // segs[j].dir_ is the direction of the beam segment relative to the stem
          // (ie. segs[j].dir_ == LEFT if the beam segment sticks out to the left of
          // its stem) whereas event_dir refers to the edge of the beam segment that
          // we are currently looking at (ie. if segs[j].dir_ == event_dir then we
          // are looking at that edge of the beam segment that is furthest from its
          // stem).
          Direction event_dir = LEFT;
          Beam_stem_segment const& seg = segs[j];
          do
            {
              Beam_stem_segment const& neighbor_seg = segs[j + event_dir];
              // TODO: make names clearer? --jneem
              // on_line_bound: whether the current segment is on the boundary of the WHOLE beam
              // on_beam_bound: whether the current segment is on the boundary of just that part
              //   of the beam with the current beam_rank
              bool on_line_bound = (seg.dir_ == LEFT) ? seg.stem_index_ == 0
                : seg.stem_index_ == stems.size() - 1;
              bool on_beam_bound = (event_dir == LEFT) ? j == 0 :
                j == segs.size () - 1;
              bool inside_stem = (event_dir == LEFT)
                ? seg.stem_index_ > 0
                : seg.stem_index_ + 1 < stems.size () ;

              bool event = on_beam_bound
                || abs (seg.rank_ - neighbor_seg.rank_) > 1
                || (abs (vertical_count) >= seg.max_connect_
                    || abs (vertical_count) >= neighbor_seg.max_connect_);

              if (!event)
                // Then this edge of the current segment is irrelevent because it will
                // be connected with the next segment in the event_dir direction.
                continue;

              current.vertical_count_ = vertical_count;
              current.horizontal_[event_dir] = seg.stem_x_;
              if (seg.dir_ == event_dir)
                // then we are examining the edge of a beam segment that is furthest
                // from its stem.
                {
                  if (on_line_bound
                      && me->get_bound (event_dir)->break_status_dir ())
                    {
                      current.horizontal_[event_dir]
                        = (robust_relative_extent (me->get_bound (event_dir),
                                                   commonx, X_AXIS)[RIGHT]
                           + event_dir * break_overshoot[event_dir]);
                    }
                  else
                    {
                      Grob *stem = stems[seg.stem_index_];
                      Drul_array<Real> beamlet_length =
                        robust_scm2interval (stem->get_property ("beamlet-default-length"), Interval (1.1, 1.1));
                      Drul_array<Real> max_proportion =
                        robust_scm2interval (stem->get_property ("beamlet-max-length-proportion"), Interval (0.75, 0.75));
                      Real length = beamlet_length[seg.dir_];

                      if (inside_stem)
                        {
                          Grob *neighbor_stem = stems[seg.stem_index_ + event_dir];
                          Real neighbor_stem_x = neighbor_stem->relative_coordinate (commonx, X_AXIS);

                          length = min (length,
                                        fabs (neighbor_stem_x - seg.stem_x_) * max_proportion[seg.dir_]);
                        }
                      current.horizontal_[event_dir] += event_dir * length;
                    }
                }
              else
                // we are examining the edge of a beam segment that is closest
                // (ie. touching, unless there is a gap) its stem.
                {
                  current.horizontal_[event_dir] += event_dir * seg.width_/2;
                  if (seg.gapped_)
                    {
                      current.horizontal_[event_dir] -= event_dir * gap_length;

                      if (Stem::is_invisible (seg.stem_))
                        {
                          /*
                            Need to do this in case of whole notes. We don't want the
                            heads to collide with the beams.
                           */
                          extract_grob_set (seg.stem_, "note-heads", heads);

                          for (vsize k = 0; k < heads.size (); k ++)
                            current.horizontal_[event_dir]
                              = event_dir * min  (event_dir * current.horizontal_[event_dir],
                                                  - gap_length/2
                                                  + event_dir
                                                    * heads[k]->extent (commonx,
                                                                        X_AXIS)[-event_dir]);
                        }
                    }
                }

              if (event_dir == RIGHT)
                {
                  segments.push_back (current);
                  current = Beam_segment ();
                }
            }
          while (flip (&event_dir) != LEFT);
        }

    }

  return segments;
}

MAKE_SCHEME_CALLBACK (Beam, print, 1);
SCM
Beam::print (SCM grob)
{
  Spanner *me = unsmob_spanner (grob);
  Grob *commonx = 0;
  vector<Beam_segment> segments = get_beam_segments (me, &commonx);

  Interval span;
  if (normal_stem_count (me))
    {
      span[LEFT] = first_normal_stem (me)->relative_coordinate (commonx, X_AXIS);
      span[RIGHT] = last_normal_stem (me)->relative_coordinate (commonx, X_AXIS);
    }
  else
    {
      extract_grob_set (me, "stems", stems);
      span[LEFT] = stems[0]->relative_coordinate (commonx, X_AXIS);
      span[RIGHT] = stems.back ()->relative_coordinate (commonx, X_AXIS);
    }

  Real blot = me->layout ()->get_dimension (ly_symbol2scm ("blot-diameter"));

  SCM posns = me->get_property ("quantized-positions");
  Interval pos;
  if (!is_number_pair (posns))
    {
      programming_error ("no beam positions?");
      pos = Interval (0, 0);
    }
  else
    pos = ly_scm2realdrul (posns);

  scale_drul (&pos, Staff_symbol_referencer::staff_space (me));

  Real dy = pos[RIGHT] - pos[LEFT];
  Real slope = (dy && span.length ()) ? dy / span.length ()  : 0;

  Real beam_thickness = get_beam_thickness (me);
  Real beam_dy = get_beam_translation (me);

