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# Source: https://github.com/rstudio/shiny/blob/master/R/stack.R
# License: GPL-3
# Relicensed a MIT with permission.
# A Stack object backed by a list. The backing list will grow or shrink as
# the stack changes in size.
Stack <- R6Class(
"Stack",
class = FALSE,
public = list(
initialize = function(init = 20L) {
# init is the initial size of the list. It is also used as the minimum
# size of the list as it shrinks.
private$stack <- vector("list", init)
private$init <- init
private$count <- 0L
},
push = function(..., .list = NULL) {
args <- c(list(...), .list)
new_size <- private$count + length(args)
# Grow if needed; double in size
while (new_size > length(private$stack)) {
private$stack[length(private$stack) * 2L] <- list(NULL)
}
private$stack[private$count + seq_along(args)] <- args
private$count <- new_size
invisible(self)
},
size = function() {
private$count
},
# Return the entire stack as a list, where the first item in the list is the
# oldest item in the stack, and the last item is the most recently added.
as_list = function() {
private$stack[seq_len(private$count)]
}
),
private = list(
stack = NULL, # A list that holds the items
count = 0L, # Current number of items in the stack
init = 20L # Initial and minimum size of the stack
)
)
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