Doc-es: various updates.

[lilypond.git] / lily / spring.cc

diff --git a/lily/spring.cc b/lily/spring.cc
index 1f250546db354d31f41a7cfb344a011281af7524..d1640e72b9790fb2d96991a743cfebef63f62f81 100644 (file)
--- a/lily/spring.cc
+++ b/lily/spring.cc
@@ -1,9 +1,37 @@
 /*
-  spring.cc -- declare Spring
+  This file is part of LilyPond, the GNU music typesetter.
 
-  source file of the GNU LilyPond music typesetter
+  Copyright (C) 2007--2015 Joe Neeman <joeneeman@gmail.com>
 
-  (c) 2007 Joe Neeman <joeneeman@gmail.com>
+  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/>.
+*/
+
+/*
+  Springs help chains of objects, such as the notes in a line of music,
+  distribute themselves evenly.
+  Each spring decides the length from the reference point of one object
+  along the line to the reference point of the next, based on a force
+  applied to the entire chain (see Spring::length() for details):
+     length = distance_ + flexibility * force
+
+  distance_  is the ideal separation between reference points
+  inverse_stretch_strength_ is the flexibility when the force is stretching
+  inverse_compress_strength_ is the flexibility when the force is compressing
+  min_distance_ sets a lower limit on length
+
+  Typically, the force applied to a list of objects ranges from about
+  -1 to about 1, though there are no set limits.
 */
 
 #include "spring.hh"
@@ -22,6 +50,8 @@ Spring::Spring (Real dist, Real min_dist)
 {
   distance_ = 1.0;
   min_distance_ = 1.0;
+  inverse_stretch_strength_ = 1.0;
+  inverse_compress_strength_ = 1.0;
 
   set_distance (dist);
   set_min_distance (min_dist);
@@ -32,23 +62,36 @@ Spring::Spring (Real dist, Real min_dist)
 void
 Spring::update_blocking_force ()
 {
-  if (distance_ == min_distance_)
-    blocking_force_ = 0.0;
-  else
+  // blocking_force_ is the value of force
+  //   below which length(force) is constant, and
+  //   above which length(force) varies according to inverse_*_strength.
+  // Simple_spacer::compress_line() depends on the condition above.
+  // We assume inverse_*_strength are non-negative.
+  if (min_distance_ > distance_)
+    if (inverse_stretch_strength_ > 0.0)
+      blocking_force_ = (min_distance_ - distance_) / inverse_stretch_strength_;
+    else
+      // Conceptually, this should be +inf, but 0.0 meets the requirements
+      //  of Simple_spacer and creates fewer cases of 0.0*inf to handle.
+      blocking_force_ = 0.0;
+  else if (inverse_compress_strength_ > 0.0)
     blocking_force_ = (min_distance_ - distance_) / inverse_compress_strength_;
+  else
+    blocking_force_ = 0.0;
 }
 
 /* scale a spring, but in a way that doesn't violate min_distance */
 void
-Spring::operator*= (Real r)
+Spring::operator *= (Real r)
 {
   distance_ = max (min_distance_, distance_ * r);
-  inverse_compress_strength_ = distance_ - min_distance_;
-  inverse_stretch_strength_ *= 0.8;
+  inverse_compress_strength_ = max (0.0, distance_ - min_distance_);
+  inverse_stretch_strength_ *= r;
+  update_blocking_force ();
 }
 
 bool
-Spring::operator> (Spring const &other) const
+Spring::operator > (Spring const &other) const
 {
   return blocking_force_ > other.blocking_force_;
 }
@@ -71,9 +114,9 @@ merge_springs (vector<Spring> const &springs)
       min_distance = max (springs[i].min_distance (), min_distance);
     }
 
-  avg_stretch /= springs.size ();
-  avg_compress /= springs.size ();
-  avg_distance /= springs.size ();
+  avg_stretch /= Real (springs.size ());
+  avg_compress /= Real (springs.size ());
+  avg_distance /= Real (springs.size ());
   avg_distance = max (min_distance + 0.3, avg_distance);
 
   Spring ret = Spring (avg_distance, min_distance);
@@ -90,7 +133,6 @@ Spring::set_distance (Real d)
     programming_error ("insane spring distance requested, ignoring it");
   else
     {
-      min_distance_ = min (min_distance_, d);
       distance_ = d;
       update_blocking_force ();
     }
@@ -104,11 +146,16 @@ Spring::set_min_distance (Real d)
   else
     {
       min_distance_ = d;
-      distance_ = max (distance_, min_distance_);
       update_blocking_force ();
     }
 }
 
+void
+Spring::ensure_min_distance (Real d)
+{
+  set_min_distance (max (d, min_distance_));
+}
+
 void
 Spring::set_inverse_stretch_strength (Real f)
 {
@@ -116,6 +163,8 @@ Spring::set_inverse_stretch_strength (Real f)
     programming_error ("insane spring constant");
   else
     inverse_stretch_strength_ = f;
+
+  update_blocking_force ();
 }
 
 void
@@ -140,15 +189,27 @@ Spring::set_blocking_force (Real f)
 
   blocking_force_ = -infinity_f;
   min_distance_ = length (f);
-  distance_ = max (distance_, min_distance_);
   update_blocking_force ();
 }
 
 void
 Spring::set_default_strength ()
 {
-  inverse_compress_strength_ = distance_ - min_distance_;
-  inverse_stretch_strength_ = distance_ - min_distance_;
+  set_default_stretch_strength ();
+  set_default_compress_strength ();
+}
+
+void
+Spring::set_default_compress_strength ()
+{
+  inverse_compress_strength_ = (distance_ >= min_distance_) ? distance_ - min_distance_ : 0;
+  update_blocking_force ();
+}
+
+void
+Spring::set_default_stretch_strength ()
+{
+  inverse_stretch_strength_ = distance_;
 }
 
 Real
@@ -163,5 +224,9 @@ Spring::length (Real f) const
       force = 0.0;
     }
 
-  return distance_ + force * inv_k;
+  // There is a corner case here: if min_distance_ is larger than
+  // distance_ but the spring is fixed, then inv_k will be zero
+  // and we need to make sure that we return min_distance_.
+  return max (min_distance_, distance_ + force * inv_k);
 }
+