/usr/share/pyshared/allmydata/util/spans.py is in tahoe-lafs 1.9.2-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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"""I represent a compressed list of booleans, one per index (an integer).
Typically, each index represents an offset into a large string, pointing
to a specific byte of a share. In this context, True means that byte has
been received, or has been requested.
Another way to look at this is maintaining a set of integers, optimized
for operations on spans like 'add range to set' and 'is range in set?'.
This is a python equivalent of perl's Set::IntSpan module, frequently
used to represent .newsrc contents.
Rather than storing an actual (large) list or dictionary, I represent my
internal state as a sorted list of spans, each with a start and a length.
My API is presented in terms of start+length pairs. I provide set
arithmetic operators, to efficiently answer questions like 'I want bytes
XYZ, I already requested bytes ABC, and I've already received bytes DEF:
what bytes should I request now?'.
The new downloader will use it to keep track of which bytes we've requested
or received already.
"""
def __init__(self, _span_or_start=None, length=None):
self._spans = list()
if length is not None:
self._spans.append( (_span_or_start, length) )
elif _span_or_start:
for (start,length) in _span_or_start:
self.add(start, length)
self._check()
def _check(self):
assert sorted(self._spans) == self._spans
prev_end = None
try:
for (start,length) in self._spans:
if prev_end is not None:
assert start > prev_end
prev_end = start+length
except AssertionError:
print "BAD:", self.dump()
raise
def add(self, start, length):
assert start >= 0
assert length > 0
#print " ADD [%d+%d -%d) to %s" % (start, length, start+length, self.dump())
first_overlap = last_overlap = None
for i,(s_start,s_length) in enumerate(self._spans):
#print " (%d+%d)-> overlap=%s adjacent=%s" % (s_start,s_length, overlap(s_start, s_length, start, length), adjacent(s_start, s_length, start, length))
if (overlap(s_start, s_length, start, length)
or adjacent(s_start, s_length, start, length)):
last_overlap = i
if first_overlap is None:
first_overlap = i
continue
# no overlap
if first_overlap is not None:
break
#print " first_overlap", first_overlap, last_overlap
if first_overlap is None:
# no overlap, so just insert the span and sort by starting
# position.
self._spans.insert(0, (start,length))
self._spans.sort()
else:
# everything from [first_overlap] to [last_overlap] overlapped
first_start,first_length = self._spans[first_overlap]
last_start,last_length = self._spans[last_overlap]
newspan_start = min(start, first_start)
newspan_end = max(start+length, last_start+last_length)
newspan_length = newspan_end - newspan_start
newspan = (newspan_start, newspan_length)
self._spans[first_overlap:last_overlap+1] = [newspan]
#print " ADD done: %s" % self.dump()
self._check()
return self
def remove(self, start, length):
assert start >= 0
assert length > 0
#print " REMOVE [%d+%d -%d) from %s" % (start, length, start+length, self.dump())
first_complete_overlap = last_complete_overlap = None
for i,(s_start,s_length) in enumerate(self._spans):
s_end = s_start + s_length
o = overlap(s_start, s_length, start, length)
if o:
o_start, o_length = o
o_end = o_start+o_length
if o_start == s_start and o_end == s_end:
# delete this span altogether
if first_complete_overlap is None:
first_complete_overlap = i
last_complete_overlap = i
elif o_start == s_start:
# we only overlap the left side, so trim the start
# 1111
# rrrr
# oo
# -> 11
new_start = o_end
new_end = s_end
assert new_start > s_start
new_length = new_end - new_start
self._spans[i] = (new_start, new_length)
elif o_end == s_end:
# we only overlap the right side
# 1111
# rrrr
# oo
# -> 11
new_start = s_start
new_end = o_start
assert new_end < s_end
new_length = new_end - new_start
self._spans[i] = (new_start, new_length)
else:
