How to find the closest value from above the current row that meets a certain criteria?

Question

I need to find the id from the closest preceding row that has level equal to one less than the current row's level. Sample data provided below.

id <- c(42,53,82,17,33,92,6,12,66)
level <- c(2,3,3,3,3,4,3,3,4)
df <- tibble(id,level)

     id level
1    42     2
2    53     3
3    82     3
4    17     3
5    33     3
6    92     4
7     6     3
8    12     3
9    66     4

The expected results are as follows:

     id level results
1    42     2      NA
2    53     3      42
3    82     3      42
4    17     3      42
5    33     3      42
6    92     4      33
7     6     3      42
8    12     3      42
9    66     4      12

I was looking at: Find closest value above current row that meets condition and apply function for each group.

I could not edit the answers from the above question to get the desired result. Specifically I am having difficulty referencing the level from the current row to compare to preceding rows.

Answer 1

A solution using a rolling join:

library(dplyr)

df <- df |> mutate(rn = row_number())

df |> 
  left_join(df, by = join_by(closest(level > level), closest(rn > rn))) |> 
  select("id" = id.x, "level" = level.x, "results" = id.y)

# A tibble: 9 × 3
     id level results
  <dbl> <dbl>   <dbl>
1    42     2      NA
2    53     3      42
3    82     3      42
4    17     3      42
5    33     3      42
6    92     4      33
7     6     3      42
8    12     3      42
9    66     4      12