  Direction feather_dir = to_dir (me->get_property ("grow-direction"));

  Interval placements = robust_scm2interval (me->get_property ("normalized-endpoints"), Interval (0.0, 0.0));

  Stencil the_beam;

  int extreme = (segments[0].vertical_count_ == 0
                 ? segments[0].vertical_count_
                 : segments.back ().vertical_count_);

  for (vsize i = 0; i < segments.size (); i ++)
    {
      Real local_slope = slope;
      /*
        Makes local slope proportional to the ratio of the length of this beam
        to the total length.
      */
      if (feather_dir)
        local_slope += (feather_dir * segments[i].vertical_count_
                                    * beam_dy
                                    * placements.length ()
                        / span.length ());

      Stencil b = Lookup::beam (local_slope, segments[i].horizontal_.length (), beam_thickness, blot);

      b.translate_axis (segments[i].horizontal_[LEFT], X_AXIS);
      Real multiplier = feather_dir ? placements[LEFT] : 1.0;

      Interval weights (1 - multiplier, multiplier);

      if (feather_dir != LEFT)
        weights.swap ();

      // we need two translations: the normal one and
      // the one of the lowest segment
      int idx[] = {i, extreme};
      Real translations[2];

      for (int j = 0; j < 2; j++)
        translations[j] = slope
                          * (segments[idx[j]].horizontal_[LEFT] - span.linear_combination (CENTER))
                          + pos.linear_combination (CENTER)
                          + beam_dy * segments[idx[j]].vertical_count_;

      Real weighted_average = translations[0] * weights[LEFT] + translations[1] * weights[RIGHT];

      /*
        Tricky.  The manipulation of the variable `weighted_average' below ensures
        that beams with a RIGHT grow direction will start from the position of the
        lowest segment at 0, and this error will decrease and decrease over the
        course of the beam.  Something with a LEFT grow direction, on the other
        hand, will always start in the correct place but progressively accrue
        error at broken places.  This code shifts beams up given where they are
        in the total span length (controlled by the variable `multiplier').  To
        better understand what it does, try commenting it out: you'll see that
        all of the RIGHT growing beams immediately start too low and get better
        over line breaks, whereas all of the LEFT growing beams start just right
        and get worse over line breaks.
      */
      Real factor = Interval (multiplier, 1 - multiplier).linear_combination (feather_dir);

      if (segments[0].vertical_count_ < 0 && feather_dir)
        weighted_average += beam_dy * (segments.size () - 1) * factor;

      b.translate_axis (weighted_average, Y_AXIS);

      the_beam.add_stencil (b);

    }

#if (DEBUG_BEAM_SCORING)
  SCM annotation = me->get_property ("annotation");
  if (scm_is_string (annotation))
    {
      extract_grob_set (me, "stems", stems);

      /*
        This code prints the demerits for each beam. Perhaps this
        should be switchable for those who want to twiddle with the
        parameters.
      */
      string str;
      SCM properties = Font_interface::text_font_alist_chain (me);

      properties = scm_cons(scm_acons (ly_symbol2scm ("font-size"), scm_from_int (-5), SCM_EOL),
                            properties);

      Direction stem_dir = stems.size () ? to_dir (stems[0]->get_property ("direction")) : UP;

      Stencil score = *unsmob_stencil (Text_interface::interpret_markup
                                       (me->layout ()->self_scm (), properties, annotation));

      if (!score.is_empty ())
        {
          score.translate_axis (me->relative_coordinate(commonx, X_AXIS), X_AXIS);
          the_beam.add_at_edge (Y_AXIS, stem_dir, score, 1.0);
        }
    }
#endif

  the_beam.translate_axis (-me->relative_coordinate (commonx, X_AXIS), X_AXIS);
  return the_beam.smobbed_copy ();
}

Direction
Beam::get_default_dir (Grob *me)
{
  extract_grob_set (me, "stems", stems);

  Drul_array<Real> extremes (0.0, 0.0);
  for (iterof (s, stems); s != stems.end (); s++)
    {
      Interval positions = Stem::head_positions (*s);
      Direction d = DOWN;
      do
        {
          if (sign (positions[d]) == d)
            extremes[d] = d * max (d * positions[d], d * extremes[d]);
        }
      while (flip (&d) != DOWN);
    }

  Drul_array<int> total (0, 0);
  Drul_array<int> count (0, 0);

  bool force_dir = false;
  for (vsize i = 0; i < stems.size (); i++)
    {
      Grob *s = stems[i];
      Direction stem_dir = CENTER;
      SCM stem_dir_scm = s->get_property_data ("direction");
      if (is_direction (stem_dir_scm))
        {
          stem_dir = to_dir (stem_dir_scm);
          force_dir = true;
        }
      else
        stem_dir = to_dir (s->get_property ("default-direction"));

      if (!stem_dir)
        stem_dir = to_dir (s->get_property ("neutral-direction"));

      if (stem_dir)
        {
          count[stem_dir] ++;
          total[stem_dir] += max (int (- stem_dir * Stem::head_positions (s) [-stem_dir]), 0);
        }
    }


  if (!force_dir)
    {
      if (abs (extremes[UP]) > -extremes[DOWN])
        return DOWN;
      else if (extremes[UP] < -extremes[DOWN])
        return UP;
    }

  Direction dir = CENTER;
  Direction d = CENTER;
  if ((d = (Direction) sign (count[UP] - count[DOWN])))
    dir = d;
  else if (count[UP]
           && count[DOWN]
           && (d = (Direction)  sign (total[UP] / count[UP] - total[DOWN]/count[DOWN])))
    dir = d;
  else if ((d = (Direction)  sign (total[UP] - total[DOWN])))
    dir = d;
  else
    dir = to_dir (me->get_property ("neutral-direction"));

  return dir;
}

/* Set all stems with non-forced direction to beam direction.
   Urg: non-forced should become `without/with unforced' direction,
   once stem gets cleaned-up. */
void
Beam::set_stem_directions (Grob *me, Direction d)
{
  extract_grob_set (me, "stems", stems);

  for (vsize i = 0; i < stems.size (); i++)
    {
      Grob *s = stems[i];

      SCM forcedir = s->get_property_data ("direction");
      if (!to_dir (forcedir))
        set_grob_direction (s, d);
    }
}

/*
  Only try horizontal beams for knees.  No reliable detection of
  anything else is possible here, since we don't know funky-beaming
  settings, or X-distances (slopes!)  People that want sloped
  knee-beams, should set the directions manually.


  TODO:

  this routine should take into account the stemlength scoring
  of a possible knee/nonknee beam.
*/
void
Beam::consider_auto_knees (Grob *me)
{
  SCM scm = me->get_property ("auto-knee-gap");
  if (!scm_is_number (scm))
    return;

  Interval_set gaps;

  gaps.set_full ();

  extract_grob_set (me, "normal-stems", stems);

  Grob *common = common_refpoint_of_array (stems, me, Y_AXIS);
  Real staff_space = Staff_symbol_referencer::staff_space (me);

  vector<Interval> head_extents_array;
  for (vsize i = 0; i < stems.size (); i++)
    {
      Grob *stem = stems[i];

      Interval head_extents = Stem::head_positions (stem);
      if (!head_extents.is_empty ())
        {
          head_extents[LEFT] += -1;
          head_extents[RIGHT] += 1;
          head_extents *= staff_space * 0.5;

          /*
            We could subtract beam Y position, but this routine only
            sets stem directions, a constant shift does not have an
            influence.
          */
          head_extents += stem->pure_relative_y_coordinate (common, 0, INT_MAX);

          if (to_dir (stem->get_property_data ("direction")))
            {
              Direction stemdir = to_dir (stem->get_property ("direction"));
              head_extents[-stemdir] = -stemdir * infinity_f;
            }
        }
      head_extents_array.push_back (head_extents);

      gaps.remove_interval (head_extents);
    }

  Interval max_gap;
  Real max_gap_len = 0.0;

  for (vsize i = gaps.allowed_regions_.size () -1; i != VPOS ;i--)
    {
      Interval gap = gaps.allowed_regions_[i];

      /*
        the outer gaps are not knees.
      */
      if (isinf (gap[LEFT]) || isinf (gap[RIGHT]))
        continue;

      if (gap.length () >= max_gap_len)
        {
          max_gap_len = gap.length ();
          max_gap = gap;
        }
    }

  Real beam_translation = get_beam_translation (me);
  Real beam_thickness = Beam::get_beam_thickness (me);
  int beam_count = Beam::get_beam_count (me);
  Real height_of_beams = beam_thickness / 2
    + (beam_count - 1) * beam_translation;
  Real threshold = scm_to_double (scm) + height_of_beams;

  if (max_gap_len > threshold)
    {
      int j = 0;
      for (vsize i = 0; i < stems.size (); i++)
        {
          Grob *stem = stems[i];
          Interval head_extents = head_extents_array[j++];

          Direction d = (head_extents.center () < max_gap.center ())
            ? UP : DOWN;

          stem->set_property ("direction", scm_from_int (d));

          head_extents.intersect (max_gap);
          assert (head_extents.is_empty () || head_extents.length () < 1e-6);
        }
    }
}

/* Set stem's shorten property if unset.

TODO:
take some y-position (chord/beam/nearest?) into account
scmify forced-fraction

This is done in beam because the shorten has to be uniform over the
entire beam.
*/



void
set_minimum_dy (Grob *me, Real *dy)
{
  if (*dy)
    {
      /*
        If dy is smaller than the smallest quant, we
        get absurd direction-sign penalties.
      */

      Real ss = Staff_symbol_referencer::staff_space (me);
      Real beam_thickness = Beam::get_beam_thickness (me) / ss;
      Real slt = Staff_symbol_referencer::line_thickness (me) / ss;
      Real sit = (beam_thickness - slt) / 2;
      Real inter = 0.5;
      Real hang = 1.0 - (beam_thickness - slt) / 2;

      *dy = sign (*dy) * max (fabs (*dy),
                              min (min (sit, inter), hang));
    }
}



MAKE_SCHEME_CALLBACK (Beam, calc_stem_shorten, 1)
SCM
Beam::calc_stem_shorten (SCM smob)
{
  Grob *me = unsmob_grob (smob);

  /*
    shortening looks silly for x staff beams
  */
  if (is_knee (me))
    return scm_from_int (0);

  Real forced_fraction = 1.0 * forced_stem_count (me)
    / normal_stem_count (me);

  int beam_count = get_beam_count (me);

  SCM shorten_list = me->get_property ("beamed-stem-shorten");
  if (shorten_list == SCM_EOL)
    return scm_from_int (0);

  Real staff_space = Staff_symbol_referencer::staff_space (me);

  SCM shorten_elt
    = robust_list_ref (beam_count -1, shorten_list);
  Real shorten = scm_to_double (shorten_elt) * staff_space;

  shorten *= forced_fraction;


  if (shorten)
    return scm_from_double (shorten);

  return scm_from_double (0.0);
}


Interval
Beam::no_visible_stem_positions (Grob *me, Interval default_value)
{
  extract_grob_set (me, "stems", stems);
  if (stems.empty ())
    return default_value;

