fix realloc when initial size was 0 (bug reported by Jens Mueller)

Eigen/src/Sparse/SparseMatrix.h

@@ -244,15 +244,23 @@ class SparseMatrix
       float reallocRatio = 1;
       if (m_data.allocatedSize()<=m_data.size())
       {
-        // we need to reallocate the data, to reduce multiple reallocations
-        // we use a smart resize algorithm based on the current filling ratio
-        // in addition, we use float to avoid integers overflows
-        float nnzEstimate = float(m_outerIndex[outer])*float(m_outerSize)/float(outer+1);
-        reallocRatio = (nnzEstimate-float(m_data.size()))/float(m_data.size());
-        // furthermore we bound the realloc ratio to:
-        //   1) reduce multiple minor realloc when the matrix is almost filled
-        //   2) avoid to allocate too much memory when the matrix is almost empty
-        reallocRatio = std::min(std::max(reallocRatio,1.5f),8.f);
+        // if there is no preallocated memory, let's reserve a minimum of 32 elements
+        if (m_data.size()==0)
+        {
+          m_data.reserve(32);
+        }
+        else
+        {
+          // we need to reallocate the data, to reduce multiple reallocations
+          // we use a smart resize algorithm based on the current filling ratio
+          // in addition, we use float to avoid integers overflows
+          float nnzEstimate = float(m_outerIndex[outer])*float(m_outerSize)/float(outer+1);
+          reallocRatio = (nnzEstimate-float(m_data.size()))/float(m_data.size());
+          // furthermore we bound the realloc ratio to:
+          //   1) reduce multiple minor realloc when the matrix is almost filled
+          //   2) avoid to allocate too much memory when the matrix is almost empty
+          reallocRatio = std::min(std::max(reallocRatio,1.5f),8.f);
+        }
       }
       m_data.resize(m_data.size()+1,reallocRatio);