dplyr / R cumulative sum with reset

Question

I'd like to generate cumulative sums with a reset if the "current" sum exceeds some threshold, using dplyr. In the below, I want to cumsum over 'a'.

library(dplyr)
library(tibble)

tib <- tibble(
  t = c(1,2,3,4,5,6),
  a = c(2,3,1,2,2,3)
)

# what I want
## thresh = 5
# A tibble: 6 x 4
#         t     a     g     c
#      <dbl> <dbl> <int> <dbl>
#   1  1.00  2.00     0  2.00
#   2  2.00  3.00     0  5.00
#   3  3.00  1.00     1  1.00
#   4  4.00  2.00     1  3.00
#   5  5.00  2.00     1  5.00
#   6  6.00  3.00     2  3.00

# what I want
## thresh = 4
# A tibble: 6 x 4
#         t     a     g     c
#      <dbl> <dbl> <int> <dbl>
#   1  1.00  2.00     0  2.00
#   2  2.00  3.00     0  5.00
#   3  3.00  1.00     1  1.00
#   4  4.00  2.00     1  3.00
#   5  5.00  2.00     1  5.00
#   6  6.00  3.00     2  3.00

# what I want
## thresh = 6
# A tibble: 6 x 4
#         t     a     g     c
#      <dbl> <dbl> <int> <dbl>
#   1  1.00  2.00     0  2.00
#   2  2.00  3.00     0  5.00
#   3  3.00  1.00     0  6.00
#   4  4.00  2.00     1  2.00
#   5  5.00  2.00     1  4.00
#   6  6.00  3.00     1  7.00

I've examined many of the similar questions here (such as resetting cumsum if value goes to negative in r) and have gotten what I hoped was close, but no.

I've tried variants of

thresh <-5
tib %>%
  group_by(g = cumsum(lag(cumsum(a) >= thresh, default = FALSE))) %>%
  mutate(c = cumsum(a)) %>%
  ungroup()

which returns

# A tibble: 6 x 4
      t     a     g     c
  <dbl> <dbl> <int> <dbl>
1  1.00  2.00     0  2.00
2  2.00  3.00     0  5.00
3  3.00  1.00     1  1.00
4  4.00  2.00     2  2.00
5  5.00  2.00     3  2.00
6  6.00  3.00     4  3.00

You can see that the "group" is not getting reset after the first time.

Answer 1

I think you can use accumulate() here to help. And i've also made a wrapper function to use for different thresholds

sum_reset_at <- function(thresh) {
  function(x) {
    accumulate(x, ~if_else(.x>=thresh, .y, .x+.y))
  }  
}

tib %>% mutate(c = sum_reset_at(5)(a))
#       t     a     c
#   <dbl> <dbl> <dbl>
# 1     1     2     2
# 2     2     3     5
# 3     3     1     1
# 4     4     2     3
# 5     5     2     5
# 6     6     3     3
tib %>% mutate(c = sum_reset_at(4)(a))
#       t     a     c
#   <dbl> <dbl> <dbl>
# 1     1     2     2
# 2     2     3     5
# 3     3     1     1
# 4     4     2     3
# 5     5     2     5
# 6     6     3     3
tib %>% mutate(c = sum_reset_at(6)(a))
#       t     a     c
#   <dbl> <dbl> <dbl>
# 1     1     2     2
# 2     2     3     5
# 3     3     1     6
# 4     4     2     2
# 5     5     2     4
# 6     6     3     7