import array
import collections
# Placeholder constants
FREE = -1
DUMMY = -2
class Dict(collections.MutableMapping):
'Space efficient dictionary with fast iteration and cheap resizes.'
@staticmethod
def _gen_probes(hashvalue, n):
'Same sequence of probes used in the current dictionary design'
PERTURB_SHIFT = 5
if hashvalue < 0:
hashvalue = int(2 ** 64 - hashvalue)
i = hashvalue & (n-1)
yield int(i)
perturb = hashvalue
while True:
i = 5 * i + perturb + 1
yield int(i & (n-1))
perturb >>= PERTURB_SHIFT
def _lookup(self, key, hashvalue):
'Same lookup logic as currently used in real dicts'
assert len(self.keylist) < len(self.indices) # At least one open slot
freeslot = None
for i in self._gen_probes(hashvalue, len(self.indices)):
index = self.indices[i]
if index == FREE:
return (FREE, i) if freeslot is None else (DUMMY, freeslot)
elif index == DUMMY:
freeslot = i
elif (self.keylist[index] is key or
self.hashlist[index] == hashvalue
and self.keylist[index] == key):
return (index, i)
@staticmethod
def _make_index(n):
'New sequence of indices using the smallest possible datatype'
if n <= 2**7: return array.array('b', [FREE]) * n # signed char
if n <= 2**15: return array.array('h', [FREE]) * n # signed short
if n <= 2**31: return array.array('l', [FREE]) * n # signed long
return [FREE] * n # python integers
def _resize(self, n):
'''Reindex the existing hash/key/value entries.
Entries do not get moved, they only get new indices.
No calls are made to hash() or __eq__().
'''
n = 2 ** n.bit_length() # round-up to power-of-two
self.indices = self._make_index(n)
for index, hashvalue in enumerate(self.hashlist):
for i in Dict._gen_probes(hashvalue, n):
if self.indices[i] == FREE:
break
self.indices[i] = index
self.filled = self.size
def __init__(self, *args, **kwds):
self.indices = self._make_index(8)
self.hashlist = []
self.keylist = []
self.valuelist = []
self.size = 0
self.filled = 0 # size + dummies
self.update(*args, **kwds)
def __getitem__(self, key):
hashvalue = hash(key)
index, i = self._lookup(key, hashvalue)
if index < 0:
raise KeyError(key)
return self.valuelist[index]
def __setitem__(self, key, value):
hashvalue = hash(key)
index, i = self._lookup(key, hashvalue)
if index < 0:
self.indices[i] = self.size
self.hashlist.append(hashvalue)
self.keylist.append(key)
self.valuelist.append(value)
self.size += 1
if index == FREE:
self.filled += 1
if self.filled * 3 > len(self.indices) * 2:
self._resize(2 * len(self))
else:
self.valuelist[index] = value
def __delitem__(self, key):
hashvalue = hash(key)
index, i = self._lookup(key, hashvalue)
if index < 0:
raise KeyError(key)
self.indices[i] = DUMMY
self.size -= 1
# If needed, swap with the lastmost entry to avoid leaving a "hole"
if index != self.size:
lasthash = self.hashlist[-1]
lastkey = self.keylist[-1]
lastvalue = self.valuelist[-1]
lastindex, j = self._lookup(lastkey, lasthash)
assert lastindex >= 0 and i != j
self.indices[j] = index
self.hashlist[index] = lasthash
self.keylist[index] = lastkey
self.valuelist[index] = lastvalue
# Remove the lastmost entry
self.hashlist.pop()
self.keylist.pop()
self.valuelist.pop()
def __iter__(self):
for key in self.keylist:
yield key
def __len__(self):
return self.size
def __repr__(self):
return 'Dict(%r)' % self.items()
def show_structure(self):
'Diagnostic method. Not part of the API.'
print '=' * 50
print self
print 'Indices:', self.indices
for i, row in enumerate(zip(self.hashlist, self.keylist, self.valuelist)):
print i, row
print '-' * 50
if __name__ == '__main__':
d = Dict([('timmy', 'red'), ('barry', 'green'), ('guido', 'blue')])
d.show_structure()
Diff to Previous Revision
--- revision 13 2012-12-24 20:17:27
+++ revision 14 2012-12-26 19:56:23
@@ -87,7 +87,7 @@
self.size += 1
if index == FREE:
self.filled += 1
- if self.filled * 2 > len(self.indices) * 3:
+ if self.filled * 3 > len(self.indices) * 2:
self._resize(2 * len(self))
else:
self.valuelist[index] = value
