#' @title Leanifies a Method
#'
#' @description
#' Moves a single method of an R6Class Generator to its package namespace.
#' The R6Class's method is reduced to a call to  the moved function.
#'
#' This creates a function named `.__<CLASSNAME>__<FUNCTIONNAME>` inside `env`.
#'
#' leanificate_method is called by [leanify_r6] for each function of an [R6] class.
#'
#' Leanification also removes the `"srcref"` attribute of functions (sets it to `NULL`), because
#' 1) it is incorrect and
#' 2) srcrefs can lead to exploding object sizes.
#'
#' However, because `roxygen2` needs the srcrefs to create the documentation for R6 classes, leanification
#' is skipped during `roxygenize()`, i.e. when the `ROXYGEN_PKG` environment variable is set.
#'
#' @param cls (`R6ClassGenerator`)\cr
#'   R6Class object (i.e. R6 object generator) to modify.
#' @param name (`character(1)`)\cr
#'   Name of the function
#' @param env (`environment`)\cr
#'   The target environment where the function should be stored.
#'   Should be either `cls$parent_env` or one of its parent environments,
#'   otherwise the stump function will not find the moved (original code) function.
#' @return NULL
#' @noRd
leanificate_method = function(cls, fname, env = cls$parent_env) {
  cname = cls$classname

  # find out where the function is stored: public, private, active. This is also
  # relevant for the `cls$set()` call at the end: we need to set the function to
  # the right protection kind as it was before.
  for (function_kind_container in c("public_methods", "private_methods", "active")) {
    fn = cls[[function_kind_container]][[fname]]
    if (!is.null(fn)) break
  }
  if (is.null(fn)) {
    stop(sprintf("Could not find function %s in class %s", fname, cname))
  }

  exportfname = sprintf(".__%s__%s", cname, fname)
  origformals = formals(fn)
  origattributes = attributes(fn)
  formals(fn) = c(pairlist(self = substitute(), private = substitute(), super = substitute()), formals(fn))
  attributes(fn) = origattributes
  assign(exportfname, fn, env)
  replacingfn = eval(call("function", origformals,
    as.call(c(list(as.symbol(exportfname)), sapply(names(formals(fn)), as.symbol, simplify = FALSE)))))
  environment(replacingfn) = environment(fn)

  function_kind = switch(function_kind_container, public_methods = "public", private_methods = "private", active = "active")
  # We remove the srcref (which exists when installing with option --with-keep.source) because:
  # (1) incorrect rendering of leanified functions
  # (2) use up memory unnecessarily
  origattributes$srcref = NULL
  attributes(replacingfn) = origattributes
  cls$set(function_kind, fname, replacingfn, overwrite = TRUE)
}

#' @title Move all methods of an R6 Class to an environment
#'
#' @description
#' `leanify_r6` moves the content of an [`R6::R6Class`]'s functions to an environment,
#' usually the package's namespace, to save space during serialization of R6 objects.
#' `leanify_package` move all methods of *all* R6 Classes to an environment.
#'
#' The function in the class (i.e. the object generator) is replaced by a stump
#' function that does nothing except calling the original function that now resides
#' somewhere else.
#'
#' It is possible to call this function after the definition of an [R6::R6]
#' class inside a package, but it is preferred to use [leanify_package()]
#' to just leanify all [R6::R6] classes inside a package.
#'
#' @param cls ([R6::R6Class])\cr
#'   Class generator to modify.
#' @param env (`environment`)\cr
#'   The target environment where the function should be stored.
#'   This should be either `cls$parent_env` (default) or one of its
#'   parent environments, otherwise the stump function will not find the moved
#'   (original code) function.
#' @return `NULL`.
#' @export
leanify_r6 = function(cls, env = cls$parent_env) {
  assert_class(cls, "R6ClassGenerator")
  assert_environment(env)
  for (assignwhich in c("public_methods", "private_methods", "active")) {
    for (fname in names(cls[[assignwhich]])) {
      leanificate_method(cls, fname, env = env)
    }
  }
}

#' @rdname leanify_r6
#'
#' @param pkg_env :: `environment`\cr
#'   The namespace from which to leanify all R6 classes. Does not have to be a
#'   package namespace, but this is the intended usecase.
#' @param skip_if :: `function`\cr
#'   Function with one argument: Is called for each individual [`R6::R6Class`].
#'   If it returns `TRUE`, the class is skipped. Default function evaluating to `FALSE`
#'   always (i.e. skipping no classes).
#' @export
leanify_package = function(pkg_env = parent.frame(), skip_if = function(x) FALSE) {
  assert_environment(pkg_env)
  assert_function(skip_if)

  # otherwise creation of R6 docs fails
  pkg = Sys.getenv("ROXYGEN_PKG")
  if (nzchar(pkg)) {
    messagef("Skipping leanification of package '%s' during roxygenize.", pkg)
    return(NULL)
  }

  for (varname in names(pkg_env)) {
    content = get(varname, envir = pkg_env, inherits = FALSE)
    if (R6::is.R6Class(content) && !isTRUE(skip_if(content))) {
      leanify_r6(content)
    }
  }
}
