Exclude message for flat concentrations in half.life calculation (#503)

R/half.life.R

@@ -180,6 +180,11 @@ pk.calc.half.life <- function(conc, time, tmax, tlast,
     # as.numeric is for units handling
     dfK <- data[as.numeric(data$time) > as.numeric(ret$tmax), ]
   }
+  # When all concentrations are the same (non-zero) value cannot compute half-life (#503)
+  if (sd(data$log_conc, na.rm = TRUE) == 0) {
+    attr(ret, "exclude") <- "No point variability in concentrations for half-life calculation"
+    return(ret)
+  }
   if (manually.selected.points) {
     if (nrow(data) > 0) {
       fit <- fit_half_life(data=data, tlast=ret$tlast, conc_units=conc_units)
@@ -266,6 +271,7 @@ pk.calc.half.life <- function(conc, time, tmax, tlast,
       class = "pknca_halflife_too_few_points"
     )
   }
+
   # Drop the inputs of tmax and tlast, if given.
   if (!missing(tmax))
     ret$tmax <- NULL

tests/testthat/test-half.life.R

@@ -506,3 +506,20 @@ test_that("get_halflife_points method (1: PKNCAdata, 2: PKNCAresults)", {
     info = "get_halflife_points uses lambda.z.time.last, not tlast"
   )
 })
+
+test_that("half-life has a exclude message when it cannot be calculated for flat data (#503)", {
+  result <- suppressMessages(
+    pk.calc.half.life(
+      conc = c(1, 1, 1, 1),
+      time = c(0, 1, 2, 3),
+      min.hl.points = 3,
+      allow.tmax.in.half.life = TRUE,
+      adj.r.squared.factor = 0.0001,
+      check = FALSE
+    )
+  )
+  expect_equal(
+    attr(result, "exclude"),
+    "No point variability in concentrations for half-life calculation"
+  )
+})