// Const access and load on demand
inline const ItemType& operator()( const int i )
{
assert( i < static_cast<int>( forward_ptr.size() ) );
if ( -1 == forward_ptr[i] ) {
assert( LAZY_BUFFER_INPUT == type );
if ( -1 != limit && static_cast<int>( backward_ptr.size() ) >= limit ) {
int pos = fl.discard();
int unload = forward_ptr[pos];
forward_ptr[pos] = -1;
assert( -1 != unload );
fseek( fdata, offsets[i], SEEK_SET );
items[unload] = std::move( ItemType( fdata ) );
forward_ptr[i] = unload;
backward_ptr[unload] = i;
} else {
fseek( fdata, offsets[i], SEEK_SET );
items.push_back( std::move( ItemType( fdata ) ) );
forward_ptr[i] = static_cast<int>( items.size() - 1 );
backward_ptr.push_back( i );
assert( backward_ptr.size() == items.size() );
}
fl.promote( i );
} else {
assert( ( 0 <= forward_ptr[i] ) && ( forward_ptr[i] < static_cast<int>( items.size() ) ) );
}
return items[forward_ptr[i]];
}
inline void evacuate()
{
assert( LAZY_BUFFER_INPUT == type );
for ( auto& ele : backward_ptr ) {
forward_ptr[ele] = -1;
fl.discard();
}
backward_ptr.clear();
items.clear();
}