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diff --git a/libs/cassowary/ClFDBinaryOneWayConstraint.cc b/libs/cassowary/ClFDBinaryOneWayConstraint.cc
new file mode 100644
index 0000000000..e7bf7f1089
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+++ b/libs/cassowary/ClFDBinaryOneWayConstraint.cc
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+// $Id$
+//
+// Cassowary Incremental Constraint Solver
+// Original Smalltalk Implementation by Alan Borning
+// This C++ Implementation by Greg J. Badros, <gjb@cs.washington.edu>
+// http://www.cs.washington.edu/homes/gjb
+// (C) 1998, 1999 Greg J. Badros and Alan Borning
+// See ../LICENSE for legal details regarding this software
+//
+// ClFDBinaryOneWayConstraint.cc
+
+
+#ifdef HAVE_CONFIG_H
+#include <config.h>
+#define CONFIG_H_INCLUDED
+#endif
+
+#include <cassowary/ClFDBinaryOneWayConstraint.h>
+#include <cassowary/ClLinearConstraint.h>
+#include <cassowary/ClTypedefs.h>
+#include <cassowary/ClLinearExpression.h>
+
+
+void 
+ClFDBinaryOneWayConstraint::EnsurePreconditionsForCn(const ClConstraint &cn)
+{
+  ClVarSet setRO = cn.ReadOnlyVars();
+  if (setRO.size() > 1) 
+    throw ExCLTooDifficultSpecial("Only 0 or 1 read only variables are allowed");
+  const ClLinearExpression &expr = cn.Expression();
+  const ClVarToNumberMap &terms = expr.Terms();
+  if (terms.size() > 2)
+    throw ExCLTooDifficultSpecial("Cannot have more than 2 variables");
+  if (terms.size() == 0)
+    throw ExCLTooDifficultSpecial("Must have at least 1 variable");
+  if (terms.size() == 2 && setRO.size() == 0)
+    throw ExCLTooDifficultSpecial("Both variables cannot be read-write, one must be read-only");
+  if (terms.size() == 1 && setRO.size() == 1)
+    throw ExCLTooDifficultSpecial("Single read-only variable in LinearConstraint -- must not be read-only.");
+  ClVariable clv = (*terms.begin()).first;
+  /* GJB:FIXME:: iterate over all the variables */
+  if (!clv->IsFDVariable()) {
+    throw ExCLTooDifficultSpecial("FD constraint contains non-FD variables");
+  }
+}
+
+bool 
+ClFDBinaryOneWayConstraint::FCanConvertCn(const ClConstraint &cn)
+{
+  try {
+    EnsurePreconditionsForCn(cn);
+    return true;
+  } catch (...) {
+    return false;
+  }
+}
+
+
+ClFDBinaryOneWayConstraint::ClFDBinaryOneWayConstraint(const ClConstraint &cn)
+    :ClFDConstraint(cn.strength(), cn.weight())
+{
+  EnsurePreconditionsForCn(cn);
+  list<FDNumber> l;
+  /* GJB:FIXME:: varargs inteface, with sentinel as first arg? */
+  l.push_back(9);
+  l.push_back(10);
+  l.push_back(12);
+  l.push_back(14);
+  l.push_back(20);
+
+  ClVarSet setRO = cn.ReadOnlyVars();
+
+  ClVariable clvRO = clvNil;
+  ClVariable clvROLinear = clvNil;
+  Number coeffRO = 0;
+
+  ClVariable clvRW = clvNil;
+  Number coeffRW = 0;
+
+  if (setRO.size() == 1) {
+    const ClVariable &clv = *(setRO.begin());
+    if (clv->IsFDVariable())
+      clvRO = clv;
+    else
+      clvRO = new ClFDVariable(clv.Name(),clv.IntValue(),l);
+    clvROLinear = clv;
+  }
+  const ClLinearExpression &expr = cn.Expression();
+  const ClVarToNumberMap &terms = expr.Terms();
+
+  for (ClVarToNumberMap::const_iterator it = terms.begin();
+       it != terms.end();
+       ++it) {
+    ClVariable clv = (*it).first;
+    if (clv == clvROLinear) {
+      coeffRO = (*it).second;
+    } else {
+      if (clv->IsFDVariable())
+        clvRW = clv;
+      else
+        clvRW = new ClFDVariable(clv.Name(),clv.Value(),l);
+      coeffRW = (*it).second;
+    }
+  }
+  assert(!clvRW.IsNil());
+  if (coeffRW == 0) {
+    throw ExCLTooDifficultSpecial("RW variable's coefficient must be non-zero");
+  }
+
+  bool fInequality = cn.IsInequality();
+  bool fStrictInequality = cn.IsStrictInequality();
+  double rhs_constant = expr.Constant();
+
+  // now we have:
+  // coeffRW * clvRW + coeffRO * clvRO <REL> rhs_constant
+  //   where <REL> is >= if fInequality, or = if !fInequality
+  // 
+  // need:
+  //   clvRW <REL> coefficient * clvRO + constant
+  // 
+  // so:
+  // coefficient = -coeffRO/coeffRW
+  // constant = rhs_constant/coeffRW
+
+  if (fStrictInequality)
+    _rel = cnGT;
+  else if (fInequality)
+    _rel = cnGEQ;
+  else
+    _rel = cnEQ;
+
+  if (coeffRW < 0)
+    _rel = ReverseInequality(_rel);
+
+  _coefficient = -coeffRO/coeffRW;
+  _constant = -rhs_constant/coeffRW;
+  _vRW = clvRW;
+  _vRO = clvRO;
+  return;
+}