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#' Parse a boosted tree model text dump
#' 
#' Parse a boosted tree model text dump into a \code{data.table} structure.
#' 
#' @param feature_names character vector of feature names. If the model already
#'          contains feature names, those would be used when \code{feature_names=NULL} (default value).
#'          Non-null \code{feature_names} could be provided to override those in the model.
#' @param model object of class \code{xgb.Booster}
#' @param text \code{character} vector previously generated by the \code{xgb.dump} 
#'          function  (where parameter \code{with_stats = TRUE} should have been set).
#'          \code{text} takes precedence over \code{model}.
#' @param trees an integer vector of tree indices that should be parsed.
#'          If set to \code{NULL}, all trees of the model are parsed.
#'          It could be useful, e.g., in multiclass classification to get only
#'          the trees of one certain class. IMPORTANT: the tree index in xgboost models
#'          is zero-based (e.g., use \code{trees = 0:4} for first 5 trees).
#' @param use_int_id a logical flag indicating whether nodes in columns "Yes", "No", "Missing" should be
#'          represented as integers (when FALSE) or as "Tree-Node" character strings (when FALSE).
#' @param ... currently not used.
#'
#' @return 
#' A \code{data.table} with detailed information about model trees' nodes.
#'
#' The columns of the \code{data.table} are:
#' 
#' \itemize{
#'  \item \code{Tree}: integer ID of a tree in a model (zero-based index)
#'  \item \code{Node}: integer ID of a node in a tree (zero-based index)
#'  \item \code{ID}: character identifier of a node in a model (only when \code{use_int_id=FALSE})
#'  \item \code{Feature}: for a branch node, it's a feature id or name (when available);
#'              for a leaf note, it simply labels it as \code{'Leaf'}
#'  \item \code{Split}: location of the split for a branch node (split condition is always "less than")
#'  \item \code{Yes}: ID of the next node when the split condition is met
#'  \item \code{No}: ID of the next node when the split condition is not met
#'  \item \code{Missing}: ID of the next node when branch value is missing
#'  \item \code{Quality}: either the split gain (change in loss) or the leaf value
#'  \item \code{Cover}: metric related to the number of observation either seen by a split
#'                      or collected by a leaf during training.
#' } 
#' 
#' When \code{use_int_id=FALSE}, columns "Yes", "No", and "Missing" point to model-wide node identifiers
#' in the "ID" column. When \code{use_int_id=TRUE}, those columns point to node identifiers from 
#' the corresponding trees in the "Node" column.
#' 
#' @examples
#' # Basic use:
#' 
#' data(agaricus.train, package='xgboost')
#' 
#' bst <- xgboost(data = agaricus.train$data, label = agaricus.train$label, max_depth = 2, 
#'                eta = 1, nthread = 2, nrounds = 2,objective = "binary:logistic")
#' 
#' (dt <- xgb.model.dt.tree(colnames(agaricus.train$data), bst))
#' 
#' # This bst model already has feature_names stored with it, so those would be used when 
#' # feature_names is not set:
#' (dt <- xgb.model.dt.tree(model = bst))
#' 
#' # How to match feature names of splits that are following a current 'Yes' branch:
#' 
#' merge(dt, dt[, .(ID, Y.Feature=Feature)], by.x='Yes', by.y='ID', all.x=TRUE)[order(Tree,Node)]
#'  
#' @export
xgb.model.dt.tree <- function(feature_names = NULL, model = NULL, text = NULL,
                              trees = NULL, use_int_id = FALSE, ...){
  check.deprecation(...)
  
  if (!inherits(model, "xgb.Booster") && !is.character(text)) {
    stop("Either 'model' must be an object of class xgb.Booster\n",
         "  or 'text' must be a character vector with the result of xgb.dump\n",
         "  (or NULL if 'model' was provided).")
  }
  
  if (is.null(feature_names) && !is.null(model) && !is.null(model$feature_names))
    feature_names <- model$feature_names
  
  if (!(is.null(feature_names) || is.character(feature_names))) {