Flag run-length of grouped intervals

I have a dataframe grouped by grp:

df <- data.frame(
  v = rnorm(25),
  grp = c(rep("A",10), rep("B",15)),
  size = 2)

I want to flag the run-length of intervals determined by size. For example, for grp == "A", size is 2, and the number of rows is 10. So the interval should have length 10/2 = 5. This code, however, creates intervals with length 2:

df %>%
  group_by(grp) %>%
  mutate(
    interval = (row_number() -1) %/% size)
# A tibble: 25 × 4
# Groups:   grp [2]
        v grp    size interval
    <dbl> <chr> <dbl>    <dbl>
 1 -0.166 A         2        0
 2 -1.12  A         2        0
 3  0.941 A         2        1
 4 -0.913 A         2        1
 5  0.486 A         2        2
 6 -1.80  A         2        2
 7 -0.370 A         2        3
 8 -0.209 A         2        3
 9 -0.661 A         2        4
10 -0.177 A         2        4
# … with 15 more rows

How can I flag the correct run-length of the size-determined intervals? The desired output is this:

# A tibble: 25 × 4
# Groups:   grp [2]
        v grp    size interval
    <dbl> <chr> <dbl>    <dbl>
 1 -0.166 A         2        0
 2 -1.12  A         2        0
 3  0.941 A         2        0
 4 -0.913 A         2        0
 5  0.486 A         2        0
 6 -1.80  A         2        1
 7 -0.370 A         2        1
 8 -0.209 A         2        1
 9 -0.661 A         2        1
10 -0.177 A         2        1
# … with 15 more rows

>Solution :

If I interpreted your question correctly, this small change should do the trick?

df %>%
  group_by(grp) %>%
  mutate(
    interval = (row_number() -1) %/% (n()/size))