require_relative 'path_scanner'

module Bootsnap
  module LoadPathCache
    class Path
      # A path is considered 'stable' if it is part of a Gem.path or the ruby
      # distribution. When adding or removing files in these paths, the cache
      # must be cleared before the change will be noticed.
      def stable?
        stability == STABLE
      end

      # A path is considered volatile if it doesn't live under a Gem.path or
      # the ruby distribution root. These paths are scanned for new additions
      # more frequently.
      def volatile?
        stability == VOLATILE
      end

      attr_reader :path

      def initialize(path)
        @path = path
      end

      # Return a list of all the requirable files and all of the subdirectories
      # of this +Path+.
      def entries_and_dirs(store)
        if stable?
          # the cached_mtime field is unused for 'stable' paths, but is
          # set to zero anyway, just in case we change the stability heuristics.
          _, entries, dirs = store.get(path)
          return [entries, dirs] if entries # cache hit
          entries, dirs = scan!
          store.set(path, [0, entries, dirs])
          return [entries, dirs]
        end

        cached_mtime, entries, dirs = store.get(path)

        current_mtime = latest_mtime(path, dirs || [])
        return [[], []]        if current_mtime == -1 # path does not exist
        return [entries, dirs] if cached_mtime == current_mtime

        entries, dirs = scan!
        store.set(path, [current_mtime, entries, dirs])
        [entries, dirs]
      end

      private

      def scan! # (expensive) returns [entries, dirs]
        PathScanner.call(path)
      end

      # last time a directory was modified in this subtree. +dirs+ should be a
      # list of relative paths to directories under +path+. e.g. for /a/b and
      # /a/b/c, pass ('/a/b', ['c'])
      def latest_mtime(path, dirs)
        max = -1
        ["", *dirs].each do |dir|
          curr = begin
            File.mtime("#{path}/#{dir}").to_i
          rescue Errno::ENOENT
            -1
          end
          max = curr if curr > max
        end
        max
      end

      # a Path can be either stable of volatile, depending on how frequently we
      # expect its contents may change. Stable paths aren't rescanned nearly as
      # often.
      STABLE   = :stable
      VOLATILE = :volatile

      RUBY_PREFIX = RbConfig::CONFIG['prefix']

      def stability
        @stability ||= begin
          if Gem.path.detect { |p| path.start_with?(p) }
            STABLE
          elsif path.start_with?(RUBY_PREFIX)
            STABLE
          else
            VOLATILE
          end
        end
      end
    end
  end
end