transforming & adding new column in r

I have currently have a data frame that is taken from a data feed of events that happened in chronological order. I would like to add a new column onto to each row of my data the corresponds to the previous event's endx if the prior event type is 1 & the previous event's x if the prior event type is not 1

e.g

player_id <- c(12, 17, 26, 3)
event_type <- c(1, 3, 1, 10)
x <- c(65, 34, 43, 72)
endx <- c(68, NA, 47, NA)
df <- data.frame(player_id, event_type, outcome, x, endx)
df
     player_id  event_type  x  endx
1        12          1      65   68
2        17          3      34   NA
3        26          1      43   47
4         3         10      72   NA

so end result

  player_id event_type   x     endx  previous
1        12          1   65     68     NA
2        17          3   34     NA     68
3        26          1   43     47     34 
4         3         10   72     NA     47