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FileMap.py
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FileMap.py
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#!/usr/bin/python
import sys
from struct import unpack_from as old_unpack_from
from struct import unpack_from as old_unpack
from struct import calcsize
from collections import OrderedDict
# From: http://code.activestate.com/recipes/577197-sortedcollection/
from SortedCollection import SortedCollection
MEGABYTE = 1024 * 1024
class LRUQueue(object):
"""
LRUQueue is a data structure that orders objects by
their insertion time, and supports an update/touch operation
that resets an item to the newest slot.
This is an example of a priority queue, ordered by
insertion time, with explicit support for "touch".
"""
def __init__(self, key=lambda n: n):
"""
The `key` parameter may be provided if the
items in the queue are complex.
The `key` parameter should select a unique "id" field from
each item.
"""
super(LRUQueue, self).__init__()
self._q = OrderedDict()
self._key = key
def push(self, v):
k = self._key(v)
self._q[k] = v
def pop(self):
return self._q.popitem(last=False)[1]
def touch(self, v):
"""
Reset the given value back to the newest slot.
"""
k = self._key(v)
del self._q[k]
self._q[k] = v
def size(self):
return len(self._q)
def __len__(self):
return self.size()
@staticmethod
def test():
q = LRUQueue()
assert q.size() == 0
assert len(q) == 0
q.push(0)
assert q.size() == 1
assert len(q) == 1
assert q.pop() == 0
assert q.size() == 0
assert len(q) == 0
q.push(0)
q.push(1)
assert q.pop() == 0
assert q.pop() == 1
q.push(0)
q.push(1)
q.touch(0)
assert q.pop() == 1
assert q.pop() == 0
q = LRUQueue(key=lambda n: n[0])
q.push([0])
assert q.pop() == [0]
q.push([0])
q.push([1])
assert q.pop() == [0]
assert q.pop() == [1]
return True
class BoundedLRUQueue(object):
"""
BoundedLRUQueue is a LRUQueue with a finite capacity.
When an item is pushed that causes the capacity to be exceeded,
the LRU item is automatically popped.
Otherwise, this class behaves just like the LRUQueue.
"""
def __init__(self, capacity, key=lambda n: n):
"""
The `key` parameter may be provided if the
items in the queue are complex.
The `key` parameter should select a unique "id" field from
each item.
"""
super(BoundedLRUQueue, self).__init__()
self._q = LRUQueue(key)
self._capacity = capacity
def push(self, v):
self._q.push(v)
if len(self._q) > self._capacity:
return self._q.pop()
def pop(self):
return self._q.pop()
def touch(self, v):
self._q.touch(v)
def size(self):
return len(self._q)
def __len__(self):
return self.size()
@staticmethod
def test():
q = BoundedLRUQueue(5)
assert q.size() == 0
assert len(q) == 0
q.push(0)
assert q.size() == 1
assert len(q) == 1
assert q.pop() == 0
assert q.size() == 0
assert len(q) == 0
q.push(0)
q.push(1)
assert q.pop() == 0
assert q.pop() == 1
q.push(0)
q.push(1)
q.touch(0)
assert q.pop() == 1
assert q.pop() == 0
q = BoundedLRUQueue(5, key=lambda n: n[0])
q.push([0])
assert q.pop() == [0]
q.push([0])
q.push([1])
assert q.pop() == [0]
assert q.pop() == [1]
q = BoundedLRUQueue(2)
assert q.push(0) is None
assert q.push(1) is None
assert q.push(2) == 0
assert q.pop() == 1
assert q.pop() == 2
return True
class RangeCache(object):
"""
RangeCache is a data structure that tracks a finite set of
ranges (a range is a 2-tuple consisting of a numeric start
and numeric length). New ranges can be added via the `push`
method, and if such a call causes the capacity to be exceeded,
then the "oldest" range is removed. The `get` method implements
an efficient lookup for a single value that may be found within
one of the ranges.
"""
def __init__(self, capacity,
start_key=lambda o: o[0],
length_key=lambda o: o[1]):
"""
@param key: A function that fetches the range start from an item.
"""
super(RangeCache, self).__init__()
self._ranges = SortedCollection(key=start_key)
self._lru = BoundedLRUQueue(capacity, key=start_key)
self._start_key = start_key
self._length_key = length_key
def push(self, o):
"""
Add a range to the cache.
If `key` is not provided to the constructor, then
`o` should be a 3-tuple:
- range start (numeric)
- range length (numeric)
- range item (object)
"""
self._ranges.insert(o)
popped = self._lru.push(o)
if popped is not None:
self._ranges.remove(popped)
def touch(self, o):
self._lru.touch(o)
def get(self, value):
"""
Search for the numeric `value` within the ranges
tracked by this cache.
@raise ValueError: if the value is not found in the range cache.
"""
hit = self._ranges.find_le(value)
if value < self._start_key(hit) + self._length_key(hit):
return hit
raise ValueError("%s not found in range cache" % value)
@staticmethod
def test():
q = RangeCache(2)
x = None
try: x = q.get(0)
except ValueError: pass
assert x is None
x = None
try: x = q.get(1)
except ValueError: pass
assert x is None
q.push((1, 1, [0]))
x = None
try: x = q.get(0)
except ValueError: pass
assert x is None
assert q.get(1) == (1, 1, [0])
assert q.get(1.99) == (1, 1, [0])
x = None
try: x = q.get(2.01)
except ValueError: pass
assert x is None
q.push((3, 1, [1]))
assert q.get(1) == (1, 1, [0])
assert q.get(3) == (3, 1, [1])
q.push((5, 1, [2]))
x = None
try: x = q.get(1)
except ValueError: pass
assert x is None
assert q.get(3) == (3, 1, [1])
assert q.get(5) == (5, 1, [2])
q.touch((3, 1, [1]))
q.push((7, 1, [3]))
assert q.get(3) == (3, 1, [1])
assert q.get(7) == (7, 1, [3])
x = None
try: x = q.get(5)
except ValueError: pass
assert x is None
return True
class FileMap(object):
"""
FileMap is a wrapper for a file-like object that satisfies the
buffer interface. This is essentially the inverse of StringIO.
