#' Add or Remove a Global 'progression' Handler
#'
#' @param action (character string)
#' If `"add"`, a global handler is added.
#' If `"remove"`, it is removed, if it exists.
#' If `"query"`, checks whether a handler is registered or not.
#'
#' @return Returns TRUE if a handler is registered, otherwise FALSE.
#' If `action = "query"`, the value is visible, otherwise invisible.
#'
#' @section Requirements:
#' This function requires R (>= 4.0.0) - the version in which global calling
#' handlers where introduces.
#'
#' @example incl/register_global_progression_handler.R
#'
#' @keywords internal
register_global_progression_handler <- function(action = c("add", "remove", "query")) {
  action <- match.arg(action[1], choices = c("add", "remove", "query", "status"))

  if (getRversion() < "4.0.0") {
    warning("register_global_progression_handler() requires R (>= 4.0.0)")
    return(invisible(FALSE))
  }

  ## All existing handlers
  handlers <- globalCallingHandlers()

  exists <- vapply(handlers, FUN = identical, global_progression_handler, FUN.VALUE = FALSE)
  if (sum(exists) > 1L) {
    warning("Detected more than one registered 'global_progression_handler'. Did you register it manually?")
  }
  
  if (action == "add") {
    if (!any(exists)) {
      globalCallingHandlers(condition = global_progression_handler)
    }
    invisible(TRUE)
  } else if (action == "remove") {
    global_progression_handler(control_progression("shutdown"))
    handlers <- handlers[!exists]
    ## Remove all
    globalCallingHandlers(NULL)
    ## Add back the ones we didn't drop
    globalCallingHandlers(handlers)
    invisible(FALSE)
  } else if (action == "query") {
    any(exists)
  } else if (action == "status") {
    global_progression_handler(control_progression("status"))
  }
}


#' A Global Calling Handler For 'progression':s
#'
#' @param condition A logical scalar or a condition object.
#'
#' @return Nothing.
#'
#' @section Requirements:
#' This function requires R (>= 4.0.0) - the version in which global calling
#' handlers where introduces.
#' 
#' @keywords internal
global_progression_handler <- local({
  active <- TRUE
  
  current_progressor_uuid <- NULL
  calling_handler <- NULL
  delays <- NULL
  stdout_file <- NULL
  capture_conditions <- NA
  conditions <- list()
  genv <- globalenv()

  update_calling_handler <- function() {
    handlers <- handlers()
    # Nothing to do?
    if (length(handlers) == 0L) return(NULL)

    handlers <- as_progression_handler(handlers)

    # Nothing to do?
    if (length(handlers) == 0L) return(NULL)

    ## Do we need to buffer?
    delays <<- use_delays(handlers)

    calling_handler <<- make_calling_handler(handlers)
  }

  interrupt_calling_handler <- function(progression, debug = FALSE) {
    if (is.null(calling_handler)) return()
    
    ## Don't capture conditions that are produced by progression handlers
    capture_conditions <<- FALSE
    on.exit(capture_conditions <<- TRUE)
  
    ## Any buffered output to flush?
    if (isTRUE(attr(delays$terminal, "flush"))) {
      if (length(conditions) > 0L || has_buffered_stdout(stdout_file)) {
        calling_handler(control_progression("hide"))
        stdout_file <<- flush_stdout(stdout_file, close = FALSE)
        conditions <<- flush_conditions(conditions)
      }
    }
  
    calling_handler(progression)
  }

  finish <- function(progression = control_progression("shutdown"), debug = FALSE) {
    finished <- FALSE
    
    ## Is progress handler active?
    if (!is.null(current_progressor_uuid)) {
      if (debug) message(" - shutdown progression handlers")
      if (!is.null(calling_handler)) {
        stdout_file <<- delay_stdout(delays, stdout_file = stdout_file)
        finished <- calling_handler(progression)
        ## Note that we might not be able to close 'stdout_file' due
        ## to blocking, non-balanced sinks
        stdout_file <<- flush_stdout(stdout_file, close = TRUE, must_work = FALSE)
        conditions <<- flush_conditions(conditions)
        delays <<- NULL
        if (debug) message(" - finished: ", finished)
      } else {
        finished <- TRUE
      }
    } else {
      if (debug) message(" - no active global progression handler")
    }

