#ifndef _POLARDECOMPOSITION_H_
#define _POLARDECOMPOSITION_H_

namespace vega
{
/*
  Polar decomposition of a general 3x3 matrix

  This code uses the polar decomposition implementation provided as a companion to the book "Graphics Gems IV": 
  Decompose.c 
  Ken Shoemake, 1993 
  Polar Decomposition of 3x3 matrix in 4x4, M = QS.  
  The Graphics Gems IV implementation is available at: 
  http://tog.acm.org/GraphicsGems/

  The above website states that "All code here (on the GraphicsGems website) can be used without restrictions". It also lists the following EULA:
  "EULA: The Graphics Gems code is copyright-protected. In other words, you cannot claim the text of the code as your own and resell it. Using the code is permitted in any program, product, or library, non-commercial or commercial. Giving credit is not required, though is a nice gesture. The code comes as-is, and if there are any flaws or problems with any Gems code, nobody involved with Gems - authors, editors, publishers, or webmasters - are to be held responsible. Basically, don't be a jerk, and remember that anything free comes with no guarantee."

  Jernej Barbic made some adaptions to the polar decomposition code (wrap into a C++ class, some change in input/output format, etc.). 
  He releases his adaptions of the polar decomposition code into the public domain, free of charge. The above EULA still applies, of course.
*/

class PolarDecomposition
{
public:

  // Computes the Polar Decomposition of a general 3x3 matrix M.
  // M = Q * S
  // M is 3x3 input matrix
  // Q is 3x3 orthogonal output matrix, Q Q^T = Q^T Q = I 
  // S is 3x3 symmetric output matrix
  // Note: det(Q)=sgn(det(M)); this sign can be 1 or -1, depending on M
  // M is not modified
  // All matrices are row-major
  static double Compute(const double * M, double * Q, double * S, double tolerance = 1E-6);

protected:

  // one-norm of a 3 x 3 matrix
  static double oneNorm(const double * A);

  // infinity-norm of a 3 x 3 matrix
  static double infNorm(const double * A);

  // a, b, c are 3-vectors
  // compute cross product c = a x b
  inline static void crossProduct(const double * a, const double * b, double * c);
};

// cross product: c = a x b
inline void PolarDecomposition::crossProduct(const double * a, const double * b, double * c)
{
  c[0] = a[1] * b[2] - a[2] * b[1];
  c[1] = a[2] * b[0] - a[0] * b[2];
  c[2] = a[0] * b[1] - a[1] * b[0];
}
}
#endif

