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+#' Set non-detects in residue series without replicates
+
+#' Sets non-detects directly before or directly after detects to NA. Values between
+#' lod and loq are set to their mean value if an loq is specified.
+#' If 'time_zero' is set to TRUE, the residue series is assumed to start with time
+#' zero, and non-detects at time zero are set to 'time_zero_nd_value'. For the
+#' set_nd_focus variant, this is zero, otherwise this argument has NA as default
+#' value.
+#' If stopping after the first non-detection is requested, as in in the FOCUS
+#' variant of the function, an loq has to be specified in order to decide
+#' if any later detections are above the loq.
+
+#' @param r A character vector of sequential residues without replicates, with
+#' non-detects specified as 'nd' and unquantified values above the limit of
+#' detection specified as 'nq', otherwise coercible to numeric
+#' @param lod Limit of detection (numeric)
+#' @param loq Limit of quantification(numeric). Must be specified if the FOCUS rule to
+#' stop after the first non-detection is to be applied
+#' @param time_zero Is the first value in the series a time zero value?
+#' @param time_zero_nd_value Which value should we use for non-detects at time zero?
+#' @param stop_after_first_nondetect Should we really stop after the first non-detection?
+#' @references FOCUS (2014) p. 75, 76, 131, 132
+#' @export
+#' @examples
+#' parent_1 <- c(.12, .09, .05, .03, "nd", "nd", "nd", "nd", "nd", "nd")
+#' set_nd(parent_1, 0.02)
+#' parent_2 <- c(.12, .09, .05, .03, "nd", "nd", .03, "nd", "nd", "nd")
+#' set_nd(parent_2, 0.02)
+#' set_nd_focus(parent_2, 0.02, loq = 0.05)
+#' parent_3 <- c(.12, .09, .05, .03, "nd", "nd", .06, "nd", "nd", "nd")
+#' set_nd(parent_3, 0.02)
+#' set_nd_focus(parent_3, 0.02, loq = 0.05)
+#' metabolite <- c("nd", "nd", "nd", 0.03, 0.06, 0.10, 0.11, 0.10, 0.09, 0.05, 0.03, "nd", "nd")
+#' set_nd(metabolite, 0.02)
+set_nd <- function(r, lod, loq = NA,
+  time_zero = TRUE, time_zero_nd_value = NA, stop_after_first_nondetect = FALSE)
+{
+
+  if (stop_after_first_nondetect & is.na(loq)) {
+    stop("You need to specify an loq to decide if the curve should be cut off after the first non-detect")
+  }
+
+  result <- r
+
+  # Handle nq values
+  if (!missing(loq)) {
+    nq = 0.5 * (lod + loq)
+    result[r == "nq"] <- nq
+  } else {
+    if (any(r == "nq")) stop("You need to specify lod and loq")
+  }
+
+  #  Handle nd values
+  if (time_zero) {
+    if (r[1] == "nd") {
+      residues_present = FALSE
+      result[1] <- time_zero_nd_value
+    } else {
+      residues_present = TRUE
+    }
+    start_i <- 2
+  } else {
+    residues_present <- if (r[1] == "nd") FALSE else TRUE
+    start_i <- 1
+  }
+
+  for (i in start_i:length(r)) {
+
+    # residues_in_next
+    if (i < length(r)) {
+      next_value <- r[i + 1]
+      if (next_value == "nd") residues_in_next = FALSE
+      else residues_in_next = TRUE
+    } else {
+      residues_in_next = FALSE
+    }
+
+    if (r[i] == "nd") {
+      if (residues_present | residues_in_next) {
+        result[i] <- 0.5 * lod
+      } else {
+        result[i] <- NA
+      }
+
+      if (stop_after_first_nondetect) {
+        if (residues_present & !residues_in_next) {
+          remaining <- (i + 1):length(r)
+          if (!any(suppressWarnings(as.numeric(r[remaining])) > loq, na.rm = TRUE)) {
+            result[remaining] <- NA
+            return(as.numeric(result))
+          }
+        }
+
+      }
+      if (!residues_in_next) residues_present <- FALSE
+      else residues_present <- TRUE
+    }
+
+  }
+  return(as.numeric(result))
+}
+
+#' @describeIn set_nd Set non-detects in residues series according to FOCUS rules
+#' @export
+set_nd_focus <- function(r, lod, loq = NA, time_zero = TRUE) {
+  result <- set_nd(r, lod, loq = loq, time_zero = time_zero,
+    time_zero_nd_value = 0, stop_after_first_nondetect = TRUE)
+  return(result)
+}