Work graph construction ease-of-use

[hcedwar]

Constructing the WorkGraphPolicies Crs data structure requires a count-allocate-fill pattern.
An ease-of-use interface could accept a user's fill-lambda, apply a dry run of that lambda to count, then allocate, then apply the lambda again to fill.

[ibaned]

Need to prevent user lambda from filling before there is anything to fill. Proposed concept:

IndexType user_lambda(IndexType work_item, bool fill, CrsType const& graph) {
  IndexType count = 0;
  for (faces) {
    if (is_dependent(face)) {
      ++count;
      if (fill) graph.entries(graph.row_map(work_item) + count) = next_work_item;
    }
  }
  return count;
}

[hcedwar]

Even simpler:

  [=]( int work_item , int * fill ) {
    int count = 0 ;
    for ( faces ) {
      if ( is_dependent(face) ) {
        if ( fill ) fill[count] = other_work_item(face);
        ++count ;
      }
    }
    return count ;
  }