/// \file

/// \ingroup tutorial_dataframe

/// \notebook -draw

/// This tutorial shows how the content of a data frame can be cached in memory

/// in form of a data frame. The content of the columns is stored in memory in

/// contiguous slabs of memory and is "ready to use", i.e. no ROOT IO operation

/// is performed.

///

/// Creating a cached data frame storing all of its content deserialised and uncompressed

/// in memory is particularly useful when dealing with datasets of a moderate size

/// (small enough to fit the RAM) over which several explorative loops need to be

/// performed at as fast as possible. In addition, caching can be useful when no file

/// on disk needs to be created as a side effect of checkpointing part of the analysis.

///

/// All steps in the caching are lazy, i.e. the cached data frame is actually filled

/// only when the event loop is triggered on it.

///

/// \macro_code

/// \macro_image

///

/// \date June 2018

/// \author Danilo Piparo

void df019_Cache()

{

// We create a data frame on top of the hsimple example

auto hsimplePath = gROOT->GetTutorialDir();

hsimplePath += "/hsimple.root";

ROOT::RDataFrame df("ntuple", hsimplePath.Data());

// We apply a simple cut and define a new column

auto df_cut = df.Filter([](float py) { return py > 0.f; }, {"py"})

.Define("px_plus_py", [](float px, float py) { return px + py; }, {"px", "py"});

// We cache the content of the dataset. Nothing has happened yet: the work to accomplish

// has been described. As for `Snapshot`, the types and columns can be written out explicitly

// or left for the jitting to handle (`df_cached` is intentionally unused - it shows how to

// to create a *cached* data frame specifying column types explicitly):

auto df_cached = df_cut.Cache<float, float>({"px_plus_py", "py"});

auto df_cached_implicit = df_cut.Cache();

auto h = df_cached_implicit.Histo1D<float>("px_plus_py");

// Now the event loop on the cached dataset is triggered. This event triggers the loop

// on the `df` data frame lazily.

h->DrawCopy();

}