#' @title Seasonal naive model
#'
#' @description Train a seasonal naive model (SNAIVE2).
#'
#' @param .data Input data as tsibble.
#' @param specials Specials as list defined in \code{specials_snaive2}.
#' @param ... Currently not in use.
#'
#' @return An object of class \code{SNAIVE2}.
train_snaive2 <- function(.data,
specials,
...){
if (length(measured_vars(.data)) > 1) {
abort("Only univariate responses are supported by SNAIVE2.")
}
periods <- common_periods(.data)
lag_day <- periods["day"]
lag_week <- periods["week"]
model_fit <- .data %>%
mutate(day_of_week = wday(
x = !!sym(index_var(.data)),
week_start = getOption("lubridate.week.start", 1))) %>%
mutate(fitted = ifelse(
.data$day_of_week %in% c(2, 3, 4, 5),
dplyr::lag(!!sym(measured_vars(.data)), n = lag_day),
dplyr::lag(!!sym(measured_vars(.data)), n = lag_week))) %>%
mutate(resid = !!sym(measured_vars(.data)) - fitted)
fitted <- model_fit[["fitted"]]
resid <- model_fit[["resid"]]
sigma <- sd(resid, na.rm = TRUE)
model_spec <- list(
data = .data,
lag_day = lag_day,
lag_week = lag_week
)
structure(
list(
model = model_spec,
fitted = fitted,
resid = resid,
sigma = sigma),
class = "SNAIVE2"
)
}
#' @title Forecast a trained seasonal naive model
#'
#' @description Forecast a trained seasonal naive model.
#'
#' @param object An object of class \code{SNAIVE2}.
#' @param new_data Forecast horizon (n-step ahead forecast)
#' @param specials Specials are currently not in use.
#' @param ... Additional arguments for forecast method.
#'
#' @return An object of class \code{fable}.
#' @export
forecast.SNAIVE2 <- function(object,
new_data,
specials = NULL,
...){
# Forecast horizon
n_ahead <- nrow(new_data)
# Extract model
data <- object$model$data
lag_day <- object$model$lag_day
lag_week <- object$model$lag_week
# Seasonal naive forecast
fcst <- data %>%
append_row(n = n_ahead) %>%
mutate(day_of_week = wday(
x = !!sym(index_var(data)),
week_start = getOption("lubridate.week.start", 1))) %>%
mutate(point = ifelse(
.data$day_of_week %in% c(2, 3, 4, 5),
dplyr::lag(!!sym(measured_vars(data)), n = lag_day),
dplyr::lag(!!sym(measured_vars(data)), n = lag_week))) %>%
slice_tail(n = n_ahead)
point <- as.numeric(fcst[["point"]])
sd <- as.numeric(rep(object$sigma, times = n_ahead))
# Return forecasts
dist_normal(point, sd)
}
specials_snaive2 <- new_specials()
#' @title Seasonal naive model
#'
#' @description Automatic train a seasonal naive model (SNAIVE2)..
#'
#' @param formula Model specification (see "Specials" section, currently not in use ...).
#' @param ... Further arguments.
#'
#' @return snaive2_model An object of class \code{SNAIVE2}.
#' @export
SNAIVE2 <- function(formula, ...){
snaive2_model <- new_model_class(
model = "SNAIVE2",
train = train_snaive2,
specials = specials_snaive2)
new_model_definition(
snaive2_model,
!!enquo(formula),
...)
}
#' @title Extract fitted values from a trained seasonal naive model
#'
#' @description Extract fitted values from a trained seasonal naive model.
#'
#' @param object An object of class \code{SNAIVE2}.
#' @param ... Currently not in use.
#'
#' @return Fitted values as tsibble.
#' @export
fitted.SNAIVE2 <- function(object, ...){
object[["fitted"]]
}
#' @title Extract residuals from a trained seasonal naive model
#'
#' @description Extract residuals from a trained seasonal naive model.
#'
#' @param object An object of class \code{SNAIVE2}.
#' @param ... Currently not in use.
#'
#' @return Fitted values as tsibble.
#' @export
residuals.SNAIVE2 <- function(object, ...){
object[["resid"]]
}
#' @title Provide a succinct summary of a trained seasonal naive model
#'
#' @description Provide a succinct summary of a trained seasonal naive model.
#'
#' @param object An object of class \code{SNAIVE2}.
#'
#' @return Model summary as character value.
#' @export
model_sum.SNAIVE2 <- function(object){
"SNAIVE2"
}
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