From b3d381750d347a7a4eecf04052f4e4658ee0d555 Mon Sep 17 00:00:00 2001
From: Andrey Aralov <andrey.aralov@meshinspector.com>
Date: Thu, 8 Aug 2024 17:14:53 +0300
Subject: [PATCH] Add `solveWithFixedDegree` wrapper (#3146)

* Add `solveWithFixedDegree` wrapper, that removes leading zeros from the polynomial before passing it to the actual solver.

* Fix build.
---
 source/MRMesh/MRBestFitPolynomial.cpp | 36 +++++++++++++++++++++++++--
 source/MRMesh/MRBestFitPolynomial.h   |  1 +
 2 files changed, 35 insertions(+), 2 deletions(-)

diff --git a/source/MRMesh/MRBestFitPolynomial.cpp b/source/MRMesh/MRBestFitPolynomial.cpp
index caa487a6996c..f1096d1a4823 100644
--- a/source/MRMesh/MRBestFitPolynomial.cpp
+++ b/source/MRMesh/MRBestFitPolynomial.cpp
@@ -124,6 +124,39 @@ struct Solver<T, 4>
 };
 
 
+template <typename T, size_t degree>
+std::vector<std::complex<T>> solveWithFixedDegree( auto coeffsBegin, auto coeffsEnd )
+{
+    assert( coeffsEnd - coeffsBegin == degree + 1 );
+    if constexpr ( degree == 0 )
+    {
+        return {};
+    }
+    else
+    {
+        if ( *std::prev( coeffsEnd ) != 0 )
+        {
+            Eigen::Vector<T, degree + 1> c;
+            for ( size_t i = 0; i < degree + 1; ++i )
+                c[i] = *( coeffsBegin + i );
+
+            Solver<T, degree> s;
+            auto roots = s( c );
+
+            std::vector<std::complex<T>> res;
+            for ( auto v : roots )
+                res.push_back( v );
+            return res;
+        }
+        else
+        {
+            return solveWithFixedDegree<T, degree - 1>( coeffsBegin, std::prev( coeffsEnd ) );
+        }
+    }
+}
+
+
+
 }
 
 namespace MR
@@ -152,8 +185,7 @@ std::vector<T> Polynomial<T, degree>::solve( T tol ) const
     if constexpr ( canSolvePolynomial( degree ) )
     {
 #endif
-    Solver<T, degree> solver;
-    auto r_c = solver( a );
+    auto r_c = solveWithFixedDegree<T, degree>( a.begin(), a.end() );
     std::vector<T> r;
     for ( std::complex<T> c : r_c )
         if ( std::abs( c.imag() ) < tol )
diff --git a/source/MRMesh/MRBestFitPolynomial.h b/source/MRMesh/MRBestFitPolynomial.h
index 9df0b0022678..fc86ffa77174 100644
--- a/source/MRMesh/MRBestFitPolynomial.h
+++ b/source/MRMesh/MRBestFitPolynomial.h
@@ -98,6 +98,7 @@ class BestFitPolynomial
 
     MRMESH_API void addPoint( T x, T y, T weight );
 
+    /// @note The result might have leading coefficient equal to zero.
     MRMESH_API Polynomial<T, degree> getBestPolynomial() const;
 
 private: