R: Define starting condition for continous value - r

Question

I´m trying to set up two new variables to incorporate into an existing data.frame which should be a running value starting at 1 (0) if a condition is met with respect to the IDs in the data.frame. So the data.frame is of similar structure to this:
ID Var1
1 0
1 2
1 5
1 12
2 0
2 2
2 NA
2 11
and I want to get to:
ID Var1 start stop
1 0 0 0
1 2 0 1
1 5 1 2
1 12 2 3
2 0 0 0
2 2 0 1
2 NA 1 2
2 11 2 3
Start should be a running value, starting once Var1 > 0 for the first time and stop should operate the same way. Start´s starting value should be 0 and stop´s starting value should be 1. It should further continue running, if Var1 takes on NA or 0 again in the course of the data.frame. I have tried doing the following:
df %>%
group_by(ID) %>%
mutate(stop = ifelse(Var1 > 0,
0:nrow(df), 0))
But the variable it returns doesn´t start with 0, but with the number of the row the condition is first met in.

Answer 1

Sorry, I don't speak dplyr but you can easily adapt this, since data.table is only used for group-by.
DF <- read.table(text = "ID Var1
1 0
1 2
1 5
1 12
2 0
2 2
2 NA
2 11", header = TRUE)
foo <- function(x) {
#quantify leading zeros:
x[is.na(x)] <- 0
lead0 <- cumsum(x > 0)
nlead0 <- sum(lead0 == 0)
#create result using sequence:
list(c(rep.int(0, nlead0), sequence(length(x) - nlead0) - 1),
c(rep.int(0, nlead0), sequence(length(x) - nlead0)))
}
library(data.table)
setDT(DF)
DF[, c("start", "stop") := foo(Var1), by = ID]
# ID Var1 start stop
#1: 1 0 0 0
#2: 1 2 0 1
#3: 1 5 1 2
#4: 1 12 2 3
#5: 2 0 0 0
#6: 2 2 0 1
#7: 2 NA 1 2
#8: 2 11 2 3

Answer 2

Here is base R option using ave + replace
transform(df,
Start = ave(ave(replace(Var1, is.na(Var1), 0) > 0, ID, FUN = cumsum) > 0, ID, FUN = function(x) cumsum(c(0, x))[-(length(x) + 1)]),
Stop = ave(ave(replace(Var1, is.na(Var1), 0) > 0, ID, FUN = cumsum) > 0, ID, FUN = cumsum)
)
or
transform(df,
Start = ave(ave(ave(replace(Var1, is.na(Var1), 0) > 0, ID, FUN = cumsum), ID, FUN = cumsum) > 1, ID, FUN = cumsum),
Stop = ave(ave(replace(Var1, is.na(Var1), 0) > 0, ID, FUN = cumsum) > 0, ID, FUN = cumsum)
)
which gives
ID Var1 Start Stop
1 1 0 0 0
2 1 2 0 1
3 1 5 1 2
4 1 12 2 3
5 2 0 0 0
6 2 2 0 1
7 2 NA 1 2
8 2 11 2 3