#' @title LOO-related Indices for Bayesian regressions.
#' @name looic
#'
#' @description Compute LOOIC (leave-one-out cross-validation (LOO) information
#'   criterion) and ELPD (expected log predictive density) for Bayesian
#'   regressions. For LOOIC and ELPD, smaller and larger values are respectively
#'   indicative of a better fit.
#'
#' @param model A Bayesian regression model.
#' @inheritParams model_performance.lm
#'
#' @return A list with four elements, the ELPD, LOOIC and their standard errors.
#'
#' @examplesIf require("rstanarm")
#' \donttest{
#' model <- suppressWarnings(rstanarm::stan_glm(
#'   mpg ~ wt + cyl,
#'   data = mtcars,
#'   chains = 1,
#'   iter = 500,
#'   refresh = 0
#' ))
#' looic(model)
#' }
#' @export
looic <- function(model, verbose = TRUE) {
  insight::check_if_installed("loo")

  algorithm <- insight::find_algorithm(model)

  if (algorithm$algorithm != "sampling") {
    if (verbose) {
      insight::format_warning(
        "`looic()` only available for models fit using the 'sampling' algorithm."
      )
    }
    return(NA)
  }

  res_loo <- tryCatch(
    {
      loo::loo(model)
    },
    error = function(e) {
      if (inherits(e, c("simpleError", "error"))) {
        insight::print_color(e$message, "red")
        cat("\n")
      }
      NULL
    }
  )
  loo_df <- res_loo$estimates

  if (is.null(loo_df)) {
    return(NULL)
  }

  out <- list(
    ELPD = loo_df["elpd_loo", "Estimate"],
    ELPD_SE = loo_df["elpd_loo", "SE"],
    LOOIC = loo_df["looic", "Estimate"],
    LOOIC_SE = loo_df["looic", "SE"]
  )
  attr(out, "loo") <- res_loo[c("pointwise", "diagnostics")]

  # Leave p_loo as I am not sure it is an index of performance

  structure(class = "looic", out)
}



# methods --------------------------

#' @export
as.data.frame.looic <- function(x, row.names = NULL, ...) {
  data.frame(
    ELPD = x$ELPD,
    ELPD_SE = x$ELPD_SE,
    LOOIC = x$LOOIC,
    LOOIC_SE = x$LOOIC_SE,
    stringsAsFactors = FALSE,
    row.names = row.names,
    ...
  )
}


#' @export
print.looic <- function(x, digits = 2, ...) {
  insight::print_color("# LOOIC and ELPD with Standard Error\n\n", "blue")

  out <- paste0(
    c(
      sprintf("  LOOIC: %.*f [%.*f]", digits, x$LOOIC, digits, x$LOOIC_SE),
      sprintf("   ELPD: %.*f [%.*f]", digits, x$ELPD, digits, x$ELPD_SE)
    ),
    collapse = "\n"
  )

  cat(out)
  cat("\n")
  invisible(x)
}
