64 lines
1.7 KiB
Python
64 lines
1.7 KiB
Python
"""
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Generate constant hash tables.
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The `constant_hash` module can generate constant pre-populated hash tables. We
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don't attempt perfect hashing, but simply generate an open addressed
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quadratically probed hash table.
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"""
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from __future__ import absolute_import
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from cdsl import next_power_of_two
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try:
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from typing import Any, List, Iterable, Callable # noqa
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except ImportError:
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pass
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def simple_hash(s):
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# type: (str) -> int
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"""
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Compute a primitive hash of a string.
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Example:
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>>> "0x%x" % simple_hash("Hello")
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'0x2fa70c01'
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>>> "0x%x" % simple_hash("world")
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'0x5b0c31d5'
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"""
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h = 5381
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for c in s:
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h = ((h ^ ord(c)) + ((h >> 6) + (h << 26))) & 0xffffffff
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return h
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def compute_quadratic(items, hash_function):
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# type: (Iterable[Any], Callable[[Any], int]) -> List[Any]
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"""
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Compute an open addressed, quadratically probed hash table containing
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`items`. The returned table is a list containing the elements of the
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iterable `items` and `None` in unused slots.
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:param items: Iterable set of items to place in hash table.
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:param hash_function: Hash function which takes an item and returns a
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number.
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Simple example (see hash values above, they collide on slot 1):
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>>> compute_quadratic(['Hello', 'world'], simple_hash)
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[None, 'Hello', 'world', None]
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"""
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items = list(items)
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# Table size must be a power of two. Aim for >20% unused slots.
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size = next_power_of_two(int(1.20*len(items)))
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table = [None] * size # type: List[Any]
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for i in items:
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h = hash_function(i) % size
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s = 0
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while table[h] is not None:
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s += 1
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h = (h + s) % size
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table[h] = i
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return table
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