Add a solver option to not calculate the failing constraints if

we're inconsistent (singular Jacobian). That's slow, so we should
provide a library interface to disable it.

[git-p4: depot-paths = "//depot/solvespace/": change = 1946]

exposed/example.c

@@ -1,3 +1,15 @@
+//-----------------------------------------------------------------------------
+// Some sample code for slvs.lib. We draw some geometric entities, provide
+// initial guesses for their positions, and then constrain them. The solver
+// calculates their new positions, in order to satisfy the constraints.
+//
+// This code is provide for evaluation purposes only. To purchase a license,
+// please visit:
+//
+//     http://solvespace.com/
+//
+// Copyright 2009, Jonathan Westhues
+//-----------------------------------------------------------------------------
 #include <windows.h>
 #include <stdio.h>
 
@@ -141,14 +153,19 @@ void Example2d(void)
                                             200,
                                             15.0,
                                             301, 101, 0, 0);
-
-    // And the distance from one endpoint to the origin is 15.0 units.
+/*
+    // And same for the other endpoint; so if you add this constraint then
+    // the sketch is overconstrained and will signal an error.
     sys.constraint[sys.constraints++] = Slvs_MakeConstraint(
                                             5, g,
                                             SLVS_C_PT_PT_DISTANCE,
                                             200,
                                             18.0,
-                                            302, 101, 0, 0);
+                                            302, 101, 0, 0); */
+
+    // If the solver fails, then ask it to report which constraints caused
+    // the problem.
+    sys.calculateFaileds = 1;
 
     // And solve.
     Slvs_Solve(&sys, g);

exposed/lib.cpp

@@ -187,7 +187,8 @@ default: dbp("bad constraint type %d", sc->type); return;
     ZERO(&bad);
 
     // Now we're finally ready to solve!
-    int how = SYS.Solve(&g, &(ssys->dof), &bad, false);
+    bool andFindBad = ssys->calculateFaileds ? true : false;
+    int how = SYS.Solve(&g, &(ssys->dof), &bad, andFindBad, false);
 
     switch(how) {
         case System::SOLVED_OKAY:

exposed/slvs.h

@@ -128,7 +128,14 @@ typedef struct {
     // Unused members of this array should be set to zero.
     Slvs_hParam         dragged[4];
 
-    //// OUTPUT PARAMETERS
+    // If the solver fails, then it can determine which constraints are
+    // causing the problem. But this is a relatively slow process (for
+    // a system with n constraints, about n times as long as just solving).
+    // If calculateFaileds is set, then the solver will do so, otherwise
+    // not.
+    int                 calculateFaileds;
+
+    //// OUTPUT VARIABLES
     // 
     // If the solver fails, then it can report which constraints are causing
     // the problem. The caller should allocate the array failed[], and pass

generate.cpp

@@ -410,7 +410,7 @@ void SolveSpace::SolveGroup(hGroup hg, bool andFindFree) {
     MarkDraggedParams();
     g->solved.remove.Clear();
     int how = sys.Solve(g, &(g->solved.dof),
-                           &(g->solved.remove), andFindFree);
+                           &(g->solved.remove), true, andFindFree);
     if((how != System::SOLVED_OKAY) ||
        (how == System::SOLVED_OKAY && g->solved.how != System::SOLVED_OKAY))
     {

solvespace.h

@@ -271,7 +271,8 @@ public:
     static const int DIDNT_CONVERGE       = 10;
     static const int SINGULAR_JACOBIAN    = 11;
     static const int TOO_MANY_UNKNOWNS    = 20;
-    int Solve(Group *g, int *dof, List<hConstraint> *bad, bool andFindFree);
+    int Solve(Group *g, int *dof, List<hConstraint> *bad,
+                bool andFindBad, bool andFindFree);
 };
 
 class TtfFont {

system.cpp

@@ -363,7 +363,7 @@ void System::FindWhichToRemoveToFixJacobian(Group *g, List<hConstraint> *bad) {
 }
 
 int System::Solve(Group *g, int *dof, List<hConstraint> *bad, 
-                  bool andFindFree)
+                  bool andFindBad, bool andFindFree)
 {
     WriteEquationsExceptFor(Constraint::NO_CONSTRAINT, g);
 
@@ -420,7 +420,9 @@ int System::Solve(Group *g, int *dof, List<hConstraint> *bad,
 
     int rank = CalculateRank();
     if(rank != mat.m) {
-        FindWhichToRemoveToFixJacobian(g, bad);
+        if(andFindBad) {
+            FindWhichToRemoveToFixJacobian(g, bad);
+        }
         return System::SINGULAR_JACOBIAN;
     }
     // This is not the full Jacobian, but any substitutions or single-eq