Add modified gradient method sample

src/Makefile.am

@@ -2,10 +2,12 @@
 SUBDIRS =
 
 noinst_PROGRAMS = \
+	bisection-example \
 	bisection-simpler-example \
 	bisection-simpler-example2 \
 	bisection-simpler-example2-gradient
 
+bisection_example_SOURCES = bisection-example.cc
 bisection_simpler_example_SOURCES = bisection-simpler-example.cc
 bisection_simpler_example2_SOURCES = bisection-simpler-example2.cc
 bisection_simpler_example2_gradient_SOURCES = bisection-simpler-example2-gradient.cc

src/bisection-example.cc

+/* -*- mode:c++; mode: flymake -*- */
+
+#ifdef HAVE_CONFIG_H
+#include "config.h"
+#endif
+
+#include <dune/common/exceptions.hh>
+#include <dune/common/stdstreams.hh>
+
+#include <dune/fufem/interval.hh>
+
+#include <dune/tnnmg/nonlinearities/smallfunctional.hh>
+#include <dune/tnnmg/problem-classes/bisection.hh>
+
+#include <cassert>
+#include <cstdlib>
+#include <limits>
+
+template <int dimension> class SampleFunctional {
+public:
+  typedef Dune::FieldVector<double, dimension> SmallVector;
+  typedef Dune::FieldMatrix<double, dimension, dimension> SmallMatrix;
+
+  // FIXME: hardcoded function H
+  SampleFunctional(SmallMatrix A, SmallVector b) : A_(A), b_(b) {}
+
+  double operator()(const SmallVector v) const {
+    SmallVector y;
+    A_.mv(v, y);                    // y = Av
+    y /= 2;                         // y = 1/2 Av
+    y -= b_;                        // y = 1/2 Av - b
+    return y * v + H(v.two_norm()); // <1/2 Av - b,v> + H(|v|)
+  }
+
+  double directionalDerivative(const SmallVector x,
+                               const SmallVector dir) const {
+    if (x == SmallVector(0.0))
+      // Well, not in this way -- but can we compute them?
+      DUNE_THROW(Dune::Exception,
+                 "Directional derivatives cannot be computed at zero.");
+
+    if (x * dir > 0)
+      return PlusGrad(x) * dir;
+    else
+      return MinusGrad(x) * dir;
+  }
+
+  SmallVector ModifiedGradient(const SmallVector x) const {
+    if (x == SmallVector(0.0))
+      // TODO
+      DUNE_THROW(Dune::Exception, "The case x = 0 is not yet handled.");
+
+    SmallVector const pg = PlusGrad(x);
+    SmallVector const mg = MinusGrad(x);
+    SmallVector ret;
+    // TODO: collinearity checks suck
+    if (pg * x == pg.two_norm() * x.two_norm() &&
+        -(mg * x) == mg.two_norm() * x.two_norm()) {
+      return SmallVector(0);
+    } else if (pg * x >= 0 && mg * x >= 0) {
+      ret = pg;
+    } else if (pg * x <= 0 && mg * x <= 0) {
+      ret = mg;
+    } else {
+      ret = project(SmoothGrad(x), x);
+    }
+    ret *= -1;
+    return ret;
+  }
+
+  SmallVector project(const SmallVector z, const SmallVector x) const {
+    SmallVector y = z;
+    y.axpy(-(z * x) / x.two_norm2(), x);
+    return y;
+  }
+
+  SmallVector minimise(const SmallVector x, unsigned int iterations) const {
+    SmallVector descDir = ModifiedGradient(x);
+
+    Dune::dverb << "Starting at x with J(x) = " << operator()(x) << std::endl;
+    Dune::dverb << "Minimizing in direction w with dJ(x,w) = "
+                << directionalDerivative(x, descDir) << std::endl;
+
+    double l = 0;
+    double r = 1;
+    SmallVector tmp;
+    while (true) {
+      tmp = x;
+      tmp.axpy(r, descDir);
+      if (directionalDerivative(tmp, descDir) >= 0)
+        break;
+
+      l = r;
+      r *= 2;
+      Dune::dverb << "Widened interval!" << std::endl;
+    }
+    Dune::dverb << "Interval now [" << l << "," << r << "]" << std::endl;
+
+    // Debugging
+    {
+      SmallVector tmpl = x;
+      tmpl.axpy(l, descDir);
+      SmallVector tmpr = x;
+      tmpr.axpy(r, descDir);
+      assert(directionalDerivative(tmpl, descDir) < 0);
+      assert(directionalDerivative(tmpr, descDir) > 0);
+    }
+
+    double m = l / 2 + r / 2;
+    SmallVector middle;
+    for (unsigned int count = 0; count < iterations; ++count) {
+      Dune::dverb << "now at m = " << m << std::endl;
+      Dune::dverb << "Value of J here: " << operator()(x + middle) << std::endl;
+
+      middle = descDir;
+      middle *= m;
+
+      double derivative = directionalDerivative(x + middle, descDir);
+
+      if (derivative < 0) {
+        l = m;
+        m = (m + r) / 2;
+      } else if (derivative > 0) {
+        r = m;
+        m = (l + m) / 2;
+      } else
+        break;
+    }
+    return middle;
+  }
+
+private:
+  SmallMatrix A_;
+  SmallVector b_;
+
+  double H(double s) const { return (s < 1) ? s : (2 * s - 1); }
+
+  double HPrimeMinus(double s) const { return (s <= 1) ? 1 : 2; }
+
+  double HPrimePlus(double s) const { return (s < 1) ? 1 : 2; }
+
+  // Gradient of the smooth part
+  SmallVector SmoothGrad(const SmallVector x) const {
+    SmallVector y;
+    A_.mv(x, y); // y = Av
+    y -= b_;     // y = Av - b
+    return y;
+  }
+
+  SmallVector PlusGrad(const SmallVector x) const {
+    SmallVector y = SmoothGrad(x);
+    y.axpy(HPrimePlus(x.two_norm()) / x.two_norm(), x);
+    return y;
+  }
+
+  SmallVector MinusGrad(const SmallVector x) const {
+    SmallVector y = SmoothGrad(x);
+    y.axpy(HPrimeMinus(x.two_norm()) / x.two_norm(), x);
+    return y;
+  }
+};
+
+int main() {
+  try {
+    int const dim = 2;
+    typedef SampleFunctional<dim> SampleFunctional;
+
+    SampleFunctional::SmallMatrix A;
+    A[0][0] = 3;
+    A[0][1] = 0;
+    A[1][0] = 0;
+    A[1][1] = 3;
+    SampleFunctional::SmallVector b;
+    b[0] = 1;
+    b[1] = 2;
+
+    SampleFunctional J(A, b);
+
+    std::cout << J.directionalDerivative(b, b) << std::endl;
+    assert(J.directionalDerivative(b, b) == 10 + 2 * sqrt(5));
+
+    SampleFunctional::SmallVector descDir = J.ModifiedGradient(b);
+
+    SampleFunctional::SmallVector start = b;
+    start *= 17;
+    SampleFunctional::SmallVector correction = J.minimise(start, 20);
+    assert(J(start + correction) <= J(start));
+    assert(std::abs(J(start + correction) + 0.254644) < 1e-8);
+    std::cout << J(start + correction) << std::endl;
+
+    return 0;
+  }
+  catch (Dune::Exception &e) {
+    Dune::derr << "Dune reported error: " << e << std::endl;
+  }
+}