/*
Description:
  Gets the indices of the trims and edges used by a brep face.
Parameters:
  brep              - [in] The brep.
  face_index        - [in] Index of the face in brep.m_F[].
  face_trim_indices - [out] Indices into brep.m_T[] of the face's trims.
  face_edge_indices - [out] Indices into brep.m_E[] of the face's edges.
Returns:
  true if successful, false if face_index is out of range.
*/
bool GetFaceTrimsAndEdges(
  const ON_Brep& brep,
  int face_index,
  ON_SimpleArray<int>& face_trim_indices,
  ON_SimpleArray<int>& face_edge_indices
)
{
  face_trim_indices.Empty();
  face_edge_indices.Empty();

  // Get the face at the given index
  const ON_BrepFace* face = brep.Face(face_index);
  if (nullptr == face)
    return false;

  // Walk the loops of the face
  for (int fli = 0; fli < face->LoopCount(); fli++)
  {
    const ON_BrepLoop* loop = face->Loop(fli);
    if (nullptr == loop)
      continue;

    // Walk the trims of the loop
    for (int lti = 0; lti < loop->TrimCount(); lti++)
    {
      const ON_BrepTrim* trim = loop->Trim(lti);
      if (nullptr == trim)
        continue;
      face_trim_indices.Append(trim->m_trim_index);

      // Get the edge the trim uses.
      // Singular trims have no edge, so trim->Edge() is nullptr for them.
      const ON_BrepEdge* edge = trim->Edge();
      if (nullptr == edge)
        continue;
      face_edge_indices.Append(edge->m_edge_index);
    }
  }

  return true;
}