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#  File src/library/base/R/split.R
#  Part of the R package, https://www.R-project.org
#
#  Copyright (C) 1995-2018 The R Core Team
#
#  This program is free software; you can redistribute it and/or modify
#  it under the terms of the GNU General Public License as published by
#  the Free Software Foundation; either version 2 of the License, or
#  (at your option) any later version.
#
#  This program is distributed in the hope that it will be useful,
#  but WITHOUT ANY WARRANTY; without even the implied warranty of
#  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
#  GNU General Public License for more details.
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#  A copy of the GNU General Public License is available at
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split <- function(x, f, drop = FALSE, ...) UseMethod("split")

split.default <- function(x, f, drop = FALSE, sep = ".", lex.order = FALSE, ...)
{
    if(!missing(...)) .NotYetUsed(deparse(...), error = FALSE)

    if (is.list(f))
    f <- interaction(f, drop = drop, sep = sep, lex.order = lex.order)
    else if (!is.factor(f)) f <- as.factor(f) # docs say as.factor
    else if (drop) f <- factor(f) # drop extraneous levels
    storage.mode(f) <- "integer"  # some factors have had double in the past
    if (is.null(attr(x, "class")))
    return(.Internal(split(x, f)))
    ## else
    ind <- .Internal(split(seq_along(x), f))
    lapply(ind, function(i) x[i])
}

## This is documented to work for matrices too
split.data.frame <- function(x, f, drop = FALSE, ...)
{
    ## If formula, maybe should check that there is no LHS?
    if (inherits(f, "formula"))
        f <- eval(attr(stats::terms(f), "variables"), x, environment(f))
    lapply(split(x = seq_len(nrow(x)), f = f, drop = drop, ...),
           function(ind) x[ind, , drop = FALSE])
}

`split<-` <- function(x, f, drop = FALSE, ..., value) UseMethod("split<-")

`split<-.default` <- function(x, f, drop = FALSE, ..., value)
{
    ix <- split(seq_along(x), f, drop = drop, ...)
    n <- length(value)
    j <- 0
    for (i in ix) {
        j <- j %% n + 1
        x[i] <- value[[j]]
    }
    x
}

## This is documented to work for matrices too
`split<-.data.frame` <- function(x, f, drop = FALSE, ..., value)
{
    if (inherits(f, "formula")) f <- eval(attr(stats::terms(f), "variables"), x, environment(f))
    ix <- split(seq_len(nrow(x)), f, drop = drop, ...)
    n <- length(value)
    j <- 0
    for (i in ix) {
        j <- j %% n + 1
        x[i,] <- value[[j]]
    }
    x
}

## (Note: use rep(NA_integer_, len) for indexing here. Logical NA confuses 
## tibbles and only coincidentally does the right thing otherwise, 
## because len is longer than value[[1]]. Otherwise recycling would kick in.) 
unsplit <- function (value, f, drop = FALSE)
{
    len <- length(if (is.list(f)) f[[1L]] else f)
    if (is.data.frame(value[[1L]])) {
        x <- value[[1L]][rep(NA_integer_, len),, drop = FALSE]
        rownames(x) <- unsplit(lapply(value, rownames), f, drop = drop)
    } else
        x <- value[[1L]][rep(NA_integer_, len)]
    split(x, f, drop = drop) <- value
    x
}