# A context object for tracking a cache that needs to be dirtied when a set of
# files changes on disk. Each time the cache is dirtied, the set of files is
# cleared. Therefore, the set of files needs to be re-built each time the cached
# code executes. This approach allows for dynamic dependency graphs.
CacheContext <- R6Class(
  'CacheContext',
  portable = FALSE,
  class = FALSE,
  public = list(
    .dirty = TRUE,
    # List of functions that return TRUE if dirty
    .tests = list(),

    addDependencyFile = function(file) {
      if (.dirty)
        return()

      file <- normalizePath(file)

      mtime <- file.info(file)$mtime
      .tests <<- c(.tests, function() {
        newMtime <- try(file.info(file)$mtime, silent=TRUE)
        if (inherits(newMtime, 'try-error'))
          return(TRUE)
        return(!identical(mtime, newMtime))
      })
      invisible()
    },
    forceDirty = function() {
      .dirty <<- TRUE
      .tests <<- list()
      invisible()
    },
    isDirty = function() {
      if (.dirty)
        return(TRUE)

      for (test in .tests) {
        if (test()) {
          forceDirty()
          return(TRUE)
        }
      }

      return(FALSE)
    },
    reset = function() {
      .dirty <<- FALSE
      .tests <<- list()
    },
    with = function(func) {
      oldCC <- .currentCacheContext$cc
      .currentCacheContext$cc <- self
      on.exit(.currentCacheContext$cc <- oldCC)

      return(func())
    }
  )
)

.currentCacheContext <- new.env()

# Indicates to Shiny that the given file path is part of the dependency graph
# for whatever is currently executing (so far, only ui.R). By default, ui.R only
# gets re-executed when it is detected to have changed; this function allows the
# caller to indicate that it should also re-execute if the given file changes.
#
# If NULL or NA is given as the argument, then ui.R will re-execute next time.
dependsOnFile <- function(filepath) {
  if (is.null(.currentCacheContext$cc))
    return()

  if (is.null(filepath) || is.na(filepath))
    .currentCacheContext$cc$forceDirty()
  else
    .currentCacheContext$cc$addDependencyFile(filepath)
}


#' @export
DiskCache <- R6Class("DiskCache",
  public = list(
    initialize = function(dir = tempfile("DiskCache-"),
                          prune = function(dir) {},
                          reset_on_finalize = TRUE)
    {
      if (!dirExists(dir)) {
        message("Creating ", dir)
        dir.create(dir, recursive = TRUE, mode = "0700")
        private$dir_was_created <- TRUE
      }
      private$dir <- absolutePath(dir)
      private$reset_on_finalize <- reset_on_finalize
      # Save in a list to avoid problems when cloning
      private$prune_ <- list(prune)
    },

    # TODO: Should call has() and return some sentinal object if not present
    # Should be atomic ot avoid race conditions.
    get = function(key) {
      if (!self$has(key)) {
        stop("Key not available: ", key)
      }
      value <- readRDS(private$key_to_filename(key))
      self$prune()
      value
    },

    mget = function(keys) {
    },

    mset = function(..., .list = NULL) {
    },

    set = function(key, value) {
      # TODO: Make sure key is a safe string
      self$prune()
      saveRDS(value, file = private$key_to_filename(key))
      invisible(self)
    },

    has = function(key) {
      file.exists(private$key_to_filename(key))
    },

    # Return all keys in the cache
    keys = function() {

    },

    remove = function(key) {
      file.remove(private$key_to_filename(key))
      invisible(self)
    },

    reset = function() {
      file.remove(dir(private$dir, "*.rds", full.names = TRUE))
      invisible(self)
    },

    prune = function() {
      private$prune_[[1]](private$dir)
      invisible(self)
    },

    size = function() {
    },

    # Resets the cache and destroys the containing folder so that no
    # one else who shares the data back end can use it anymore.
    destroy = function() {
    },

    finalize = function() {
      if (private$reset_on_finalize) {
        self$reset()
        if (private$dir_was_created) {
          message("Removing ", private$dir)
          dirRemove(private$dir)
        }
      }
    }
  ),
  private = list(
    dir = NULL,
    prune_ = NULL,
    dir_was_created = FALSE,
    reset_on_finalize = NULL,
    key_to_filename = function(key) {
      if (! (is.character(key) && length(key)==1) ) {
        stop("Key must be a character vector of length 1.")
      }
      file.path(private$dir, paste0(key, ".rds"))
    }
  )
)

# Safely hash an object. If it has any weird things in it that might change, like
# functions or xptrs (?), throw an error.
safe_hash <- function(x) {

}


disk_pruner <- function(max_size = 5 * 1024^2, max_age = Inf,
                        discard = c("oldest", "newest"),
                        timetype = c("ctime", "atime", "mtime"))
{
  discard  <- match.arg(discard)
  timetype <- match.arg(timetype)

  function(path) {
    files <- file.info(dir(path, "*.rds", full.names = TRUE))
    files <- files[files$isdir == FALSE, ]
    files$name <- rownames(files)
    rownames(files) <- NULL

    time <- Sys.time()
    # Remove any files where the age exceeds max age.
    files$timediff <- as.numeric(Sys.time() - files[[timetype]], units = "secs")
    files$rm <- files$timediff > max_age
    if (any(files$rm)) {
      message("Removing ", paste(files$name[files$rm], collapse = ", "))
    }
    file.remove(files$name[files$rm])

    # Remove rows of files that were deleted
    files <- files[!files$rm, ]

    # Sort the files by time, get a cumulative sum of size, and remove any
    # files where the cumlative size exceeds max_size.
    if (sum(files$size) > max_size) {
      sort_decreasing <- (discard == "oldest")

      files <- files[order(files[[timetype]], decreasing = sort_decreasing), ]
      files$cum_size <- cumsum(files$size)
      files$rm <- files$cum_size > max_size
      if (any(files$rm)) {
        message("Removing ", paste(files$name[files$rm], collapse = ", "))
      }
      file.remove(files$name[files$rm])
    }
  }
}