#' Cross Validation
#'
#' The cross validation function of xgboost
#'
#' @param params the list of parameters. The complete list of parameters is
#'   available in the \href{http://xgboost.readthedocs.io/en/latest/parameter.html}{online documentation}. Below
#'   is a shorter summary:
#' \itemize{
#'   \item \code{objective} objective function, common ones are
#'   \itemize{
#'     \item \code{reg:squarederror} Regression with squared loss.
#'     \item \code{binary:logistic} logistic regression for classification.
#'     \item See \code{\link[=xgb.train]{xgb.train}()} for complete list of objectives.
#'   }
#'   \item \code{eta} step size of each boosting step
#'   \item \code{max_depth} maximum depth of the tree
#'   \item \code{nthread} number of thread used in training, if not set, all threads are used
#' }
#'
#'   See \code{\link{xgb.train}} for further details.
#'   See also demo/ for walkthrough example in R.
#' @param data takes an \code{xgb.DMatrix}, \code{matrix}, or \code{dgCMatrix} as the input.
#' @param nrounds the max number of iterations
#' @param nfold the original dataset is randomly partitioned into \code{nfold} equal size subsamples.
#' @param label vector of response values. Should be provided only when data is an R-matrix.
#' @param missing is only used when input is a dense matrix. By default is set to NA, which means
#'        that NA values should be considered as 'missing' by the algorithm.
#'        Sometimes, 0 or other extreme value might be used to represent missing values.
#' @param prediction A logical value indicating whether to return the test fold predictions
#'        from each CV model. This parameter engages the \code{\link{xgb.cb.cv.predict}} callback.
#' @param showsd \code{boolean}, whether to show standard deviation of cross validation
#' @param metrics, list of evaluation metrics to be used in cross validation,
#'   when it is not specified, the evaluation metric is chosen according to objective function.
#'   Possible options are:
#' \itemize{
#'   \item \code{error} binary classification error rate
#'   \item \code{rmse} Rooted mean square error
#'   \item \code{logloss} negative log-likelihood function
#'   \item \code{mae} Mean absolute error
#'   \item \code{mape} Mean absolute percentage error
#'   \item \code{auc} Area under curve
#'   \item \code{aucpr} Area under PR curve
#'   \item \code{merror} Exact matching error, used to evaluate multi-class classification
#' }
#' @param obj customized objective function. Returns gradient and second order
#'        gradient with given prediction and dtrain.
#' @param feval customized evaluation function. Returns
#'        \code{list(metric='metric-name', value='metric-value')} with given
#'        prediction and dtrain.
#' @param stratified a \code{boolean} indicating whether sampling of folds should be stratified
#'        by the values of outcome labels.
#' @param folds \code{list} provides a possibility to use a list of pre-defined CV folds
#'        (each element must be a vector of test fold's indices). When folds are supplied,
#'        the \code{nfold} and \code{stratified} parameters are ignored.
#' @param train_folds \code{list} list specifying which indicies to use for training. If \code{NULL}
#'        (the default) all indices not specified in \code{folds} will be used for training.
#' @param verbose \code{boolean}, print the statistics during the process
#' @param print_every_n Print each n-th iteration evaluation messages when \code{verbose>0}.
#'        Default is 1 which means all messages are printed. This parameter is passed to the
#'        \code{\link{xgb.cb.print.evaluation}} callback.
#' @param early_stopping_rounds If \code{NULL}, the early stopping function is not triggered.
#'        If set to an integer \code{k}, training with a validation set will stop if the performance
#'        doesn't improve for \code{k} rounds.
#'        Setting this parameter engages the \code{\link{xgb.cb.early.stop}} callback.
#' @param maximize If \code{feval} and \code{early_stopping_rounds} are set,
#'        then this parameter must be set as well.
#'        When it is \code{TRUE}, it means the larger the evaluation score the better.
#'        This parameter is passed to the \code{\link{xgb.cb.early.stop}} callback.
#' @param callbacks a list of callback functions to perform various task during boosting.
#'        See \code{\link{xgb.Callback}}. Some of the callbacks are automatically created depending on the
#'        parameters' values. User can provide either existing or their own callback methods in order
#'        to customize the training process.
#' @param ... other parameters to pass to \code{params}.
#'
#' @details
#' The original sample is randomly partitioned into \code{nfold} equal size subsamples.
#'
#' Of the \code{nfold} subsamples, a single subsample is retained as the validation data for testing the model,
#' and the remaining \code{nfold - 1} subsamples are used as training data.
#'
#' The cross-validation process is then repeated \code{nrounds} times, with each of the
#' \code{nfold} subsamples used exactly once as the validation data.
#'
#' All observations are used for both training and validation.
#'
#' Adapted from \url{https://en.wikipedia.org/wiki/Cross-validation_\%28statistics\%29}
#'
#' @return
#' An object of class \code{xgb.cv.synchronous} with the following elements:
#' \itemize{
#'   \item \code{call} a function call.
#'   \item \code{params} parameters that were passed to the xgboost library. Note that it does not
#'         capture parameters changed by the \code{\link{xgb.cb.reset.parameters}} callback.
#'   \item \code{evaluation_log} evaluation history stored as a \code{data.table} with the
#'         first column corresponding to iteration number and the rest corresponding to the
#'         CV-based evaluation means and standard deviations for the training and test CV-sets.
#'         It is created by the \code{\link{xgb.cb.evaluation.log}} callback.
#'   \item \code{niter} number of boosting iterations.
#'   \item \code{nfeatures} number of features in training data.
#'   \item \code{folds} the list of CV folds' indices - either those passed through the \code{folds}
#'         parameter or randomly generated.
#'   \item \code{best_iteration} iteration number with the best evaluation metric value
#'         (only available with early stopping).
#' }
#'
#' Plus other potential elements that are the result of callbacks, such as a list `cv_predict` with
#' a sub-element `pred` when passing `prediction = TRUE`, which is added by the \link{xgb.cb.cv.predict}
#' callback (note that one can also pass it manually under `callbacks` with different settings,
#' such as saving also the models created during cross validation); or a list `early_stop` which
#' will contain elements such as `best_iteration` when using the early stopping callback (\link{xgb.cb.early.stop}).
#'
#' @examples
#' data(agaricus.train, package='xgboost')
#' dtrain <- with(agaricus.train, xgb.DMatrix(data, label = label, nthread = 2))
#' cv <- xgb.cv(data = dtrain, nrounds = 3, nthread = 2, nfold = 5, metrics = list("rmse","auc"),
#'              max_depth = 3, eta = 1, objective = "binary:logistic")
#' print(cv)
#' print(cv, verbose=TRUE)
#'
#' @export
xgb.cv <- function(params = list(), data, nrounds, nfold, label = NULL, missing = NA,
                   prediction = FALSE, showsd = TRUE, metrics = list(),
                   obj = NULL, feval = NULL, stratified = TRUE, folds = NULL, train_folds = NULL,
                   verbose = TRUE, print_every_n = 1L,
                   early_stopping_rounds = NULL, maximize = NULL, callbacks = list(), ...) {

