timeline_cache.py

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import bisect
import Queue
import threading
import time


class TimelineCache(threading.Thread):
    """
    Caches items for timeline renderers
    """
    def __init__(self, loader, listener=None, max_cache_size=100):
        threading.Thread.__init__(self)

        self.loader = loader
        self.listener = listener
        self.stop_flag = False
        self.lock = threading.RLock()
        self.items = {}  # topic -> [(timestamp, items), ...]
        self.last_accessed = {}  # topic -> [(access time, timestamp), ...]
        self.item_access = {}  # topic -> timestamp -> access time
        self.max_cache_size = max_cache_size  # max number of items to cache (per topic)
        self.queue = Queue.Queue()
        self.setDaemon(True)
        self.start()

    def run(self):
        while not self.stop_flag:
            # Get next item to load
            entry = self.queue.get()
            # self used to signal a change in stop_flag
            if entry == self:
                continue
            # Check we haven't already cached it
            topic, stamp, time_threshold, item_details = entry

            if not self.get_item(topic, stamp, time_threshold):
                # Load the item
                msg_stamp, item = self.loader(topic, stamp, item_details)
                if item:
                    # Store in the cache
                    self.cache_item(topic, msg_stamp, item)

                    if self.listener:
                        self.listener(topic, msg_stamp, item)
#                else:
#                    try:
#                        qWarning('Failed to load:%s' % entry)
#                    except:
#                        qWarning('Failed to load cache item')
            self.queue.task_done()

    def enqueue(self, entry):
        self.queue.put(entry)

    def cache_item(self, topic, t, item):
        with self.lock:
            if topic not in self.items:
                self.items[topic] = []
            topic_cache = self.items[topic]

            cache_entry = (t.to_sec(), item)
            cache_index = bisect.bisect_right(topic_cache, cache_entry)
            topic_cache.insert(cache_index, cache_entry)

            self._update_last_accessed(topic, t.to_sec())

            self._limit_cache()

    def get_item(self, topic, stamp, time_threshold):
        with self.lock:
            # Attempt to get a item from the cache that's within time_threshold secs from stamp
            topic_cache = self.items.get(topic)
            if topic_cache:
                cache_index = max(0, bisect.bisect_right(topic_cache, (stamp, None)) - 1)

                if cache_index <= len(topic_cache) - 1:
                    # Get cache entry before (or at) timestamp, and entry after
                    (cache_before_stamp, cache_before_item) = topic_cache[cache_index]
                    if cache_index < len(topic_cache) - 1:
                        cache_after_stamp, cache_after_item = topic_cache[cache_index + 1]
                    else:
                        cache_after_stamp = None

                    # Find closest entry
                    cache_before_dist = abs(stamp - cache_before_stamp)
                    if cache_after_stamp:
                        cache_after_dist = abs(cache_after_stamp - stamp)

                    if cache_after_stamp and cache_after_dist < cache_before_dist:
                        cache_dist, cache_stamp, cache_item = cache_after_dist, cache_after_stamp, cache_after_item
                    else:
                        cache_dist, cache_stamp, cache_item = cache_before_dist, cache_before_stamp, cache_before_item

                    # Check entry is close enough
                    if cache_dist <= time_threshold:
                        self._update_last_accessed(topic, cache_stamp)
                        return cache_item
            return None

    def _update_last_accessed(self, topic, stamp):
        """
        Maintains a sorted list of cache accesses by timestamp for each topic.
        """
        with self.lock:
            access_time = time.time()

            if topic not in self.last_accessed:
                self.last_accessed[topic] = [(access_time, stamp)]
                self.item_access[topic] = {stamp: access_time}
                return

            topic_last_accessed = self.last_accessed[topic]
            topic_item_access = self.item_access[topic]

            if stamp in topic_item_access:
                last_access = topic_item_access[stamp]

                index = bisect.bisect_left(topic_last_accessed, (last_access, None))
                assert(topic_last_accessed[index][1] == stamp)

                del topic_last_accessed[index]

            topic_last_accessed.append((access_time, stamp))
            topic_item_access[stamp] = access_time

    def _limit_cache(self):
        """
        Removes LRU's from cache until size of each topic's cache is <= max_cache_size.
        """
        with self.lock:
            for topic, topic_cache in self.items.items():
                while len(topic_cache) > self.max_cache_size:
                    lru_stamp = self.last_accessed[topic][0][1]

                    cache_index = bisect.bisect_left(topic_cache, (lru_stamp, None))
                    assert(topic_cache[cache_index][0] == lru_stamp)

                    del topic_cache[cache_index]
                    del self.last_accessed[topic][0]
                    del self.item_access[topic][lru_stamp]

    def stop(self):
        self.stop_flag = True
        self.queue.put(self)