tidyr::fill() with sequential integers rather than a repeated value - r

Question

After grouping by id I wish to replace the NAs in dist_from_top with sequential values such that dist_from_top becomes c(5,4,3,2,1,5,4,3,2). I am using the one dist_from_top value within each id grouping as a seed of sorts to fill in the values of dist_from_top that are above and below.
tidyr::fill() can fill in the same value throughout the grouping, but I can't think of a way to make it increase and decrease by 1 as it fills. Any help is greatly appreciated.
library(dplyr)
library(tidyr)
df <-
tribble(
~id, ~mgr, ~dist_from_top,
"A", "B", NA,
"A", "C", NA,
"A", "D", 3,
"A", "E", NA,
"A", "F", NA,
"B", "C", NA,
"B", "D", 4,
"B", "E", NA,
"B", "F", NA
)
An "almost there" solution using fill()
df %>%
group_by(id) %>%
fill(dist_from_top, .direction = "up") %>%
fill(dist_from_top, .direction = "down")

Answer 1

Create a column that counts downwards in each group, from any starting point:
... %>% mutate(rn = -row_number())
Add the offset that is defined by the difference between dist_from_top and rn for the one row where dist_from_top is not NA:
... %>% mutate(dist_from_top = rn + max(dist_from_top - rn, na.rm = TRUE))
This uses max() merely to pick one value, assuming there is only one value that isn't NA.
Both mutate() operations operate on groups:
df %>%
group_by(id) %>%
mutate(rn = ...) %>%
mutate(dist_from_top = ...) %>%
ungroup() %>%
select(-rn)
If there is an all-NA group, you'll see a warning.