slide_index(.x, .i, .f, ..., .before = 0L, .after = 0L, .complete = FALSE)

slide_index_vec(
  .x,
  .i,
  .f,
  ...,
  .before = 0L,
  .after = 0L,
  .complete = FALSE,
  .ptype = NULL
)

slide_index_dbl(.x, .i, .f, ..., .before = 0L, .after = 0L, .complete = FALSE)

slide_index_int(.x, .i, .f, ..., .before = 0L, .after = 0L, .complete = FALSE)

slide_index_lgl(.x, .i, .f, ..., .before = 0L, .after = 0L, .complete = FALSE)

slide_index_chr(.x, .i, .f, ..., .before = 0L, .after = 0L, .complete = FALSE)

slide_index_dfr(
  .x,
  .i,
  .f,
  ...,
  .before = 0L,
  .after = 0L,
  .complete = FALSE,
  .names_to = rlang::zap(),
  .name_repair = c("unique", "universal", "check_unique")
)

slide_index_dfc(
  .x,
  .i,
  .f,
  ...,
  .before = 0L,
  .after = 0L,
  .complete = FALSE,
  .size = NULL,
  .name_repair = c("unique", "universal", "check_unique", "minimal")
)

library(lubridate)

x <- 1:5

# In some cases, sliding over `x` with a strict window size of 2
# will fit your use case.
slide(x, ~.x, .before = 1)

# However, if this `i` is a date vector paired with `x`, when computing
# rolling calculations you might want to iterate over `x` while
# respecting the fact that `i` is an irregular sequence.
i <- as.Date("2019-08-15") + c(0:1, 4, 6, 7)

# For example, a "2 day" window should not pair `"2019-08-19"` and
# `"2019-08-21"` together, even though they are next to each other in `x`.
# `slide_index()` computes the lookback value from the current date in `.i`,
# meaning that if you are currently on `"2019-08-21"` and look back 1 day,
# it will correctly not include `"2019-08-19"`.
slide_index(i, i, ~.x, .before = 1)

# We could have equivalently used a lubridate period object for this as well,
# since `i - lubridate::days(1)` is allowed
slide_index(i, i, ~.x, .before = lubridate::days(1))

# ---------------------------------------------------------------------------
# Functions for `.before` and `.after`

# In some cases, it might not be appropriate to compute
# `.i - .before` or `.i + .after`, either because there isn't a `-` or `+`
# method defined, or because there is an alternative way to perform the
# arithmetic. For example, subtracting 1 month with `- months(1)` (using
# lubridate) can sometimes land you on an invalid date that doesn't exist.
i <- as.Date(c("2019-01-31", "2019-02-28", "2019-03-31"))

# 2019-03-31 - months(1) = 2019-02-31, which doesn't exist
i - months(1)

# These NAs create problems with `slide_index()`, which doesn't allow
# missing values in the computed endpoints
try(slide_index(i, i, identity, .before = months(1)))

# In these cases, it is more appropriate to use `%m-%`,
# which will snap to the end of the month, at least giving you something
# to work with.
i %m-% months(1)

# To use this as your `.before` or `.after`, supply an anonymous function of
# 1 argument that performs the computation
slide_index(i, i, identity, .before = ~.x %m-% months(1))

# Notice that in the `.after` case, `2019-02-28 %m+% months(1)` doesn't
# capture the end of March, so it isn't included in the 2nd result
slide_index(i, i, identity, .after = ~.x %m+% months(1))

# ---------------------------------------------------------------------------

# When `.i` has repeated values, they are always grouped together.
i <- c(2017, 2017, 2018, 2019, 2020, 2020)
slide_index(i, i, ~.x)
slide_index(i, i, ~.x, .after = 1)

# ---------------------------------------------------------------------------
# Rolling regressions

# Rolling regressions are easy with `slide_index()` because:
# - Data frame `.x` values are iterated over rowwise
# - The index is respected by using `.i`
set.seed(123)

df <- data.frame(
  y = rnorm(100),
  x = rnorm(100),
  i = as.Date("2019-08-15") + c(0, 2, 4, 6:102) # <- irregular
)

# 20 day rolling regression. Current day + 19 days back.
# Additionally, set `.complete = TRUE` to not compute partial results.
regr <- slide_index(df, df$i, ~lm(y ~ x, .x), .before = 19, .complete = TRUE)

regr[16:18]

# The first 16 slots are NULL because there is no possible way to
# look back 19 days from the 16th index position and construct a full
# window. But on the 17th index position, `""2019-09-03"`, if we look
# back 19 days we get to `""2019-08-15"`, which is the same value as
# `i[1]` so a full window can be constructed.
df$i[16] - 19 >= df$i[1] # FALSE
df$i[17] - 19 >= df$i[1] # TRUE

# ---------------------------------------------------------------------------
# Accessing the current index value

# A very simplistic version of `purrr::map2()`
fake_map2 <- function(.x, .y, .f, ...) {
  Map(.f, .x, .y, ...)
}

# Occasionally you need to access the index value that you are currently on.
# This is generally not possible with a single call to `slide_index()`, but
# can be easily accomplished by following up a `slide_index()` call with a
# `purrr::map2()`. In this example, we want to use the distance from the
# current index value (in days) as a multiplier on `x`. Values further
# away from the current date get a higher multiplier.
set.seed(123)

# 25 random days past 2000-01-01
i <- sort(as.Date("2000-01-01") + sample(100, 25))

df <- data.frame(i = i, x = rnorm(25))

weight_by_distance <- function(df, i) {
  df$weight = abs(as.integer(df$i - i))
  df$x_weighted = df$x * df$weight
  df
}

# Use `slide_index()` to just generate the rolling data.
# Here we take the current date + 5 days before + 5 days after.
dfs <- slide_index(df, df$i, ~.x, .before = 5, .after = 5)

# Follow up with a `map2()` with `i` as the second input.
# This allows you to track the current `i` value and weight accordingly.
result <- fake_map2(dfs, df$i, weight_by_distance)

head(result)