fst distance to leader

R/distance_to_leader.R

@@ -0,0 +1,35 @@
+#' Calculate distance to leader
+#'
+#' Leader as identified using group_leader by first position along
+#' group axis of movement (return_rank = TRUE)
+#'
+#' @param DT
+#' @param coords character vector of column names for x, y
+#' @param group group column name generated by eg. group_pts
+distance_to_leader <- function(DT, coords = c('x', 'y'), group = 'group') {
+  stopifnot(first(coords) %in% colnames(DT))
+  stopifnot(last(coords) %in% colnames(DT))
+  stopifnot(group %in% colnames(DT))
+  stopifnot('rank_dist_along_group_bearing' %in% colnames(DT))
+
+  DT[, temp_N_by_group := .N, by = c(group)]
+
+  check_has_leader <- DT[, .(has_leader = any(rank_dist_along_group_bearing == 1)),
+                         by = c(group)][!(has_leader)]
+
+  if (check_has_leader[, .N > 0]) {
+    warning('groups found missing leader (rank_dist_along_group_bearing == 1): \n',
+            check_has_leader[, paste(group, collapse = ', ')])
+  }
+
+  DT[!group %in% check_has_leader$group,
+     dist_leader := fifelse(
+       temp_N_by_group > 1,
+       as.matrix(dist(cbind(.SD[[1]], .SD[[2]])))[, which(.SD[[3]] == 1)],
+       0
+     ),
+     .SDcols = c(coords, 'rank_dist_along_group_bearing'),
+     by = c(group)]
+  DT[, temp_N_by_group := NULL]
+  return(DT)
+}