R - Conditional lagging - How to lag a certain amo

Been trying to solve this for weeks, but can't seem to get it.

I have the following data frame:

    post_id user_id
1    post-1   user1
2    post-2   user2
3 comment-1   user1
4 comment-2   user3
5 comment-3   user4
6    post-3   user2
7 comment-4   user2

And want to create a new variable parent_id. So that for each observation it should perform the following steps:

Check if post_id is either post or comment
If post_id is post then parent_id should equal the earliest post_id of the whole data frame.
If post_id is the first post then parent_id should equal NA
If post_id is comment then parent_id should equal to the first post_id it encounters.

The output should look something like:

    post_id user_id parent_id_man
1    post-1   user1            NA
2    post-2   user2        post-1
3 comment-1   user1        post-2
4 comment-2   user3        post-2
5 comment-3   user4        post-2
6    post-3   user2        post-1
7 comment-4   user2        post-3

I have tried the following:

#Prepare data
df <- df %>% separate(post_id, into=c("type","number"), sep="-", remove=FALSE)
df$number <- as.numeric(df$number)
df <- df %>% mutate(comment_number = ifelse(type == "comment",number,99999))
df <- df %>% mutate(post_number = ifelse(type == "post",number,99999))

#Create parent_id column
df <- df %>% mutate(parent_id = ifelse(type == "post",paste("post-",min(post_number), sep=""),0))
df <- df %>% mutate(parent_id = ifelse(parent_id == post_id,"NA",parent_id))
df <- df %>% select(-comment_number, -post_number)

With that code I am able to perform Steps 1, 2 and 3, but step 4 is beyond me. I get the feeling that a certain type of conditional lagging based should be able to solve it, but can't come up with how to do it.

Any ideas would be very much appreciated!

Building on your solution,

x <- which(df$type == 'post')
z <- which(df$type == 'comment')
df$parent_id[df$parent_id == 0] <- df$post_id[x[sapply(z, function(i) findInterval(i, x))]]
df$parent_id
#[1] "NA"     "post-1" "post-2" "post-2" "post-2" "post-1" "post-3"