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#
# Reference Huffman coding
# Copyright (c) Project Nayuki
#
# https://www.nayuki.io/page/reference-huffman-coding
# https://github.com/nayuki/Reference-Huffman-coding
#
import heapq, sys
python3 = sys.version_info.major >= 3
# ---- Huffman coding core classes ----
# Encodes symbols and writes to a Huffman-coded bit stream.
class HuffmanEncoder(object):
# Constructs a Huffman encoder based on the given bit output stream.
def __init__(self, bitout):
# The underlying bit output stream
self.output = bitout
# The code tree to use in the next write() operation. Must be given a suitable value
# value before calling write(). The tree can be changed after each symbol encoded, as long
# as the encoder and decoder have the same tree at the same point in the code stream.
self.codetree = None
# Encodes the given symbol and writes to the Huffman-coded output stream.
def write(self, symbol):
if not isinstance(self.codetree, CodeTree):
raise ValueError("Invalid current code tree")
bits = self.codetree.get_code(symbol)
for b in bits:
self.output.write(b)
# Reads from a Huffman-coded bit stream and decodes symbols.
class HuffmanDecoder(object):
# Constructs a Huffman decoder based on the given bit input stream.
def __init__(self, bitin):
# The underlying bit input stream
self.input = bitin
# The code tree to use in the next read() operation. Must be given a suitable value
# value before calling read(). The tree can be changed after each symbol decoded, as long
# as the encoder and decoder have the same tree at the same point in the code stream.
self.codetree = None
# Reads from the input stream to decode the next Huffman-coded symbol.
def read(self):
if not isinstance(self.codetree, CodeTree):
raise ValueError("Invalid current code tree")
currentnode = self.codetree.root
while True:
temp = self.input.read_no_eof()
if temp == 0: nextnode = currentnode.leftchild
elif temp == 1: nextnode = currentnode.rightchild
else: raise AssertionError("Invalid value from read_no_eof()")
if isinstance(nextnode, Leaf):
return nextnode.symbol
elif isinstance(nextnode, InternalNode):
currentnode = nextnode
else:
raise AssertionError("Illegal node type")
# A table of symbol frequencies. Mutable. Symbols values are numbered
# from 0 to symbolLimit-1. A frequency table is mainly used like this:
# 0. Collect the frequencies of symbols in the stream that we want to compress.
# 1. Build a code tree that is statically optimal for the current frequencies.
class FrequencyTable(object):
# Constructs a frequency table from the given sequence of frequencies.
# The sequence length must be at least 2, and each value must be non-negative.
def __init__(self, freqs):
self.frequencies = list(freqs) # Make a copy
if len(self.frequencies) < 2:
raise ValueError("At least 2 symbols needed")
if any(x < 0 for x in self.frequencies):
raise ValueError("Negative frequency")
# Returns the number of symbols in this frequency table. The result is always at least 2.
def get_symbol_limit(self):
return len(self.frequencies)
# Returns the frequency of the given symbol in this frequency table. The result is always non-negative.
def get(self, symbol):
self._check_symbol(symbol)
return self.frequencies[symbol]
# Sets the frequency of the given symbol in this frequency table to the given value.
def set(self, symbol, freq):
self._check_symbol(symbol)
if freq < 0:
raise ValueError("Negative frequency")
self.frequencies[symbol] = freq
# Increments the frequency of the given symbol in this frequency table.
def increment(self, symbol):
self._check_symbol(symbol)
self.frequencies[symbol] += 1
# Returns silently if 0 <= symbol < len(frequencies), otherwise raises an exception.
def _check_symbol(self, symbol):
if 0 <= symbol < len(self.frequencies):
return
else:
raise ValueError("Symbol out of range")
# Returns a string representation of this frequency table,
# useful for debugging only, and the format is subject to change.
def __str__(self):
result = ""
for (i, freq) in enumerate(self.frequencies):
result += "{}\t{}\n".format(i, freq)
return result
# Returns a code tree that is optimal for the symbol frequencies in this table.
# The tree always contains at least 2 leaves (even if they come from symbols with
# 0 frequency), to avoid degenerate trees. Note that optimal trees are not unique.
def build_code_tree(self):
# Note that if two nodes have the same frequency, then the tie is broken
# by which tree contains the lowest symbol. Thus the algorithm has a
# deterministic output and does not rely on the queue to break ties.
# Each item in the priority queue is a tuple of type (int frequency,
# int lowestSymbol, Node node). As per Python rules, tuples are ordered asceding
# by the lowest differing index, e.g. (0, 0) < (0, 1) < (0, 2) < (1, 0) < (1, 1).
pqueue = []
# Add leaves for symbols with non-zero frequency
for (i, freq) in enumerate(self.frequencies):
if freq > 0:
heapq.heappush(pqueue, (freq, i, Leaf(i)))
# Pad with zero-frequency symbols until queue has at least 2 items
for (i, freq) in enumerate(self.frequencies):
if len(pqueue) >= 2:
break
if freq == 0:
heapq.heappush(pqueue, (freq, i, Leaf(i)))
assert len(pqueue) >= 2
# Repeatedly tie together two nodes with the lowest frequency
while len(pqueue) > 1:
x = heapq.heappop(pqueue) # Tuple of (frequency, lowest symbol, node object)
y = heapq.heappop(pqueue) # Tuple of (frequency, lowest symbol, node object)
z = (x[0] + y[0], min(x[1], y[1]), InternalNode(x[2], y[2])) # Construct new tuple
heapq.heappush(pqueue, z)
# Return the remaining node
return CodeTree(pqueue[0][2], len(self.frequencies))