  Interval head_positions;
  Slice multiplicity;
  for (vsize i = 0; i < stems.size(); i++)
    {
      head_positions.unite (Stem::head_positions (stems[i]));
      multiplicity.unite (Stem::beam_multiplicity (stems[i]));
    }

  Direction dir = get_grob_direction (me);
  Real y = head_positions[dir]
    * 0.5 * Staff_symbol_referencer::staff_space (me)
    + dir * get_beam_translation (me) * (multiplicity.length () + 1);

  y /= Staff_symbol_referencer::staff_space (me);
  return Interval (y,y);
}


/*
  Compute a first approximation to the beam slope.
*/
MAKE_SCHEME_CALLBACK (Beam, calc_least_squares_positions, 2);
SCM
Beam::calc_least_squares_positions (SCM smob, SCM /* posns */)
{
  Grob *me = unsmob_grob (smob);

  int count = normal_stem_count (me);
  Interval pos (0,0);
  if (count < 1)
    return ly_interval2scm (no_visible_stem_positions (me, pos));

  vector<Real> x_posns;
  extract_grob_set (me, "normal-stems", stems);
  Grob *commonx = common_refpoint_of_array (stems, me, X_AXIS);
  Grob *commony = common_refpoint_of_array (stems, me, Y_AXIS);

  Real my_y = me->relative_coordinate (commony, Y_AXIS);

  Grob *fvs = first_normal_stem (me);
  Grob *lvs = last_normal_stem (me);

  Interval ideal (Stem::get_stem_info (fvs).ideal_y_
                  + fvs->relative_coordinate (commony, Y_AXIS) - my_y,
                  Stem::get_stem_info (lvs).ideal_y_
                  + lvs->relative_coordinate (commony, Y_AXIS) - my_y);

  Real x0 = first_normal_stem (me)->relative_coordinate (commonx, X_AXIS);
  for (vsize i = 0; i < stems.size (); i++)
    {
      Grob *s = stems[i];

      Real x = s->relative_coordinate (commonx, X_AXIS) - x0;
      x_posns.push_back (x);
    }
  Real dx = last_normal_stem (me)->relative_coordinate (commonx, X_AXIS) - x0;

  Real y = 0;
  Real slope = 0;
  Real dy = 0;
  Real ldy = 0.0;
  if (!ideal.delta ())
    {
      Interval chord (Stem::chord_start_y (stems[0]),
                      Stem::chord_start_y (stems.back ()));

      /* Simple beams (2 stems) on middle line should be allowed to be
         slightly sloped.

         However, if both stems reach middle line,
         ideal[LEFT] == ideal[RIGHT] and ideal.delta () == 0.

         For that case, we apply artificial slope */
      if (!ideal[LEFT] && chord.delta () && count == 2)
        {
          /* FIXME. -> UP */
          Direction d = (Direction) (sign (chord.delta ()) * UP);
          pos[d] = get_beam_thickness (me) / 2;
          pos[-d] = -pos[d];
        }
      else
        pos = ideal;

      /*
        For broken beams this doesn't work well. In this case, the
        slope esp. of the first part of a broken beam should predict
        where the second part goes.
      */
      ldy = pos[RIGHT] - pos[LEFT];
    }
  else
    {
      vector<Offset> ideals;
      for (vsize i = 0; i < stems.size (); i++)
        {
          Grob *s = stems[i];
          ideals.push_back (Offset (x_posns[i],
                               Stem::get_stem_info (s).ideal_y_
                               + s->relative_coordinate (commony, Y_AXIS)
                               - my_y));
        }

      minimise_least_squares (&slope, &y, ideals);

      dy = slope * dx;

      set_minimum_dy (me, &dy);

      ldy = dy;
      pos = Interval (y, (y + dy));
    }

  /*
    "position" is relative to the staff.
  */
  scale_drul (&pos, 1 / Staff_symbol_referencer::staff_space (me));

  me->set_property ("least-squares-dy",  scm_from_double (ldy));
  return ly_interval2scm (pos);
}


// Assuming V is not empty, pick a 'reasonable' point inside V.
static Real
point_in_interval (Interval v, Real dist)
{
  if (isinf (v[DOWN]))
    return v[UP] - dist;
  else if (isinf (v[UP]))
    return v[DOWN] + dist;
  else
    return v.center ();
}

/*
  We can't combine with previous function, since check concave and
  slope damping comes first.

  TODO: we should use the concaveness to control the amount of damping
  applied.
*/
MAKE_SCHEME_CALLBACK (Beam, shift_region_to_valid, 2);
SCM
Beam::shift_region_to_valid (SCM grob, SCM posns)
{
  Grob *me = unsmob_grob (grob);

  /*
    Code dup.
  */
  vector<Real> x_posns;
  extract_grob_set (me, "stems", stems);
  extract_grob_set (me, "covered-grobs", covered);

  Grob *common[NO_AXES] = { me, me };
  for (Axis a = X_AXIS; a < NO_AXES; incr (a)) {
    common[a] = common_refpoint_of_array (stems, me, a);
    common[a] = common_refpoint_of_array (covered, common[a], a);
  }
  Grob *fvs = first_normal_stem (me);

  if (!fvs)
    return posns;
  Interval x_span;
  x_span[LEFT] = fvs->relative_coordinate (common[X_AXIS], X_AXIS);
  for (vsize i = 0; i < stems.size (); i++)
    {
      Grob *s = stems[i];

      Real x = s->relative_coordinate (common[X_AXIS], X_AXIS) - x_span[LEFT];
      x_posns.push_back (x);
    }

