# Space Efficient Dict¶

## Question¶

Designing a space efficient dictionary.

## Solution¶

```import array
import collections
import itertools

# Placeholder constants
FREE = -1
DUMMY = -2

class Dict(collections.MutableMapping):
'Space efficient dictionary with fast iteration and cheap resizes.'

@staticmethod
'Same sequence of probes used in the current dictionary design'
PERTURB_SHIFT = 5
if hashvalue < 0:
hashvalue = -hashvalue
yield i
perturb = hashvalue
while True:
i = (5 * i + perturb + 1)
perturb >>= PERTURB_SHIFT

def _lookup(self, key, hashvalue):
'Same lookup logic as currently used in real dicts'
assert self.filled < len(self.indices)   # At least one open slot
freeslot = None
for i in self._gen_probes(hashvalue, len(self.indices)-1):
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-1):
if self.indices[i] == FREE:
break
self.indices[i] = index
self.filled = self.used

def clear(self):
self.indices = self._make_index(8)
self.hashlist = []
self.keylist = []
self.valuelist = []
self.used = 0
self.filled = 0                                         # used + dummies

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.used
self.hashlist.append(hashvalue)
self.keylist.append(key)
self.valuelist.append(value)
self.used += 1
if index == FREE:
self.filled += 1
if self.filled * 3 > len(self.indices) * 2:
self._resize(4 * 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.used -= 1
# If needed, swap with the lastmost entry to avoid leaving a "hole"
if index != self.used:
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 __init__(self, *args, **kwds):
if not hasattr(self, 'keylist'):
self.clear()
self.update(*args, **kwds)

def __len__(self):
return self.used

def __iter__(self):
return iter(self.keylist)

def iterkeys(self):
return iter(self.keylist)

def keys(self):
return list(self.keylist)

def itervalues(self):
return iter(self.valuelist)

def values(self):
return list(self.valuelist)

def iteritems(self):
return itertools.izip(self.keylist, self.valuelist)

def items(self):
return zip(self.keylist, self.valuelist)

def __contains__(self, key):
index, i = self._lookup(key, hash(key))
return index >= 0

def get(self, key, default=None):
index, i = self._lookup(key, hash(key))
return self.valuelist[index] if index >= 0 else default

def popitem(self):
if not self.keylist:
raise KeyError('popitem(): dictionary is empty')
key = self.keylist[-1]
value = self.valuelist[-1]
del self[key]
return key, value

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()
```