Divide data in to chunks with multiple values in each chunk in R - r

Question

I have a dataframe with observations from three years time, with column df$week that indicates the week of the observation. (The week count of the second year continues from the count of the first, so the data contains 207 weeks).
I would like to divide the data to longer time periods, to df$period that would include all observations from several weeks' time.
If a period would be the length of three weeks, and I the data would include 13 observations in six weeks time, the I idea would be to divide
weeks <- c(1, 1, 1, 2, 2, 3, 3, 4, 5, 5, 6, 6, 6)
into
periods <- c(1, 1, 1, 2, 2, 3, 3), c(4, 5, 5, 6, 6, 6)
periods
[1]
1 1 1 2 2 3 3
[2]
4 5 5 6 6 6
To look something like
> df
week period
1 1 1
2 1 1
3 1 1
4 2 1
5 2 1
6 3 1
7 3 1
8 4 2
9 5 2
10 5 2
11 6 2
12 6 2
13 6 2
>
The data contains +13k rows so would need to do some sort of map in style of
mapPeriod <- function(df, fun) {
out <- vector("vector_of_weeks", length(df))
for (i in seq_along(df)) {
out[i] <- fun(df[[i]])
}
out
}
I just don't know what to include in the fun to divide the weeks to the decided sequences of periods. Can function rep be of assistance here? How?
I would be very grateful for all input and suggestions.

Answer 1

split(weeks, f = (weeks - 1) %/% 3)
$`0`
[1] 1 1 1 2 2 3 3
$`1`
[1] 4 5 5 6 6 6
from comments below
weeks <- c(1, 1, 1, 2, 2, 3, 3, 4, 5, 5, 6, 6, 6)
df <- data.frame(weeks)
library(data.table)
df$period <- data.table::rleid((weeks - 1) %/% 3)
# weeks period
# 1 1 1
# 2 1 1
# 3 1 1
# 4 2 1
# 5 2 1
# 6 3 1
# 7 3 1
# 8 4 2
# 9 5 2
# 10 5 2
# 11 6 2
# 12 6 2
# 13 6 2