#' Configure a Python Environment
#'
#' Configure a Python environment, satisfying the Python dependencies of any
#' loaded \R packages.
#'
#' Normally, this function should only be used by package authors, who want
#' to ensure that their package dependencies are installed in the active
#' Python environment. For example:
#'
#' ```
#' .onLoad <- function(libname, pkgname) {
#'   reticulate::configure_environment(pkgname)
#' }
#' ```
#'
#' If the Python session has not yet been initialized, or if the user is not
#' using the default Miniconda Python installation, no action will be taken.
#' Otherwise, `reticulate` will take this as a signal to install any required
#' Python dependencies into the user's Python environment.
#'
#' If you'd like to disable `reticulate`'s auto-configure behavior altogether,
#' you can set the environment variable:
#'
#' ```
#' RETICULATE_AUTOCONFIGURE = FALSE
#' ```
#'
#' e.g. in your `~/.Renviron` or similar.
#'
#' Note that, in the case where the Python session has not yet been initialized,
#' `reticulate` will automatically ensure your required Python dependencies
#' are installed after the Python session is initialized (when appropriate).
#'
#' @param package The name of a package to configure. When `NULL`, `reticulate`
#'   will instead look at all loaded packages and discover their associated
#'   Python requirements.
#'
#' @param force Boolean; force configuration of the Python environment? Note
#'   that `configure_environment()` is a no-op within non-interactive \R
#'   sessions. Use this if you require automatic environment configuration, e.g.
#'   when testing a package on a continuous integration service.
#'
#' @export
configure_environment <- function(package = NULL, force = FALSE) {

  # no-op when Python has not yet been initialized
  if (!is_python_initialized())
    return(FALSE)

  # allow opt-out through envvar
  auto <- Sys.getenv("RETICULATE_AUTOCONFIGURE", unset = "TRUE")
  if (auto %in% c("FALSE", "False", "0"))
    return(FALSE)

  # disallow in non-interactive R sessions unless forced
  # (even if force is set, do not allow unless user has explicitly
  # promised they're not on CRAN)
  ok <- interactive() || (force && identical(Sys.getenv("NOT_CRAN"), "true"))
  if (!ok)
    return(FALSE)

  # disallow environment configuration when not using a Python environment
  config <- py_config()
  root <- config$prefix
  ok <- is_virtualenv(root) || is_condaenv(root)
  if (!ok)
    return(FALSE)

  # find Python requirements
  reqs <- python_package_requirements(package)
  if (length(reqs) == 0)
    return(FALSE)

  pkgreqs <- unlist(
    lapply(reqs, `[[`, "packages"),
    recursive = FALSE,
    use.names = FALSE
  )

  # check for incompatible package requests
  df <- do.call(rbind.data.frame, pkgreqs)
  splat <- split(df, df$package)
  pkgreqs <- enumerate(splat, function(pkg, requests) {

    rownames(requests) <- NULL

    # check for explicit requests by version
    explicit <- requests[!is.na(requests$version), ]
    if (nrow(explicit) == 0)
      return(requests[1, ])

    # check for single explicit version request
    n <- length(unique(requests$version))
    if (n == 1)
      return(explicit[1, ])

    # otherwise warn and sort
    explicit <- explicit[order(explicit$version, decreasing = TRUE), ]
    output <- capture.output(format(explicit))

    fmt <- "WARNING: incompatible requirements for package '%s' detected!"
    messagef(fmt, pkg)

    output <- capture.output(format(requests))
    message(paste(output, collapse = "\n"))
    selected <- explicit[1, ]

    fmt <- "WARNING: %s [%s] will be used."
    messagef(fmt, selected$package, selected$version)
    selected

  })

  # split into packages to be installed with pip vs. conda
  # we'll diff the requested packages against the currently-installed
  # packages and only install packages which truly need to be updated
  pip_installed_packages <- NULL
  conda_installed_packages <- NULL

  pip_packages <- character()
  conda_packages <- character()

  for (req in pkgreqs) {

    # if no 'pip' requirement was specified, assume pip
    pip <- req$pip
    if (is.null(pip) || is.na(pip))
      pip <- TRUE

    # if this is a virtual environment, we cannot use conda
    if (nzchar(config$virtualenv %||% ""))
      pip <- TRUE

    # normalize version request
    version <- req$version
    if (is.null(version) || is.na(version))
      version <- NULL


    components <- c(req$package, version)

    if (pip) {

      # read installed packages lazily
      if (is.null(pip_installed_packages)) {
        pip_installed_packages <- pip_freeze(python = config$python)
      }

      # construct requirement string
      requirement <- paste(components, collapse = "==")

      # check to see if we satisfy this requirement already
      satisfied <-
        requirement %in% pip_installed_packages$requirement ||
        requirement %in% pip_installed_packages$package

      if (satisfied)
        next

      pip_packages[[length(pip_packages) + 1]] <- requirement

    } else {

      # read installed packages lazily
      envpath <- dirname(dirname(config$python))
      conda <- conda_binary()

      if (is.null(conda_installed_packages)) {
        conda_installed_packages <- conda_list_packages(
          envname = envpath,
          conda = conda
        )
      }

      # construct requirement string
      requirement <- paste(components, collapse = "=")

      # check to see if we satisfy this requirement already
      satisfied <-
        requirement %in% conda_installed_packages$requirement ||
        requirement %in% conda_installed_packages$package

      if (satisfied)
        next

      conda_packages[[length(conda_packages) + 1]] <- requirement

    }

  }

  if (length(pip_packages) || length(conda_packages)) {

    fmt <- "Configuring package '%s': please wait ..."
    messagef(fmt, package)

    if (length(pip_packages))
      py_install(pip_packages, pip = TRUE)

    if (length(conda_packages))
      py_install(conda_packages, pip = FALSE)

    message("Done!")

  }

  TRUE
}

python_package_requirements <- function(packages = NULL) {

  packages <- packages %||% loadedNamespaces()
  names(packages) <- packages
  reqs <- lapply(packages, function(package) {
    tryCatch(
      python_package_requirements_find(package),
      error = function(e) { warning(e); NULL }
    )
  })

  Filter(Negate(is.null), reqs)

}

python_package_requirements_find <- function(package) {

  descpath <- system.file("DESCRIPTION", package = package)
  desc <- read.dcf(descpath, all = TRUE)

  entry <- desc[["Config/reticulate"]]
  if (is.null(entry))
    return(NULL)

  spec <- eval(parse(text = entry), envir = baseenv())

  fields <- c("package", "version", "pip")
  spec$packages <- lapply(spec$packages, function(req) {

    if (is.null(req$package)) {
      warning("invalid spec provided by package '%s'", package)
      return(NULL)
    }

    data.frame(
      source  = package,
      package = as.character(req[["package"]]),
      version = as.character(req[["version"]] %||% NA),
      pip     = as.logical(req[["pip"]] %||% NA),
      stringsAsFactors = FALSE
    )

  })

  spec

}