# Hierarchical Classification

    library(tabnet)
    library(dplyr)
    library(data.tree)
    library(ggplot2)
    library(rsample)
    library(tibble)
    set.seed(202307)

## Data format

The supported data format for hierarchical classification is the `Node` object format from package `{data.tree}`.

This is a general purpose format that fits generic hierarchical tree encoding needs. Each node of the tree is associated with predictor values through the `attributes` in the data `Node` object.

{tabnet} concept | {data.tree} concept | see command
--- | --- | ---
dataset predictor | Node `attributesAll` | acme example
dataset multi-label target | Node hierarchy | print(acme)

A very basic example is the `acme` dataset to show you how the two predictors values `cost` and `p` are associates attributes of each node in the hierarchy :

    data(acme, package = "data.tree")
    acme$attributesAll
    print(acme, "cost", "p" , limit = 8)

So printing Node objects reverse the usual ordering, as target is printed first in column `levelName`, and predictors printed right of it.

As you can see, only leaf nodes of the tree gets predictors value. {tabnet} will take this into account via an `ancestor` square sparse tensor registering all possible parent-child relation among the target labels.

## Data preparation

Multiple manual or programmatic methods are available to create or update predictors. They are detailled in the `vignette("data.tree", package = "data.tree")`.

- a lot of native hierarchical data-format conversion from files to `Node` are covered by the`{data.tree}` package. You can find them in the “Create tree from a file” section of the same vignette. If needed, the `{ape}` package covers a lot of conversion format to the `philo` format. Thus you can reach the `Node` format in maybe two transformation steps…

- A quick way to achieve `Node` format from a data frame with columns being the different levels of the hierarchy consist in pasting the columns into a single string with `"/"` separator into a `pathString` column. This will be turn into the expected hierarchy by the `data.tree::as.Node()` command.

Let’s do it with `starwars` dataset as a toy example :

    data(starwars, package = "dplyr")
    head(starwars, 4)

    # erroneous Node construction
    starwars_tree <- starwars %>%
      mutate(pathString = paste("StarWars_characters", species, sex, `name`, sep = "/")) %>%
      as.Node()
    print(starwars_tree, "name","height", "mass", "eye_color", limit = 8)

You may have noticed that `name` and `height` have unexpected values according to the original data: `Human` is not part of the `name` in orginal dataset, and `height` values have been changed into the local height of the tree. This is due to some rules we will have to follow to create the Node from data frame.

## `Node` preparation rules for {tabnet} models

### Avoid `factor` predictors

As `as.Node()` will only consider the as.numeric() values of a factor(), you should turn them into characters before applying the `as.Node()` function in order for {tabnet} to properly embed them.

### Avoid column name collision with reserved {data.tree} names

`name` and `height` are both part of the `NODE_RESERVED_NAMES_CONST` reserved list of names for `Node` attributes. So they must not be used as predictor names, or the `as.Node()` function will silently discard them.

### Avoid column named `level_*` to avoid collision with output data.tree names

Your dataset hierarchy will be turn internally into multi-outcomes named `level_1` to `level_n`, n beeing the depth of your tree. Thus column names starting with `level_` should be avoided.

### Ensure the last hierarchy of the tree is the observation id

The tree only keeps a single row of attributes per tree leaf. Thus in order to transfer your complete predictors dataset into the Node object, you must keep the last level of the hierarchy to be a unique observation identifier (last resort beeing `rowid_to_column()` to achieve it).

The classification will be done removing the last level of hierarchy in any case.

### Ensure there is a root level in the hierarchy

The tree should have a single root for all nodes to be consistent. Thus you have to use a constant prefix to all `pathString`.

The classification will be done removing the first level of hierarchy in any case.

### Ensure there is no missing values in the hierarchical classes

Missing values should be replaced. Turning them by an explicit “Unknown_something” is a good approach.