It implements a caching layer over the calls to the OS seek/read
functions for improved performance.
Q: Why might you want this over mmap?
A: 1) Its pure Python
2) You can stack this over any Python file-like objects.
eg. FileMap over ZipFile gives you a random access buffer
thats backed by a compressed image on the file system.
"""
def __init__(self, filelike, block_size=MEGABYTE,
cache_size=10, size=None):
"""
If `size` is not provided, then `filelike` must have the
`seek` and `tell` methods implemented.
"""
super(FileMap, self).__init__()
if size is None:
import os
filelike.seek(0, os.SEEK_END)
size = filelike.tell()
self._f = filelike
self._block_size = block_size
self._size = size
self._block_cache = RangeCache(cache_size)
def __getitem__(self, index):
if index < 0:
index = self._size + index
block_index = index % self._block_size
block_start = index - block_index
try:
hit = self._block_cache.get(index)
buf = hit[2]
self._block_cache.touch(hit)
return buf[block_index]
except ValueError:
self._f.seek(block_start)
buf = self._f.read(self._block_size)
self._block_cache.push((block_start, self._block_size, buf))
return buf[block_index]
def _get_containing_block(self, index):
"""
Given an index, return block-aligned block that contains it,
updating the appropriate caches.
"""
block_index = index % self._block_size
block_start = index - block_index
try:
hit = self._block_cache.get(block_start)
buf = hit[2]
self._block_cache.touch(hit)
return buf
except ValueError:
self._f.seek(block_start)
buf = self._f.read(self._block_size)
self._block_cache.push((block_start, self._block_size, buf))
return buf
def __getslice__(self, start, end):
if end == sys.maxint:
end = self._size
start_block_index = start % self._block_size
start_block_start = start - start_block_index
end_block_index = end % self._block_size
end_block_start = end - end_block_index
if start_block_start == end_block_start:
# easy case, everything falls within the same block
buf = self._get_containing_block(start)
return buf[start_block_index:end_block_index]
else:
# hard case, slice goes over one or more block boundaries
ret = ""
# phase 1, start to block boundary
buf = self._get_containing_block(start_block_start)
s = start_block_index
e = start_block_start + self._block_size
ret += buf[s:e]
# phase 2, any complete blocks
cur_block_start = start_block_start + self._block_size
while cur_block_start + self._block_size < end_block_start:
buf = self._get_containing_block(cur_block_start)
ret += buf
cur_block_start += self._block_size
# phase 3, block boundary to end
buf = self._get_containing_block(cur_block_start)
s = 0
e = end_block_index or self._block_size
ret += buf[0:e]
return ret
def __len__(self):
return self._size
@staticmethod
def test():
from cStringIO import StringIO
f = StringIO("0123abcd4567efgh")
buf = FileMap(f, block_size=4, cache_size=2)
assert len(buf) == 16
assert buf[0] == "0"
assert buf[1] == "1"
assert buf[0:2] == "01"
assert buf[4] == "a"
assert buf[5] == "b"
assert buf[4:6] == "ab"
assert buf[2:6] == "23ab"
assert buf[0:8] == "0123abcd"
assert buf[0:12] == "0123abcd4567"
assert buf[0:16] == "0123abcd4567efgh"
assert buf[:] == "0123abcd4567efgh"
assert buf[-1] == "h"
assert buf[-2:] == "gh"
assert buf[-4:] == "efgh"
assert buf[-8:] == "4567efgh"
return True
def unpack_from(fmt, buffer, off=0):
"""
Shim struct.unpack_from and divert unpacking of FileMaps.
Otherwise, you'd get an exception like:
TypeError: unpack_from() argument 1 must be convertible to a buffer, not FileMap
So, we extract a true sub-buffer from the FileMap, and feed this
back into the old unpack function.
Theres an extra allocation and copy, but there's no getting
around that.
"""
if not isinstance(buffer, FileMap):
return old_unpack_from(fmt, buffer, off)
size = calcsize(fmt)
buf = buffer[off:off + size]
return old_unpack_from(fmt, buf, 0x0)
def unpack(fmt, string):
"""
Like the shimmed unpack_from, but for struct.unpack.
"""
if not isinstance(buffer, FileMap):
return old_unpack(fmt, string)
size = calcsize(fmt)
buf = string[:size]
return old_unpack(fmt, buf, 0x0)
def struct_test():
from cStringIO import StringIO
f = StringIO("\x04\x03\x02\x01")
buf = FileMap(f)
assert unpack_from("<B", buf, 0x0)[0] == 0x04
assert unpack_from("<H", buf, 0x0)[0] == 0x0304
assert unpack_from("<I", buf, 0x0)[0] == 0x01020304
def test():
if LRUQueue.test():
print "LRUQueue passed tests."
if BoundedLRUQueue.test():
print "BoundedLRUQueue passed tests."
if RangeCache.test():
print "RangeCache passed tests."
if FileMap.test():
print "FileMap passed tests."
if struct_test():
print "struct passed tests."
if __name__ == "__main__":
test()