  Grob *lvs = last_normal_stem (me);
  x_span[RIGHT] = lvs->relative_coordinate (common[X_AXIS], X_AXIS);

  Drul_array<Real> pos = ly_scm2interval (posns);

  scale_drul (&pos, Staff_symbol_referencer::staff_space (me));

  Real beam_dy = pos[RIGHT] - pos[LEFT];
  Real beam_left_y = pos[LEFT];
  Real slope = x_span.delta () ? (beam_dy / x_span.delta ()) : 0.0;

  /*
    Shift the positions so that we have a chance of finding good
    quants (i.e. no short stem failures.)
  */
  Interval feasible_left_point;
  feasible_left_point.set_full ();

  for (vsize i = 0; i < stems.size (); i++)
    {
      Grob *s = stems[i];
      if (Stem::is_invisible (s))
        continue;

      Direction d = get_grob_direction (s);
      Real left_y
        = Stem::get_stem_info (s).shortest_y_
        - slope * x_posns [i];

      /*
        left_y is now relative to the stem S. We want relative to
        ourselves, so translate:
      */
      left_y
        += + s->relative_coordinate (common[Y_AXIS], Y_AXIS)
        - me->relative_coordinate (common[Y_AXIS], Y_AXIS);

      Interval flp;
      flp.set_full ();
      flp[-d] = left_y;

      feasible_left_point.intersect (flp);
    }

  /*
    We have two intervals here, one for the up variant (beams goes
    over the collision) one for the down.
  */
  Drul_array<Interval> collision_free (feasible_left_point,
                                       feasible_left_point);

  vector<Grob*> filtered;
  /*
    We only update these for objects that are too large for quanting
    to find a workaround.  Typically, these are notes with
    stems, and timesig/keysig/clef, which take out the entire area
    inside the staff as feasible.

    The code below disregards the thickness and multiplicity of the
    beam.  This should not be a problem, as the beam quanting will
    take care of computing the impact those exactly.
  */
  Real min_y_size = 2.0;
  for (vsize i = 0; i < covered.size(); i++)
    {
      if (!covered[i]->is_live())
        continue;

      // TODO - use this logic in is_cross_staff.
      if (is_cross_staff (me)
          && Align_interface::has_interface (common_refpoint_of_array (stems, me, Y_AXIS)))
        continue;

      if (Beam::has_interface (covered[i]) && is_cross_staff (covered[i]))
        continue;

      Box b;
      for (Axis a = X_AXIS; a < NO_AXES; incr (a))
        b[a] = covered[i]->extent (common[a], a);

      if (b[X_AXIS].is_empty () || b[Y_AXIS].is_empty ())
        continue;

      if (intersection (b[X_AXIS], x_span).is_empty ())
        continue;

      filtered.push_back (covered[i]);
      Grob *head_stem = Rhythmic_head::get_stem (covered[i]);
      if (head_stem && Stem::is_normal_stem (head_stem)
          && Note_head::has_interface (covered[i]))
        {
          if (Stem::get_beam (head_stem))
            {
              /*
                We must assume that stems are infinitely long in this
                case, as asking for the length of the stem typically
                leads to circular dependencies.

                This strategy assumes that we don't want to handle the
                collision of beams in opposite non-forced directions
                with this code, where shortening the stems of both
                would resolve the problem, eg.

                 x    x
                |    | 
                =====

                =====
                |   |  
                x   x
                
                Such beams would need a coordinating grob to resolve
                the collision, since both will likely want to occupy
                the centerline.
              */
              Direction stemdir = get_grob_direction (head_stem);
              b[Y_AXIS][stemdir] = stemdir * infinity_f; 
            }
          else
            {
              // TODO - should we include the extent of the stem here?
            }
        }

      if (b[Y_AXIS].length () < min_y_size)
        continue;

      Direction d = LEFT;
      do
        {
          Real x = b[X_AXIS][d] - x_span[LEFT];
          Real dy = slope * x;

          Direction yd = DOWN;
          do
            {
              Real left_y = b[Y_AXIS][yd];

              if (left_y == yd * infinity_f)
                {
                  collision_free[yd].set_empty ();
                  continue;
                }

              left_y -= dy;

              // Translate back to beam as ref point.
              left_y -= me->relative_coordinate (common[Y_AXIS], Y_AXIS);
            
              Interval allowed;
              allowed.set_full ();

              allowed[-yd] = left_y;
              collision_free[yd].intersect (allowed);
            }
          while (flip (&yd) != DOWN);
        }
      while (flip (&d) != LEFT);
    }

  Grob_array *arr = 
    Pointer_group_interface::get_grob_array (me,
                                             ly_symbol2scm ("covered-grobs"));
  arr->set_array (filtered);

  if (collision_free[DOWN].contains (beam_left_y)
      || collision_free[UP].contains (beam_left_y))
    {
      // We're good to go. Do nothing.
    }
  else if (collision_free[DOWN].is_empty() != collision_free[UP].is_empty())
    {
      // Only one of them offers is feasible solution. Pick that one.
      Interval v =
        (!collision_free[DOWN].is_empty()) ?
        collision_free[DOWN] : 
        collision_free[UP];

      beam_left_y = point_in_interval (v, 2.0);
    }
  else if (!collision_free[DOWN].is_empty ()
           && !collision_free[UP].is_empty ())
    {
      // Above and below are candidates, take the one closest to the
      // starting solution.
      Interval best =  
        (collision_free[DOWN].distance (beam_left_y)
         < collision_free[UP].distance (beam_left_y)) ?
        collision_free[DOWN] : collision_free[UP];

