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#' Create a stack
#'
#' A `faststack` is backed by a list. The backing list will grow or shrink as
#' the stack changes in size.
#'
#' `faststack` objects have the following methods:
#'
#' \describe{
#' \item{\code{push(x)}}{
#' Push an object onto the stack.
#' }
#' \item{\code{mpush(..., .list = NULL)}}{
#' Push objects onto the stack. `.list` can be a list of objects to add.
#' }
#' \item{\code{pop(missing = missing_default)}}{
#' Remove and return the top object on the stack. If the stack is empty,
#' it will return `missing`, which defaults to the value of
#' `missing_default` that `stack()` was created with (typically, `NULL`).
#' }
#' \item{\code{mpop(n, missing = missing_default)}}{
#' Remove and return the top `n` objects on the stack, in a list. The first
#' element of the list is the top object in the stack. If `n` is greater
#' than the number of objects in the stack, any requested items beyond
#' those in the stack will be replaced with `missing` (typically, `NULL`).
#' }
#' \item{\code{peek(missing = missing_default)}}{
#' Return the top object on the stack, but do not remove it from the stack.
#' If the stack is empty, this will return `missing`.
#' }
#' \item{\code{reset()}}{
#' Reset the stack, clearing all items.
#' }
#' \item{\code{size()}}{
#' Returns the number of items in the stack.
#' }
#' \item{\code{as_list()}}{
#' Return a list containing the objects in the stack, where the first
#' element in the list is the object at the bottom of the stack, and the
#' last element in the list is the object at the top of the stack.
#' }
#' }
#'
#'
#' @param init Initial size of the list that backs the stack. This is also used
#' as the minimum size of the list; it will not shrink any smaller.
#' @param missing_default The value to return when `pop()` or `peek()` are
#' called when the stack is empty. Default is `NULL`.
#' @export
faststack <- function(init = 20, missing_default = NULL) {
force(missing_default)
# A list that represents the stack
s <- vector("list", init)
# Current size of the stack
count <- 0L
push <- function(x) {
new_size <- count + 1L
# R 3.4.0 and up will automatically grow vectors in place, if possible, so
# we don't need to explicitly grow the list here.
if (is.null(x)) {
# Special case for NULL (in the normal case, we'll avoid creating a new
# list() and then unwrapping it.)
s[new_size] <<- list(NULL)
} else {
s[[new_size]] <<- x
}
count <<- new_size
invisible()
}
mpush <- function(..., .list = NULL) {
if (is.null(.list)) {
# Fast path for common case
args <- list(...)
} else {
args <- c(list(...), .list)
}
if (length(args) == 0) {
stop("`mpush`: No items provided to push on stack.")
}
new_size <- count + length(args)
# R 3.4.0 and up will automatically grow vectors in place, if possible, so
# we don't need to explicitly grow the list here.
s[count + seq_along(args)] <<- args
count <<- new_size
invisible()
}
pop <- function(missing = missing_default) {
if (count == 0L) {
return(missing)
}
value <- s[[count]]
s[count] <<- list(NULL)
count <<- count - 1L
# Shrink list if < 1/2 of the list is used, down to a minimum size of `init`
len <- length(s)
if (len > init && count < len/2) {
new_len <- max(init, count)
s[seq.int(new_len + 1L, len)] <<- list(NULL)
}
value
}
mpop <- function(n, missing = missing_default) {
n <- as.integer(n)
if (n < 1) {
stop("`n` must be at least 1.")
}
if (n > count) {
n_pop <- count
n_extra <- n - count
} else {
n_pop <- n
n_extra <- 0L
}
idx <- seq.int(count, count - n_pop + 1L)
if (n_extra != 0) {
values <- vector("list", n)
values[seq_len(n_pop)] <- s[idx]
if (!is.null(missing)) {
values[seq.int(n_pop + 1, n)] <- missing
}
} else {
values <- s[idx]
}
s[idx] <<- list(NULL)
count <<- count - n_pop
# Shrink list if < 1/2 of the list is used, down to a minimum size of `init`
len <- length(s)
if (len > init && count < len/2) {
new_len <- max(init, count)
# Assign in place; avoids making copies
s[seq.int(new_len + 1L, len)] <<- NULL
}
values
}
peek <- function(missing = missing_default) {
if (count == 0L) {
return(missing)
}
s[[count]]
}
reset <- function() {
s <<- vector("list", init)
count <<- 0L
invisible()
}
size <- function() {
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() {
s[seq_len(count)]
}
list(
push = push,
mpush = mpush,
pop = pop,
mpop = mpop,
peek = peek,
reset = reset,
size = size,
as_list = as_list
)
}
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