get_kaplan_meier.r

## wrapper function to get an overall kaplan-meier estimate
get_kaplan_meier <- function(time, status, group=NULL, data, conf_int=FALSE,
                             conf_type="plain", conf_level=0.95,
                             cif=FALSE, fixed_t=NULL, ...) {
  # get formula
  if (is.null(group)) {
    form <- stats::as.formula(paste0("survival::Surv(", time, ", ",
                                     status, ") ~ 1"))
  } else {
    form <- stats::as.formula(paste0("survival::Surv(", time, ", ",
                                     status, ") ~ ", group))
  }

  # estimate kaplan-meier
  survf <- survival::survfit(formula=form, data=data, se.fit=conf_int,
                             conf.int=conf_level, conf.type=conf_type, ...)

  km_dat <- data.frame(time=survf$time,
                       est=survf$surv)

  # add group variable, if specified
  if (!is.null(group)) {
    km_group <- c()
    for (strat in names(survf$strata)) {
      km_group <- c(km_group, rep(strat, survf$strata[strat]))
    }
    km_group <- gsub(paste0(group, "="), "", km_group)
    km_dat$group <- factor(km_group, levels=levels(data[, group]))
  }

  # add confidence interval
  if (conf_int) {
    km_dat$ci_lower <- survf$lower
    km_dat$ci_upper <- survf$upper
  }

  # add rows at t = 0
  if (is.null(group) & conf_int) {
    row_0 <- data.frame(time=0, est=1, ci_lower=1, ci_upper=1)
  } else if (is.null(group)) {
    row_0 <- data.frame(time=0, est=1)
  } else if (!is.null(group) & conf_int) {
    row_0 <- data.frame(time=0, est=1, group=levels(data[, group]),
                        ci_lower=1, ci_upper=1)
  } else {
    row_0 <- data.frame(time=0, est=1, group=levels(data[, group]))
  }
  km_dat <- rbind(row_0, km_dat)

  # read at specific points in time
  # NOTE: this ignores confidence intervals and se, because those are never
  #       used when using fixed_t
  if (!is.null(fixed_t)) {
    km_dat <- km_at_t(data=km_dat, times=fixed_t, group=group)
  }

  # turn to cif
  if (cif) {
    km_dat$est <- 1 - km_dat$est
    if ("ci_lower" %in% colnames(km_dat)) {
      km_dat$ci_lower <- 1 - km_dat$ci_lower
      km_dat$ci_upper <- 1 - km_dat$ci_upper
    }
  }

  return(km_dat)
}

## function to get kaplan-meier estimates at t using interpolation
km_at_t <- function(data, times, group) {

  d_ref <- data[, c("time", "est")]

  if (is.null(group)) {
    d_ref <- d_ref[order(d_ref$time), ]
    d_ref <- data.frame(time=times,
                        est=vapply(X=times,
                                   FUN=read_from_step_function,
                                   FUN.VALUE=numeric(1),
                                   data=d_ref,
                                   est="est"))
  } else {
    levs <- unique(data$group)
    out <- vector(mode="list", length=length(levs))
    for (i in seq_len(length(levs))) {
      d_temp <- d_ref[data$group==levs[i], ]
      d_temp <- d_temp[order(d_temp$time), ]
      d_temp <- data.frame(time=times,
                           est=vapply(X=times,
                                      FUN=read_from_step_function,
                                      FUN.VALUE=numeric(1),
                                      data=d_temp,
                                      est="est"))
      d_temp$group <- levs[i]
      out[[i]] <- d_temp
    }
    d_ref <- dplyr::bind_rows(out)
  }

  return(d_ref)
}