  check.deprecation(...)
  if (inherits(data, "xgb.DMatrix") && .Call(XGCheckNullPtr_R, data)) {
    stop("'data' is an invalid 'xgb.DMatrix' object. Must be constructed again.")
  }

  params <- check.booster.params(params, ...)
  # TODO: should we deprecate the redundant 'metrics' parameter?
  for (m in metrics)
    params <- c(params, list("eval_metric" = m))

  check.custom.obj()
  check.custom.eval()

  # Check the labels
  if ((inherits(data, 'xgb.DMatrix') && !xgb.DMatrix.hasinfo(data, 'label')) ||
      (!inherits(data, 'xgb.DMatrix') && is.null(label))) {
    stop("Labels must be provided for CV either through xgb.DMatrix, or through 'label=' when 'data' is matrix")
  } else if (inherits(data, 'xgb.DMatrix')) {
    if (!is.null(label))
      warning("xgb.cv: label will be ignored, since data is of type xgb.DMatrix")
    cv_label <- getinfo(data, 'label')
  } else {
    cv_label <- label
  }

  # CV folds
  if (!is.null(folds)) {
    if (!is.list(folds) || length(folds) < 2)
      stop("'folds' must be a list with 2 or more elements that are vectors of indices for each CV-fold")
    nfold <- length(folds)
  } else {
    if (nfold <= 1)
      stop("'nfold' must be > 1")
    folds <- generate.cv.folds(nfold, nrow(data), stratified, cv_label, params)
  }

  # Callbacks
  tmp <- .process.callbacks(callbacks, is_cv = TRUE)
  callbacks <- tmp$callbacks
  cb_names <- tmp$cb_names
  rm(tmp)

  # Early stopping callback
  if (!is.null(early_stopping_rounds) && !("early_stop" %in% cb_names)) {
    callbacks <- add.callback(
      callbacks,
      xgb.cb.early.stop(
        early_stopping_rounds,
        maximize = maximize,
        verbose = verbose
      ),
      as_first_elt = TRUE
    )
  }
  # verbosity & evaluation printing callback:
  params <- c(params, list(silent = 1))
  print_every_n <- max(as.integer(print_every_n), 1L)
  if (verbose && !("print_evaluation" %in% cb_names)) {
    callbacks <- add.callback(callbacks, xgb.cb.print.evaluation(print_every_n, showsd = showsd))
  }
  # evaluation log callback: always is on in CV
  if (!("evaluation_log" %in% cb_names)) {
    callbacks <- add.callback(callbacks, xgb.cb.evaluation.log())
  }
  # CV-predictions callback
  if (prediction && !("cv_predict" %in% cb_names)) {
    callbacks <- add.callback(callbacks, xgb.cb.cv.predict(save_models = FALSE))
  }

