"""Helper to watch a (set) of directories for modifications."""

import os
import threading
import time


class DirWatcher(object):
  """Helper to watch a (set) of directories for modifications."""

  def __init__(self, paths):
    if isinstance(paths, basestring):
      paths = [paths]
    self._mu = threading.Lock()
    self._done = False
    self.paths = list(paths)
    self.lastrun = time.time()
    self._cache = self._calculate()

  def _calculate(self):
    """Walk over all subscribed paths, check most recent mtime."""
    most_recent_change = None
    for path in self.paths:
      if not os.path.exists(path):
        continue
      if not os.path.isdir(path):
        continue
      for root, _, files in os.walk(path):
        for f in files:
          st = os.stat(os.path.join(root, f))
          if most_recent_change is None:
            most_recent_change = st.st_mtime
          else:
            most_recent_change = max(most_recent_change, st.st_mtime)
    return most_recent_change

  def most_recent_change(self):
    self._mu.acquire()
    try:
      if time.time() - self.lastrun > 1:
        self._cache = self._calculate()
        self.lastrun = time.time()
      return self._cache
    finally:
      self._mu.release()