Now let’s have all those rules applied to the `starwars_tree` :

    # demonstration of reserved column modification in Node construction
    starwars_tree <- starwars %>%
      rename(`_name` = "name", `_height` = "height") %>%
      mutate(
        species = coalesce(species, "Unknown_Species"),
        sex     = coalesce(sex, "Unknown_Sex"),
        pathString = paste("StarWars_characters", species, sex, `_name`, sep = "/")) %>%
      as.Node()
    print(starwars_tree, "name", "_name","_height", "mass", "eye_color", limit = 8)

We can see that the reserved `name` column contains slightly different content that the original `_name` column.

## Model building

### Data set split

The `starwars` dataset contains list columns, hosting some variability in the predictor values. Thus we decide here to `unnest_longer` every list column to each of its values. This will triple the size of the `starwars` dataset.
The dataset split here will be done upfront of the transformation into `as.Node()`. We will use `rsample::initial_split()` to split with a stratification on the parent category of the first level of our hierarchy which is `species`.

    starw_split <- starwars %>%
      tidyr::unnest_longer(films) %>%
      tidyr::unnest_longer(vehicles, keep_empty = TRUE) %>%
      tidyr::unnest_longer(starships, keep_empty = TRUE) %>%
      initial_split( prop = .8, strata = "species")

In order to train a model properly, we should prevent the outcomes to be part of the predictor columns. For the sake of demonstration, the `_name` column was present in `starwars_tree` but must now be dropped.

    # correct Node construction for hierarchical modeling
    starwars_train_tree <- starw_split %>%
      training() %>%
      # avoid reserved column names
      rename(`_name` = "name", `_height` = "height") %>%
      rowid_to_column() %>%
      mutate(pathString = paste("StarWars_characters", species, sex, rowid, sep = "/")) %>%
      # remove outcomes labels from predictors
      select(-species, -sex, -`_name`, -rowid) %>%
      # turn it as hierarchical Node
      as.Node()

    starwars_test_tree <- starw_split %>%
      testing() %>%
      rename(`_name` = "name", `_height` = "height") %>%
      rowid_to_column() %>%
      mutate(pathString = paste("StarWars_characters", species, sex, rowid, sep = "/")) %>%
      select(-species, -sex, -`_name`, -rowid) %>%
      as.Node()

    starwars_train_tree$attributesAll

Now we can see that none of the predictor leaks the outcome hierarchy information.

## Model building

This `starwars_tree` can now be used as an input for `tabnet_fit()` :

    config <- tabnet_config(decision_width = 8, attention_width = 8, num_steps = 3, penalty = .003, cat_emb_dim = 2, valid_split = 0.2, learn_rate = 1e-3, lr_scheduler = "reduce_on_plateau", early_stopping_monitor = "valid_loss", early_stopping_patience = 4, verbose = FALSE)

    starw_model <- tabnet_fit(starwars_train_tree, config = config, epoch = 170, checkpoint_epochs = 25)

## Model diagnostic

We have avoid the verbose output of the model, thus very first diagnostic is the check for model over-fitting though the training loss plot.

    autoplot(starw_model)

Then global feature importance gives us a clue of model quality

    vip::vip(starw_model)

## Model inference

We can infer on the test-set

    starwars_hat <- augment(starw_model, starwars_test_tree)
    tail(starwars_hat, n = 5)

We can see in the Warnings that the dataset is a challenge as many new levels are found in a lot of predictors in the test set.
The model also here is very poor on the `level_2` ( `species` ) and on `level_3` ( `sex` ) as this is definitively not a model-intended dataset. The reason is that the input dataset not collecting large samples of distinctive observation per leaf, but rather a very diverse but limited number of characters compatible with watching a movie saga.

Despite the performance, we do have local feature importance on the complete dataset here :

    starwars_explain <- tabnet_explain(starw_model, starwars_test_tree)
    autoplot(starwars_explain)
    autoplot(starwars_explain, type = "steps")

Hopefully your own hierarchical outcome will have a better success than the one here with `starwars` dataset. But in this journey, you have learned a lot in the data format constraints and solutions, and you now have a new performing solution in your toolbox.