# we overlap the middle, so create a new span. No need to
# examine any other spans.
# 111111
# rr
# LL RR
left_start = s_start
left_end = o_start
left_length = left_end - left_start
right_start = o_end
right_end = s_end
right_length = right_end - right_start
self._spans[i] = (left_start, left_length)
self._spans.append( (right_start, right_length) )
self._spans.sort()
break
if first_complete_overlap is not None:
del self._spans[first_complete_overlap:last_complete_overlap+1]
#print " REMOVE done: %s" % self.dump()
self._check()
return self
def dump(self):
return "len=%d: %s" % (self.len(),
",".join(["[%d-%d]" % (start,start+l-1)
for (start,l) in self._spans]) )
def each(self):
for start, length in self._spans:
for i in range(start, start+length):
yield i
def __iter__(self):
for s in self._spans:
yield s
def __nonzero__(self): # this gets us bool()
return bool(self.len())
def len(self):
# guess what! python doesn't allow __len__ to return a long, only an
# int. So we stop using len(spans), use spans.len() instead.
return sum([length for start,length in self._spans])
def __add__(self, other):
s = self.__class__(self)
for (start, length) in other:
s.add(start, length)
return s
def __sub__(self, other):
s = self.__class__(self)
for (start, length) in other:
s.remove(start, length)
return s
def __iadd__(self, other):
for (start, length) in other:
self.add(start, length)
return self
def __isub__(self, other):
for (start, length) in other:
self.remove(start, length)
return self
def __and__(self, other):
if not self._spans:
return self.__class__()
bounds = self.__class__(self._spans[0][0],
self._spans[-1][0]+self._spans[-1][1])
not_other = bounds - other
return self - not_other
def __contains__(self, (start,length)):
for span_start,span_length in self._spans:
o = overlap(start, length, span_start, span_length)
if o:
o_start,o_length = o
if o_start == start and o_length == length:
return True
return False
def overlap(start0, length0, start1, length1):
# return start2,length2 of the overlapping region, or None
# 00 00 000 0000 00 00 000 00 00 00 00
# 11 11 11 11 111 11 11 1111 111 11 11
left = max(start0, start1)
right = min(start0+length0, start1+length1)
# if there is overlap, 'left' will be its start, and right-1 will
# be the end'
if left < right:
return (left, right-left)
return None
def adjacent(start0, length0, start1, length1):
if (start0 < start1) and start0+length0 == start1:
return True
elif (start1 < start0) and start1+length1 == start0:
return True
return False
class DataSpans:
"""I represent portions of a large string. Equivalently, I can be said to
maintain a large array of characters (with gaps of empty elements). I can
be used to manage access to a remote share, where some pieces have been
retrieved, some have been requested, and others have not been read.
"""
def __init__(self, other=None):
self.spans = [] # (start, data) tuples, non-overlapping, merged
if other:
for (start, data) in other.get_chunks():
self.add(start, data)
def __nonzero__(self): # this gets us bool()
return bool(self.len())
def len(self):
# return number of bytes we're holding
return sum([len(data) for (start,data) in self.spans])
def _dump(self):
# return iterator of sorted list of offsets, one per byte
for (start,data) in self.spans:
for i in range(start, start+len(data)):
yield i
def dump(self):
return "len=%d: %s" % (self.len(),
",".join(["[%d-%d]" % (start,start+len(data)-1)
for (start,data) in self.spans]) )
def get_chunks(self):
return list(self.spans)
def get_spans(self):
"""Return a Spans object with a bit set for each byte I hold"""
return Spans([(start, len(data)) for (start,data) in self.spans])
def assert_invariants(self):
if not self.spans:
return
prev_start = self.spans[0][0]
prev_end = prev_start + len(self.spans[0][1])
for start, data in self.spans[1:]:
if not start > prev_end:
# adjacent or overlapping: bad
print "ASSERTION FAILED", self.spans
raise AssertionError
def get(self, start, length):
# returns a string of LENGTH, or None
#print "get", start, length, self.spans
end = start+length
for (s_start,s_data) in self.spans:
s_end = s_start+len(s_data)
#print " ",s_start,s_end
if s_start <= start < s_end:
# we want some data from this span. Because we maintain
# strictly merged and non-overlapping spans, everything we
# want must be in this span.
offset = start - s_start
if offset + length > len(s_data):
#print " None, span falls short"
return None # span falls short
#print " some", s_data[offset:offset+length]
return s_data[offset:offset+length]
if s_start >= end:
# we've gone too far: no further spans will overlap
#print " None, gone too far"
return None
#print " None, ran out of spans"
return None
def add(self, start, data):
# first: walk through existing spans, find overlap, modify-in-place
# create list of new spans
# add new spans
# sort
# merge adjacent spans
#print "add", start, data, self.spans
end = start + len(data)
i = 0
while len(data):
#print " loop", start, data, i, len(self.spans), self.spans
if i >= len(self.spans):
#print " append and done"
# append a last span
self.spans.append( (start, data) )
break
(s_start,s_data) = self.spans[i]
# five basic cases:
# a: OLD b:OLDD c1:OLD c2:OLD d1:OLDD d2:OLD e: OLLDD
# NEW NEW NEW NEWW NEW NEW NEW
#
# we handle A by inserting a new segment (with "N") and looping,
# turning it into B or C. We handle B by replacing a prefix and
# terminating. We handle C (both c1 and c2) by replacing the
# segment (and, for c2, looping, turning it into A). We handle D
# by replacing a suffix (and, for d2, looping, turning it into
# A). We handle E by replacing the middle and terminating.
if start < s_start:
# case A: insert a new span, then loop with the remainder
#print " insert new span"
s_len = s_start-start
self.spans.insert(i, (start, data[:s_len]))
i += 1
start = s_start
data = data[s_len:]
continue
s_len = len(s_data)
s_end = s_start+s_len
if s_start <= start < s_end:
#print " modify this span", s_start, start, s_end
# we want to modify some data in this span: a prefix, a
# suffix, or the whole thing
if s_start == start:
if s_end <= end:
#print " replace whole segment"
# case C: replace this segment
self.spans[i] = (s_start, data[:s_len])
i += 1
start += s_len
data = data[s_len:]
# C2 is where len(data)>0
continue
# case B: modify the prefix, retain the suffix
#print " modify prefix"
self.spans[i] = (s_start, data + s_data[len(data):])
break
if start > s_start and end < s_end:
# case E: modify the middle
#print " modify middle"
prefix_len = start - s_start # we retain this much
suffix_len = s_end - end # and retain this much
newdata = s_data[:prefix_len] + data + s_data[-suffix_len:]
self.spans[i] = (s_start, newdata)
break
# case D: retain the prefix, modify the suffix
#print " modify suffix"
prefix_len = start - s_start # we retain this much
suffix_len = s_len - prefix_len # we replace this much
#print " ", s_data, prefix_len, suffix_len, s_len, data
self.spans[i] = (s_start,
s_data[:prefix_len] + data[:suffix_len])
i += 1
start += suffix_len
data = data[suffix_len:]
#print " now", start, data
# D2 is where len(data)>0
continue
# else we're not there yet
#print " still looking"
i += 1
continue
# now merge adjacent spans
#print " merging", self.spans
newspans = []
for (s_start,s_data) in self.spans:
if newspans and adjacent(newspans[-1][0], len(newspans[-1][1]),
s_start, len(s_data)):
newspans[-1] = (newspans[-1][0], newspans[-1][1] + s_data)
else:
newspans.append( (s_start, s_data) )
self.spans = newspans
self.assert_invariants()
#print " done", self.spans
def remove(self, start, length):
i = 0
end = start + length
#print "remove", start, length, self.spans
while i < len(self.spans):
(s_start,s_data) = self.spans[i]
if s_start >= end:
# this segment is entirely right of the removed region, and
# all further segments are even further right. We're done.
break
s_len = len(s_data)
s_end = s_start + s_len
o = overlap(start, length, s_start, s_len)
if not o:
i += 1
continue
o_start, o_len = o
o_end = o_start + o_len
if o_len == s_len:
# remove the whole segment
del self.spans[i]
continue
if o_start == s_start:
# remove a prefix, leaving the suffix from o_end to s_end
prefix_len = o_end - o_start
self.spans[i] = (o_end, s_data[prefix_len:])
i += 1
continue
elif o_end == s_end:
# remove a suffix, leaving the prefix from s_start to o_start
prefix_len = o_start - s_start
self.spans[i] = (s_start, s_data[:prefix_len])
i += 1
continue
# remove the middle, creating a new segment
# left is s_start:o_start, right is o_end:s_end
left_len = o_start - s_start
left = s_data[:left_len]
right_len = s_end - o_end
right = s_data[-right_len:]
self.spans[i] = (s_start, left)
self.spans.insert(i+1, (o_end, right))
break
#print " done", self.spans
def pop(self, start, length):
data = self.get(start, length)
if data:
self.remove(start, length)
return data
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