Impl part 3/3 for Memo

Refactor the caching policy for the Memo by evicting the elements that have been
the least-recently-accessed.

Python's heapq module default to a min-heap. By storing our heap elements
as (UnixTime, a), we can guarantee that when we call heappop, we will get the
element with the lowest UnixTime value in heap (i.e. the oldest). When we call
heappush, we use (time.time(), key) and these values -- by having the largest
UnixTime, will propogate to the bottom of the min-heap.
This commit is contained in:
William Carroll 2020-07-01 15:13:56 +01:00
parent a8b3a2d3c0
commit 155dff562a

View file

@ -1,40 +1,19 @@
import time import time
import random import random
from collections import deque from heapq import heappush, heappop
class BoundedQueue(object):
def __init__(self, size=0):
"""
Returns a queue of elements that will never exceed `size`
members. BoundedQueue evicts the oldest elements from itself before
adding new elements.
"""
self.xs = deque([None] * size)
def add(self, x):
"""
Add element `x` to the end of the queue. Evict the oldest element from
the queue.
"""
evicted = None
if self.xs:
evicted = self.xs.popleft()
self.xs.append(x)
return evicted
class Memo(object): class Memo(object):
def __init__(self, size=1): def __init__(self, size=1):
""" """
Create a key-value data-structure that will never exceed `size` Create a key-value data-structure that will never exceed `size`
members. Memo evicts the oldest elements from itself before adding members. Memo evicts the least-recently-accessed elements from itself
inserting new key-value pairs. before adding inserting new key-value pairs.
""" """
if size <= 0: if size <= 0:
raise Exception("We do not support an empty memo") raise Exception("We do not support an empty memo")
self.xs = {} self.xs = {}
self.q = BoundedQueue(size=size) self.heap = [(0, None)] * size
def contains(self, k): def contains(self, k):
""" """
@ -46,19 +25,21 @@ class Memo(object):
""" """
Return the memoized item at key `k`. Return the memoized item at key `k`.
""" """
# "touch" the element in the heap
return self.xs[k] return self.xs[k]
def set(self, k, v): def set(self, k, v):
""" """
Memoize value `v` at key `k`. Memoize value `v` at key `k`.
""" """
evicted = self.q.add(k) _, to_evict = heappop(self.heap)
if evicted != None: if to_evict != None:
del self.xs[evicted] del self.xs[to_evict]
heappush(self.heap, (time.time(), k))
self.xs[k] = v self.xs[k] = v
memo = Memo(size=3) memo = Memo(size=10)
def f(x): def f(x):