From 813a0fc10cf5f7aaa6356cd97a4258fa78b9b87f Mon Sep 17 00:00:00 2001
From: Mikko Rasa <tdb@tdb.fi>
Date: Thu, 7 Nov 2013 10:25:58 +0200
Subject: [PATCH] Better method of implementing zero gravity

---
 source/layout.cpp | 33 ++++++++++++++++++++++-----------
 source/layout.h   |  2 +-
 2 files changed, 23 insertions(+), 12 deletions(-)

diff --git a/source/layout.cpp b/source/layout.cpp
index bee972d..b8d8fb7 100644
--- a/source/layout.cpp
+++ b/source/layout.cpp
@@ -92,8 +92,8 @@ Layout::Layout():
 	row_spacing(5),
 	col_spacing(4)
 {
-	n_slack_constraints[0] = 0;
-	n_slack_constraints[1] = 0;
+	n_slack_vars[0] = 0;
+	n_slack_vars[1] = 0;
 }
 
 Layout::~Layout()
@@ -219,14 +219,17 @@ void Layout::update_slot_indices()
 			(*i)->index = -1;
 	}
 
-	n_slack_constraints[0] = 0;
-	n_slack_constraints[1] = 0;
+	n_slack_vars[0] = 0;
+	n_slack_vars[1] = 0;
 	for(list<Slot *>::iterator i=slots.begin(); i!=slots.end(); ++i)
 		if((*i)->index>=0)
 		{
+			for(unsigned j=0; j<2; ++j)
+				if(((*i)->*(pointers[j].packing)).gravity==0)
+					n_slack_vars[j] += 2;
 			for(list<Constraint>::iterator j=(*i)->constraints.begin(); j!=(*i)->constraints.end(); ++j)
 				if(j->target.index>(*i)->index && (j->type&SLACK))
-					++n_slack_constraints[j->type&1];
+					++n_slack_vars[j->type&1];
 		}
 }
 
@@ -282,6 +285,7 @@ void Layout::set_gravity(Widget &wdg, int h, int v)
 	slot.horiz_pack.gravity = h;
 	slot.vert_pack.gravity = v;
 
+	update_slot_indices();
 	update();
 }
 
@@ -338,7 +342,7 @@ void Layout::solve_constraints(int dir, SolveMode mode)
 	five columns for each widget, and one constant column.  The first and second
 	columns of a widget are its position and dimension, respectively.  The
 	remaining three are slack columns; see below for their purposes. */
-	LinearProgram linprog(n_active_slots*5+n_slack_constraints[dir]+1);
+	LinearProgram linprog(n_active_slots*5+n_slack_vars[dir]+1);
 	float weight = slots.size();
 	unsigned k = n_active_slots*5;
 	for(list<Slot *>::iterator i=slots.begin(); i!=slots.end(); ++i)
@@ -378,12 +382,19 @@ void Layout::solve_constraints(int dir, SolveMode mode)
 
 		if(((*i)->*(ptrs.packing)).gravity==0)
 		{
-			/* This forces the widget's distance from the left and right edge of
-			the container to be equal.  It's a bit of a hack, but more time and
-			thought is needed for a better solution. */
+			/* Try to keep the widget as close to the center of the container
+			as possible.  Since linear programs can't express absolute values
+			directly, use two opposing slack variables that are optimized for
+			a low value. */
 			LinearProgram::Row row = linprog.add_row();
-			row[(*i)->index*5+2] = 1;
-			row[(*i)->index*5+3] = -1;
+			row[(*i)->index*5] = 1;
+			row[(*i)->index*5+1] = 0.5;
+			row[k] = 1;
+			row[k+1] = -1;
+			row.back() = geom.*(ptrs.dim)/2;
+			objective[k] = -1;
+			objective[k+1] = -1;
+			k += 2;
 		}
 
 		{
diff --git a/source/layout.h b/source/layout.h
index 9c4e2a0..043d92c 100644
--- a/source/layout.h
+++ b/source/layout.h
@@ -172,7 +172,7 @@ private:
 	Container *container;
 	std::list<Slot *> slots;
 	unsigned n_active_slots;
-	unsigned n_slack_constraints[2];
+	unsigned n_slack_vars[2];
 	Sides margin;
 	unsigned row_spacing;
 	unsigned col_spacing;
-- 
2.43.0