  # create the booster-folds
  # train_folds
  dall <- xgb.get.DMatrix(
    data = data,
    label = label,
    missing = missing,
    weight = NULL,
    nthread = params$nthread
  )
  bst_folds <- lapply(seq_along(folds), function(k) {
    dtest  <- xgb.slice.DMatrix(dall, folds[[k]])
    # code originally contributed by @RolandASc on stackoverflow
    if (is.null(train_folds))
       dtrain <- xgb.slice.DMatrix(dall, unlist(folds[-k]))
    else
       dtrain <- xgb.slice.DMatrix(dall, train_folds[[k]])
    bst <- xgb.Booster(
      params = params,
      cachelist = list(dtrain, dtest),
      modelfile = NULL
    )
    bst <- bst$bst
    list(dtrain = dtrain, bst = bst, evals = list(train = dtrain, test = dtest), index = folds[[k]])
  })

  # extract parameters that can affect the relationship b/w #trees and #iterations
  num_class <- max(as.numeric(NVL(params[['num_class']], 1)), 1) # nolint

  # those are fixed for CV (no training continuation)
  begin_iteration <- 1
  end_iteration <- nrounds

  .execute.cb.before.training(
    callbacks,
    bst_folds,
    dall,
    NULL,
    begin_iteration,
    end_iteration
  )

  # synchronous CV boosting: run CV folds' models within each iteration
  for (iteration in begin_iteration:end_iteration) {

    .execute.cb.before.iter(
      callbacks,
      bst_folds,
      dall,
      NULL,
      iteration
    )

    msg <- lapply(bst_folds, function(fd) {
      xgb.iter.update(
        bst = fd$bst,
        dtrain = fd$dtrain,
        iter = iteration - 1,
        obj = obj
      )
      xgb.iter.eval(
        bst = fd$bst,
        evals = fd$evals,
        iter = iteration - 1,
        feval = feval
      )
    })
    msg <- simplify2array(msg)

    should_stop <- .execute.cb.after.iter(
      callbacks,
      bst_folds,
      dall,
      NULL,
      iteration,
      msg
    )

    if (should_stop) break
  }
  cb_outputs <- .execute.cb.after.training(
    callbacks,
    bst_folds,
    dall,
    NULL,
    iteration,
    msg
  )

  # the CV result
  ret <- list(
    call = match.call(),
    params = params,
    niter = iteration,
    nfeatures = ncol(dall),
    folds = folds
  )
  ret <- c(ret, cb_outputs)

  class(ret) <- 'xgb.cv.synchronous'
  return(invisible(ret))
}



#' Print xgb.cv result
#'
#' Prints formatted results of \code{xgb.cv}.
#'
#' @param x an \code{xgb.cv.synchronous} object
#' @param verbose whether to print detailed data
#' @param ... passed to \code{data.table.print}
#'
#' @details
#' When not verbose, it would only print the evaluation results,
#' including the best iteration (when available).
#'
#' @examples
#' data(agaricus.train, package='xgboost')
#' train <- agaricus.train
#' cv <- xgb.cv(data = train$data, label = train$label, nfold = 5, max_depth = 2,
#'                eta = 1, nthread = 2, nrounds = 2, objective = "binary:logistic")
#' print(cv)
#' print(cv, verbose=TRUE)
#'
#' @rdname print.xgb.cv
#' @method print xgb.cv.synchronous
#' @export
print.xgb.cv.synchronous <- function(x, verbose = FALSE, ...) {
  cat('##### xgb.cv ', length(x$folds), '-folds\n', sep = '')

  if (verbose) {
    if (!is.null(x$call)) {
      cat('call:\n  ')
      print(x$call)
    }
    if (!is.null(x$params)) {
      cat('params (as set within xgb.cv):\n')
      cat('  ',
          paste(names(x$params),
                paste0('"', unlist(x$params), '"'),
                sep = ' = ', collapse = ', '), '\n', sep = '')
    }

    for (n in c('niter', 'best_iteration')) {
      if (is.null(x$early_stop[[n]]))
        next
      cat(n, ': ', x$early_stop[[n]], '\n', sep = '')
    }

    if (!is.null(x$cv_predict$pred)) {
      cat('pred:\n')
      str(x$cv_predict$pred)
    }
  }

  if (verbose)
    cat('evaluation_log:\n')
  print(x$evaluation_log, row.names = FALSE, ...)

  if (!is.null(x$early_stop$best_iteration)) {
    cat('Best iteration:\n')
    print(x$evaluation_log[x$early_stop$best_iteration], row.names = FALSE, ...)
  }
  invisible(x)
}