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132 lines
4.1 KiB
Cython
132 lines
4.1 KiB
Cython
cimport cython
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from libcpp.queue cimport priority_queue
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from libcpp.pair cimport pair
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from ..typedefs cimport attr_t
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from .bits cimport bit_append
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from .bits cimport BitArray
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cdef class HuffmanCodec:
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"""Create a Huffman code table, and use it to pack and unpack sequences into
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byte strings. Emphasis is on efficiency, so API is quite strict:
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Messages will be encoded/decoded as indices that refer to the probability sequence.
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For instance, the sequence [5, 10, 8] indicates the 5th most frequent item,
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the 10th most frequent item, the 8th most frequent item.
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Arguments:
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weights (float[:]): A descending-sorted sequence of probabilities/weights.
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Must include a weight for an EOL symbol.
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"""
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def __init__(self, freqs):
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cdef float count
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cdef Code code
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cdef pair[float, int] item
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cdef pair[float, int] item1
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cdef pair[float, int] item2
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cdef priority_queue[pair[float, int]] queue
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cdef int i = 0
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self._map = {}
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self.leaves = []
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for word, weight in freqs:
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item.first = -weight
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item.second = -(i+1)
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queue.push(item)
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self.leaves.append(word)
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code.bits = 0
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code.length = 0
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self.codes.push_back(code)
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self._map[word] = i
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i += 1
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cdef Node node
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while queue.size() >= 2:
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item1 = queue.top(); queue.pop()
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item2 = queue.top(); queue.pop()
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node = Node(left=item1.second, right=item2.second)
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self.nodes.push_back(node)
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item.first = item1.first + item2.first
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item.second = self.nodes.size()-1
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queue.push(item)
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item = queue.top()
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self.root = self.nodes[item.second]
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cdef Code path
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path.bits = 0
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path.length = 0
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assign_codes(self.nodes, self.codes, item.second, path)
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def encode(self, msg, BitArray bits=None):
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if bits is None:
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bits = BitArray()
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cdef int i
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for word in msg:
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i = self._map[word]
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bits.extend(self.codes[i].bits, self.codes[i].length)
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return bits
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def decode(self, bits, msg):
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node = self.root
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cdef int i = 0
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cdef int n = len(msg)
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for bit in bits:
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branch = node.right if bit else node.left
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if branch >= 0:
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node = self.nodes.at(branch)
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else:
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msg[i] = self.leaves[-(branch + 1)]
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node = self.nodes.back()
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i += 1
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if i == n:
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break
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else:
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raise Exception(
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"Buffer exhausted at %d/%d symbols read." % (i, len(msg)))
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property strings:
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@cython.boundscheck(False)
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@cython.wraparound(False)
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@cython.nonecheck(False)
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def __get__(self):
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output = []
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cdef int i, j
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cdef bytes string
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cdef Code code
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for i in range(self.codes.size()):
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code = self.codes[i]
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string = b'{0:b}'.format(code.bits).rjust(code.length, '0')
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string = string[::-1]
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output.append(string)
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return output
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cdef int assign_codes(vector[Node]& nodes, vector[Code]& codes, int i, Code path) except -1:
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"""Recursively assign paths, from the top down. At the end, the entry codes[i]
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knows the bit-address of the node[j] that points to entry i in the vocabulary.
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So, to encode i, we go to codes[i] and read its bit-string. To decode, we
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navigate nodes recursively.
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"""
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cdef Code left_path = bit_append(path, 0)
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cdef Code right_path = bit_append(path, 1)
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# Assign down left branch
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if nodes[i].left >= 0:
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assign_codes(nodes, codes, nodes[i].left, left_path)
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else:
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# Leaf on left
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id_ = -(nodes[i].left + 1)
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codes[id_] = left_path
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# Assign down right branch
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if nodes[i].right >= 0:
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assign_codes(nodes, codes, nodes[i].right, right_path)
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else:
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# Leaf on right
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id_ = -(nodes[i].right + 1)
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codes[id_] = right_path
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