      beam_left_y = point_in_interval (best, 2.0);
    }
  else if (!feasible_left_point.is_empty ())
    {
      // We are somewhat screwed: we have a collision, but at least
      // there is a way to satisfy stem length constraints.
      beam_left_y = point_in_interval (feasible_left_point, 2.0);
    }
  else
    {
      // We are completely screwed.
      me->warning (_ ("no viable initial configuration found: may not find good beam slope"));
    }
  
  pos = Drul_array<Real> (beam_left_y, (beam_left_y + beam_dy));
  scale_drul (&pos, 1 / Staff_symbol_referencer::staff_space (me));

  return ly_interval2scm (pos);
}

/* This neat trick is by Werner Lemberg,
   damped = tanh (slope)
   corresponds with some tables in [Wanske] CHECKME */
MAKE_SCHEME_CALLBACK (Beam, slope_damping, 2);
SCM
Beam::slope_damping (SCM smob, SCM posns)
{
  Grob *me = unsmob_grob (smob);
  Drul_array<Real> pos = ly_scm2interval (posns);

  if (normal_stem_count (me) <= 1)
    return posns;

  SCM s = me->get_property ("damping");
  Real damping = scm_to_double (s);
  Real concaveness = robust_scm2double (me->get_property ("concaveness"), 0.0);
  if (concaveness >= 10000)
    {
      pos[LEFT] = pos[RIGHT];
      me->set_property ("least-squares-dy", scm_from_double (0));
      damping = 0;
    }

  if (damping)
    {
      scale_drul (&pos, Staff_symbol_referencer::staff_space (me));

      Real dy = pos[RIGHT] - pos[LEFT];

      Grob *fvs = first_normal_stem (me);
      Grob *lvs = last_normal_stem (me);

      Grob *commonx = fvs->common_refpoint (lvs, X_AXIS);

      Real dx = last_normal_stem (me)->relative_coordinate (commonx, X_AXIS)
        - first_normal_stem (me)->relative_coordinate (commonx, X_AXIS);

      Real slope = dy && dx ? dy / dx : 0;

      slope = 0.6 * tanh (slope) / (damping + concaveness);

      Real damped_dy = slope * dx;

      set_minimum_dy (me, &damped_dy);

      pos[LEFT] += (dy - damped_dy) / 2;
      pos[RIGHT] -= (dy - damped_dy) / 2;

      scale_drul (&pos, 1 / Staff_symbol_referencer::staff_space (me));
    }

  return ly_interval2scm (pos);
}


MAKE_SCHEME_CALLBACK (Beam, quanting, 2);
SCM
Beam::quanting (SCM smob, SCM posns)
{
  Grob *me = unsmob_grob (smob);
  Drul_array<Real> ys(0, 0);
  ys = robust_scm2drul (posns, ys);
  Beam_scoring_problem problem (me, ys);

  ys = problem.solve ();
  return ly_interval2scm (ys);
}


/*
  Report slice containing the numbers that are both in (car BEAMING)
  and (cdr BEAMING)
*/
Slice
where_are_the_whole_beams (SCM beaming)
{
  Slice l;

  for (SCM s = scm_car (beaming); scm_is_pair (s); s = scm_cdr (s))
    {
      if (scm_c_memq (scm_car (s), scm_cdr (beaming)) != SCM_BOOL_F)

        l.add_point (scm_to_int (scm_car (s)));
    }

  return l;
}

/* Return the Y position of the stem-end, given the Y-left, Y-right
   in POS for stem S.  This Y position is relative to S. */
Real
Beam::calc_stem_y (Grob *me, Grob *stem, Grob **common,
                   Real xl, Real xr, Direction feather_dir,
                   Drul_array<Real> pos, bool french)
{
  Real beam_translation = get_beam_translation (me);
  Direction stem_dir = get_grob_direction (stem);

  Real dx = xr - xl;
  Real relx = dx ? (stem->relative_coordinate (common[X_AXIS], X_AXIS) - xl)/dx : 0;
  Real xdir = 2*relx-1;

  Real stem_y = linear_combination(pos, xdir);

  SCM beaming = stem->get_property ("beaming");

  Slice beam_slice (french
                    ? where_are_the_whole_beams (beaming)
                    : Stem::beam_multiplicity (stem));
  if (beam_slice.is_empty ())
    beam_slice = Slice (0,0);
  Interval beam_multiplicity(beam_slice[LEFT],
                             beam_slice[RIGHT]);

  /*
    feather dir = 1 , relx 0->1 : factor 0 -> 1
    feather dir = 0 , relx 0->1 : factor 1 -> 1
    feather dir = -1, relx 0->1 : factor 1 -> 0
   */
  Real feather_factor = 1;
  if (feather_dir > 0)
    feather_factor = relx;
  else if (feather_dir < 0)
    feather_factor = 1 - relx;

  stem_y += feather_factor * beam_translation
    * beam_multiplicity[Direction(((french) ? DOWN : UP)*stem_dir)];
  Real id = me->relative_coordinate (common[Y_AXIS], Y_AXIS)
    - stem->relative_coordinate (common[Y_AXIS], Y_AXIS);

  return stem_y + id;
}

/*
  Hmm.  At this time, beam position and slope are determined.  Maybe,
  stem directions and length should set to relative to the chord's
  position of the beam.  */
MAKE_SCHEME_CALLBACK (Beam, set_stem_lengths, 1);
SCM
Beam::set_stem_lengths (SCM smob)
{
  Grob *me = unsmob_grob (smob);

  /* trigger callbacks. */
  (void) me->get_property ("direction");
  (void) me->get_property ("beaming");

  SCM posns = me->get_property ("positions");

  extract_grob_set (me, "stems", stems);
  if (!stems.size ())
    return posns;