    ## Note that we might not have been able to close 'stdout_file' in previous
    ## calls to finish() due to blocking, non-balanced sinks. Try again here,
    ## just in case
    if (!is.null(stdout_file)) {
      stdout_file <<- flush_stdout(stdout_file, close = TRUE, must_work = FALSE)
    }

    current_progressor_uuid <<- NULL
    calling_handler <<- NULL
    capture_conditions <<- NA
    finished <- TRUE
    stop_if_not(length(conditions) == 0L, is.null(delays), isTRUE(finished), is.na(capture_conditions))
    
    finished
  }

  handle_progression <- function(progression, debug = getOption("progressr.global.debug", FALSE)) {
    ## To please R CMD check
    calling_handler <- NULL; rm(list = "calling_handler")

    ## Don't capture conditions that are produced by progression handlers
    last_capture_conditions <- capture_conditions
    capture_conditions <<- FALSE
    on.exit({
      if (is.null(current_progressor_uuid)) {
        capture_conditions <<- NA
      } else if (!is.na(capture_conditions)) {
        capture_conditions <<- TRUE
      }
    })

    stop_if_not(inherits(progression, "progression"))
    
    assign(".Last.progression", value = progression, envir = genv, inherits = FALSE)

    if (debug) message(sprintf("*** Caught a %s condition:", sQuote(class(progression)[1])))
    progressor_uuid <- progression[["progressor_uuid"]]
    if (debug) message(" - source: ", progressor_uuid)
    
    ## Listen to this progressor?
    if (!is.null(current_progressor_uuid) &&
        !identical(progressor_uuid, current_progressor_uuid)) {
      if (debug) message(" - action: ignoring, already listening to another")
      return()
    }


    if (!is.null(calling_handler) && !is.null(delays)) {
      ## Any buffered output to flush?
      if (isTRUE(attr(delays$terminal, "flush"))) {
        if (length(conditions) > 0L || has_buffered_stdout(stdout_file)) {
          calling_handler(control_progression("hide"))
          stdout_file <<- flush_stdout(stdout_file, close = FALSE)
          stop_if_not(inherits(stdout_file, "connection"))
          conditions <<- flush_conditions(conditions)
          calling_handler(control_progression("unhide"))
        }
      }
    }

    type <- progression[["type"]]
    if (debug) message(" - type: ", type)
  
    if (type == "initiate") {
      if (identical(progressor_uuid, current_progressor_uuid)) {
        stop(sprintf("INTERNAL ERROR: Already listening to this progressor which just sent another %s request", sQuote(type)))
      }
      if (debug) message(" - start listening")
#      finished <- finish(debug = debug)
#      stop_if_not(is.null(stdout_file), length(conditions) == 0L)
      current_progressor_uuid <<- progressor_uuid
      if (debug) message(" - reset progression handlers")
      update_calling_handler()
      if (!is.null(calling_handler)) {      
        stdout_file <<- delay_stdout(delays, stdout_file = stdout_file)
        calling_handler(control_progression("reset"))
        if (debug) message(" - initiate progression handlers")
        finished <- calling_handler(progression)
        if (debug) message(" - finished: ", finished)
        if (finished) {
          finished <- finish(debug = debug)
          stop_if_not(length(conditions) == 0L, is.na(capture_conditions), isTRUE(finished))
        }
      }
    } else if (type == "update") {
      if (is.null(current_progressor_uuid)) {
        ## We might receive zero-amount progress updates after the fact that the
        ## progress has been completed
        amount <- progression$amount
        if (!is.numeric(amount) || amount > 0) {
          msg <- conditionMessage(progression)
          if (length(msg) == 0) msg <- "character(0)"
          warning(sprintf("Received a progression %s request (amount=%g; msg=%s) but is not listening to this progressor. This can happen when code signals more progress updates than it configured the progressor to do. When the progressor completes all steps, it shuts down resulting in the global progression handler to no longer listen to it. To troubleshoot this, try with progressr::handlers(\"debug\")", sQuote(type), amount, sQuote(msg)))
        }
        return()
      }

      if (debug) message(" - update progression handlers")
      if (!is.null(calling_handler)) {
        stdout_file <<- delay_stdout(delays, stdout_file = stdout_file)
        finished <- calling_handler(progression)
        if (debug) message(" - finished: ", finished)
        if (finished) {
          calling_handler(control_progression("shutdown"))
          finished <- finish(debug = debug)
          stop_if_not(length(conditions) == 0L, is.na(capture_conditions), isTRUE(finished))
        }
      }
    } else if (type == "finish") {
      finished <- finish(debug = debug)
      stop_if_not(length(conditions) == 0L, is.na(capture_conditions), isTRUE(finished))
    } else if (type == "status") {
      status <- list(
        current_progressor_uuid = current_progressor_uuid,
        calling_handler = calling_handler,
        delays = delays,
        stdout_file = stdout_file,
        capture_conditions = last_capture_conditions,
        conditions = conditions
      )
      if (debug) message(" - done")
      return(status)
    }

    if (debug) message(" - done")
  } ## handle_progression()


  function(condition) {
    if (is.logical(condition)) {
      stop_if_not(length(condition) == 1L, !is.na(condition))
      active <<- condition
      return()
    }
    
    if (!active) return()
    
    debug <- getOption("progressr.global.debug", FALSE)
    
    ## Shut down progression handling?
    if (inherits(condition, c("interrupt", "error"))) {
      if (isTRUE(getOption("progressr.interrupts", TRUE))) {
        ## Create progress message saying why the progress was interrupted
        msg <- sprintf("Progress interrupted by %s condition", class(condition)[1])
        msg <- paste(c(msg, conditionMessage(condition)), collapse = ": ")
        suspendInterrupts({
          interrupt_calling_handler(control_progression("interrupt", message = msg), debug = debug)
        })
      }

      suspendInterrupts({
        progression <- control_progression("shutdown")
        finished <- finish(debug = debug)
        stop_if_not(length(conditions) == 0L, is.na(capture_conditions), isTRUE(finished))
      })
      return()
    }

    ## A 'progression' update?
    if (inherits(condition, "progression")) {
      suspendInterrupts({
        res <- handle_progression(condition, debug = debug)
      })
      return(res)
    }

    ## Nothing do to?
    if (is.na(capture_conditions) || !isTRUE(capture_conditions)) return()

    ## Nothing do to?
    if (is.null(delays) || !inherits(condition, delays$conditions)) return()

    ## Record non-progression condition to be flushed later
    conditions[[length(conditions) + 1L]] <<- condition
    
    ## Muffle it for now
    if (inherits(condition, "message")) {
      invokeRestart("muffleMessage")
    } else if (inherits(condition, "warning")) {
      invokeRestart("muffleWarning")
    } else if (inherits(condition, "condition")) {
      ## If there is a "muffle" restart for this condition,
      ## then invoke that restart, i.e. "muffle" the condition
      restarts <- computeRestarts(condition)
      for (restart in restarts) {
        name <- restart$name
        if (is.null(name)) next
        if (!grepl("^muffle", name)) next
        invokeRestart(restart)
        break
      }
    }
  }
}) ## global_progression_handler()



if (getRversion() < "4.0.0") {
  globalCallingHandlers <- function(...) {
    stop("register_global_progression_handler() requires R (>= 4.0.0)")
  }
}


as_progression_handler <- function(handlers, drop = TRUE, debug = FALSE) {
  ## FIXME(?)
  if (!is.list(handlers)) handlers <- list(handlers)

  if (debug) {
    message("as_progression_handler() ...")
    message(sprintf("- Number of progression handlers: %d", length(handlers)))
    on.exit({
      message(sprintf("- Number of progression handlers: %d", length(handlers)))
      message("as_progression_handler() ... done")
    })
  }

  for (kk in seq_along(handlers)) {
    handler <- handlers[[kk]]
    stop_if_not(is.function(handler))
    if (!inherits(handler, "progression_handler")) {
      if (debug) message(sprintf("- Handler %d is a function; calling it", kk))
      handler <- handler()
      stop_if_not(is.function(handler),
                  inherits(handler, "progression_handler"))
      handlers[[kk]] <- handler
    }
  }

  ## Keep only enabled handlers?
  if (drop) {
    if (debug) message(sprintf("- drop = TRUE => dropping non-enabled handlers"))
    enabled <- vapply(handlers, FUN = function(h) {
      env <- environment(h)
      value <- env$enable
      isTRUE(value) || is.null(value)
    }, FUN.VALUE = TRUE)
    
    if (debug && !all(enabled)) {
      message(sprintf("- Detected %d non-enabled handlers: %s", sum(!enabled), vapply(handlers[!enabled], FUN = function(h) class(h)[1], FUN.VALUE = NA_character_)))
    }
    
    handlers <- handlers[enabled]
  }

  handlers
}