  Grob *common[2];
  for (int a = 2; a--;)
    common[a] = common_refpoint_of_array (stems, me, Axis (a));

  Drul_array<Real> pos = ly_scm2realdrul (posns);
  Real staff_space = Staff_symbol_referencer::staff_space (me);
  scale_drul (&pos, staff_space);

  bool gap = false;
  Real thick = 0.0;
  if (robust_scm2int (me->get_property ("gap-count"), 0))
    {
      gap = true;
      thick = get_beam_thickness (me);
    }

  Grob *fvs = first_normal_stem (me);
  Grob *lvs = last_normal_stem (me);

  Real xl = fvs ? fvs->relative_coordinate (common[X_AXIS], X_AXIS) : 0.0;
  Real xr = lvs ? lvs->relative_coordinate (common[X_AXIS], X_AXIS) : 0.0;
  Direction feather_dir = to_dir (me->get_property ("grow-direction"));

  for (vsize i = 0; i < stems.size (); i++)
    {
      Grob *s = stems[i];

      bool french = to_boolean (s->get_property ("french-beaming"));
      Real stem_y = calc_stem_y (me, s, common,
                                 xl, xr, feather_dir,
                                 pos, french && s != lvs && s!= fvs);

      /*
        Make the stems go up to the end of the beam. This doesn't matter
        for normal beams, but for tremolo beams it looks silly otherwise.
      */
      if (gap
          && !Stem::is_invisible (s))
        stem_y += thick * 0.5 * get_grob_direction (s);

      /*
        Do set_stemend for invisible stems too, so tuplet brackets
        have a reference point for sloping
       */
      Stem::set_stemend (s, 2 * stem_y / staff_space);
    }

  return posns;
}

void
Beam::set_beaming (Grob *me, Beaming_pattern const *beaming)
{
  extract_grob_set (me, "stems", stems);

  Direction d = LEFT;
  for (vsize i = 0; i < stems.size (); i++)
    {
      /*
        Don't overwrite user settings.
      */
      do
        {
          Grob *stem = stems[i];
          SCM beaming_prop = stem->get_property ("beaming");
          if (beaming_prop == SCM_EOL
              || index_get_cell (beaming_prop, d) == SCM_EOL)
            {
              int count = beaming->beamlet_count (i, d);
              if (i > 0
                  && i + 1 < stems.size ()
                  && Stem::is_invisible (stem))
                count = min (count, beaming->beamlet_count (i,-d));

              if ( ((i == 0 && d == LEFT)
                    || (i == stems.size ()-1 && d == RIGHT))
                   && stems.size () > 1
                   && to_boolean (me->get_property ("clip-edges")))
                count = 0;

              Stem::set_beaming (stem, count, d);
            }
        }
      while (flip (&d) != LEFT);
    }
}

int
Beam::forced_stem_count (Grob *me)
{
  extract_grob_set (me, "normal-stems", stems);

  int f = 0;
  for (vsize i = 0; i < stems.size (); i++)
    {
      Grob *s = stems[i];

      /* I can imagine counting those boundaries as a half forced stem,
         but let's count them full for now. */
      Direction defdir = to_dir (s->get_property ("default-direction"));

      if (abs (Stem::chord_start_y (s)) > 0.1
          && defdir
          && get_grob_direction (s) != defdir)
        f++;
    }
  return f;
}

int
Beam::normal_stem_count (Grob *me)
{
  extract_grob_set (me, "normal-stems", stems);
  return stems.size ();
}

Grob *
Beam::first_normal_stem (Grob *me)
{
  extract_grob_set (me, "normal-stems", stems);
  return stems.size () ? stems[0] : 0;
}

Grob *
Beam::last_normal_stem (Grob *me)
{
  extract_grob_set (me, "normal-stems", stems);
  return stems.size () ? stems.back () : 0;
}

/*
  [TODO]

  handle rest under beam (do_post: beams are calculated now)
  what about combination of collisions and rest under beam.

  Should lookup

  rest -> stem -> beam -> interpolate_y_position ()
*/
MAKE_SCHEME_CALLBACK_WITH_OPTARGS (Beam, rest_collision_callback, 2, 1, "");
SCM
Beam::rest_collision_callback (SCM smob, SCM prev_offset)
{
  Grob *rest = unsmob_grob (smob);
  if (scm_is_number (rest->get_property ("staff-position")))
    return scm_from_int (0);

  Real offset = robust_scm2double (prev_offset, 0.0);

  Grob *st = unsmob_grob (rest->get_object ("stem"));
  Grob *stem = st;
  if (!stem)
    return scm_from_double (0.0);
  Grob *beam = unsmob_grob (stem->get_object ("beam"));
  if (!beam
      || !Beam::has_interface (beam)
      || !Beam::normal_stem_count (beam))
    return scm_from_double (0.0);

  Drul_array<Real> pos (robust_scm2drul (beam->get_property ("positions"),
                                         Drul_array<Real> (0,0)));

  Real staff_space = Staff_symbol_referencer::staff_space (rest);

  scale_drul (&pos, staff_space);

  Real dy = pos[RIGHT] - pos[LEFT];

  Drul_array<Grob*> visible_stems (first_normal_stem (beam),
                                   last_normal_stem (beam));
  extract_grob_set (beam, "stems", stems);

  Grob *common = common_refpoint_of_array (stems, beam, X_AXIS);

  Real x0 = visible_stems[LEFT]->relative_coordinate (common, X_AXIS);
  Real dx = visible_stems[RIGHT]->relative_coordinate (common, X_AXIS) - x0;
  Real slope = dy && dx ? dy / dx : 0;

  Direction d = get_grob_direction (stem);
  Real stem_y = pos[LEFT]
    + (stem->relative_coordinate (common, X_AXIS) - x0) * slope;

  Real beam_translation = get_beam_translation (beam);
  Real beam_thickness = Beam::get_beam_thickness (beam);

  /*
    TODO: this is not strictly correct for 16th knee beams.
  */
  int beam_count
    = Stem::beam_multiplicity (stem).length () + 1;

  Real height_of_my_beams = beam_thickness / 2
    + (beam_count - 1) * beam_translation;
  Real beam_y = stem_y - d * height_of_my_beams;

  Grob *common_y = rest->common_refpoint (beam, Y_AXIS);

  Interval rest_extent = rest->extent (rest, Y_AXIS);
  rest_extent.translate (offset + rest->get_parent (Y_AXIS)->relative_coordinate (common_y, Y_AXIS));

  Real rest_dim = rest_extent[d];
  Real minimum_distance
    = staff_space * (robust_scm2double (stem->get_property ("stemlet-length"), 0.0)
                     + robust_scm2double (rest->get_property ("minimum-distance"), 0.0));

  Real shift = d * min (d * (beam_y - d * minimum_distance - rest_dim), 0.0);

  shift /= staff_space;
  Real rad = Staff_symbol_referencer::line_count (rest) * staff_space / 2;

  /* Always move discretely by half spaces */
  shift = ceil (fabs (shift * 2.0)) / 2.0 * sign (shift);

  /* Inside staff, move by whole spaces*/
  if ((rest_extent[d] + staff_space * shift) * d
      < rad
      || (rest_extent[-d] + staff_space * shift) * -d
      < rad)
    shift = ceil (fabs (shift)) * sign (shift);

  return scm_from_double (offset + staff_space * shift);
}

bool
Beam::is_knee (Grob *me)
{
  SCM k = me->get_property ("knee");
  if (scm_is_bool (k))
    return ly_scm2bool (k);

  bool knee = false;
  int d = 0;
  extract_grob_set (me, "stems", stems);
  for (vsize i = stems.size (); i--;)
    {
      Direction dir = get_grob_direction (stems[i]);
      if (d && d != dir)
        {
          knee = true;
          break;
        }
      d = dir;
    }

  me->set_property ("knee", ly_bool2scm (knee));

  return knee;
}

bool
Beam::is_cross_staff (Grob *me)
{
  extract_grob_set (me, "stems", stems);
  Grob *staff_symbol = Staff_symbol_referencer::get_staff_symbol (me);
  for (vsize i = 0; i < stems.size (); i++)
    if (Staff_symbol_referencer::get_staff_symbol (stems[i]) != staff_symbol)
      return true;
  return false;
}

MAKE_SCHEME_CALLBACK (Beam, calc_cross_staff, 1)
SCM
Beam::calc_cross_staff (SCM smob)
{
  return scm_from_bool (is_cross_staff (unsmob_grob (smob)));
}

int
Beam::get_direction_beam_count (Grob *me, Direction d)
{
  extract_grob_set (me, "stems", stems);
  int bc = 0;

  for (vsize i = stems.size (); i--;)
    {
      /*
        Should we take invisible stems into account?
      */
      if (get_grob_direction (stems[i]) == d)
        bc = max (bc, (Stem::beam_multiplicity (stems[i]).length () + 1));
    }

  return bc;
}

ADD_INTERFACE (Beam,
               "A beam.\n"
               "\n"
               "The @code{beam-thickness} property is the weight of beams,"
               " measured in staffspace.  The @code{direction} property is"
               " not user-serviceable.  Use the @code{direction} property"
               " of @code{Stem} instead.\n"
               "\n"
               "The following properties may be set in the @code{details}"
               " list.\n"
               "\n"
               "@table @code\n"
               "@item stem-length-demerit-factor\n"
               "Demerit factor used for inappropriate stem lengths.\n"
               "@item secondary-beam-demerit\n"
               "Demerit used in quanting calculations for multiple"
               " beams.\n"
               "@item region-size\n"
               "Size of region for checking quant scores.\n"
               "@item beam-eps\n"
               "Epsilon for beam quant code to check for presence"
               " in gap.\n"
               "@item stem-length-limit-penalty\n"
               "Penalty for differences in stem lengths on a beam.\n"
               "@item damping-direction-penalty\n"
               "Demerit penalty applied when beam direction is different"
               " from damping direction.\n"
               "@item hint-direction-penalty\n"
               "Demerit penalty applied when beam direction is different"
               " from damping direction, but damping slope is"
               " <= @code{round-to-zero-slope}.\n"
               "@item musical-direction-factor\n"
               "Demerit scaling factor for difference between"
               " beam slope and music slope.\n"
               "@item ideal-slope-factor\n"
               "Demerit scaling factor for difference between"
               " beam slope and damping slope.\n"
               "@item round-to-zero-slope\n"
               "Damping slope which is considered zero for purposes of"
               " calculating direction penalties.\n"
               "@end table\n",

               /* properties */
               "annotation "
               "auto-knee-gap "
               "beamed-stem-shorten "
               "beaming "
               "beam-thickness "
               "break-overshoot "
               "clip-edges "
               "concaveness "
               "collision-interfaces "
               "collision-voice-only "
               "covered-grobs "
               "damping "
               "details "
               "direction "
               "gap "
               "gap-count "
               "grow-direction "
               "inspect-quants "
               "knee "
               "length-fraction "
               "least-squares-dy "
               "neutral-direction "
               "normal-stems "
               "positions "
               "quantized-positions "
               "shorten "
               "stems "
               );