# A binary tree that represents a mapping between symbols
# and binary strings. There are two main uses of a code tree:
# - Read the root field and walk through the tree to extract the desired information.
# - Call getCode() to get the binary code for a particular encodable symbol.
# The path to a leaf node determines the leaf's symbol's code. Starting from the root, going
# to the left child represents a 0, and going to the right child represents a 1. Constraints:
# - The root must be an internal node, and the tree is finite.
# - No symbol value is found in more than one leaf.
# - Not every possible symbol value needs to be in the tree.
# Illustrated example:
# Huffman codes:
# 0: Symbol A
# 10: Symbol B
# 110: Symbol C
# 111: Symbol D
# Code tree:
# .
# / \
# A .
# / \
# B .
# / \
# C D
class CodeTree(object):
# Constructs a code tree from the given tree of nodes and given symbol limit.
# Each symbol in the tree must have value strictly less than the symbol limit.
def __init__(self, root, symbollimit):
# Recursive helper function
def build_code_list(node, prefix):
if isinstance(node, InternalNode):
build_code_list(node.leftchild , prefix + (0,))
build_code_list(node.rightchild, prefix + (1,))
elif isinstance(node, Leaf):
if node.symbol >= symbollimit:
raise ValueError("Symbol exceeds symbol limit")
if self.codes[node.symbol] is not None:
raise ValueError("Symbol has more than one code")
self.codes[node.symbol] = prefix
else:
raise AssertionError("Illegal node type")
if symbollimit < 2:
raise ValueError("At least 2 symbols needed")
# The root node of this code tree
self.root = root
# Stores the code for each symbol, or None if the symbol has no code.
# For example, if symbol 5 has code 10011, then codes[5] is the tuple (1,0,0,1,1).
self.codes = [None] * symbollimit
build_code_list(root, ()) # Fill 'codes' with appropriate data
# Returns the Huffman code for the given symbol, which is a sequence of 0s and 1s.
def get_code(self, symbol):
if symbol < 0:
raise ValueError("Illegal symbol")
elif self.codes[symbol] is None:
raise ValueError("No code for given symbol")
else:
return self.codes[symbol]
# Returns a string representation of this code tree,
# useful for debugging only, and the format is subject to change.
def __str__(self):
# Recursive helper function
def to_str(prefix, node):
if isinstance(node, InternalNode):
return to_str(prefix + "0", node.leftchild) + to_str(prefix + "0", node.rightchild)
elif isinstance(node, Leaf):
return "Code {}: Symbol {}\n".format(prefix, node.symbol)
else:
raise AssertionError("Illegal node type")
return to_str("", self.root)
# A node in a code tree. This class has exactly two subclasses: InternalNode, Leaf.
class Node(object):
pass
# An internal node in a code tree. It has two nodes as children.
class InternalNode(Node):
def __init__(self, left, right):
if not isinstance(left, Node) or not isinstance(right, Node):
raise TypeError()
self.leftchild = left
self.rightchild = right
# A leaf node in a code tree. It has a symbol value.
class Leaf(Node):
def __init__(self, sym):
if sym < 0:
raise ValueError("Symbol value must be non-negative")
self.symbol = sym
# A canonical Huffman code, which only describes the code length
# of each symbol. Code length 0 means no code for the symbol.
# The binary codes for each symbol can be reconstructed from the length information.
# In this implementation, lexicographically lower binary codes are assigned to symbols
# with lower code lengths, breaking ties by lower symbol values. For example:
# Code lengths (canonical code):
# Symbol A: 1
# Symbol B: 3
# Symbol C: 0 (no code)
# Symbol D: 2
# Symbol E: 3
# Sorted lengths and symbols:
# Symbol A: 1
# Symbol D: 2
# Symbol B: 3
# Symbol E: 3
# Symbol C: 0 (no code)
# Generated Huffman codes:
# Symbol A: 0
# Symbol D: 10
# Symbol B: 110
# Symbol E: 111
# Symbol C: None
# Huffman codes sorted by symbol:
# Symbol A: 0
# Symbol B: 110
# Symbol C: None
# Symbol D: 10
# Symbol E: 111
class CanonicalCode(object):
# Constructs a canonical code in one of two ways:
# - CanonicalCode(codelengths):
# Builds a canonical Huffman code from the given array of symbol code lengths.
# Each code length must be non-negative. Code length 0 means no code for the symbol.
# The collection of code lengths must represent a proper full Huffman code tree.
# Examples of code lengths that result in under-full Huffman code trees:
# * [1]
# * [3, 0, 3]
# * [1, 2, 3]
# Examples of code lengths that result in correct full Huffman code trees:
# * [1, 1]
# * [2, 2, 1, 0, 0, 0]
# * [3, 3, 3, 3, 3, 3, 3, 3]
# Examples of code lengths that result in over-full Huffman code trees:
# * [1, 1, 1]
# * [1, 1, 2, 2, 3, 3, 3, 3]
# - CanonicalCode(tree, symbollimit):
# Builds a canonical code from the given code tree.
def __init__(self, codelengths=None, tree=None, symbollimit=None):
if codelengths is not None and tree is None and symbollimit is None:
# Check basic validity
if len(codelengths) < 2:
raise ValueError("At least 2 symbols needed")
if any(cl < 0 for cl in codelengths):
raise ValueError("Illegal code length")
# Copy once and check for tree validity
codelens = sorted(codelengths, reverse=True)
currentlevel = codelens[0]
numnodesatlevel = 0
for cl in codelens:
if cl == 0:
break
while cl < currentlevel:
if numnodesatlevel % 2 != 0:
raise ValueError("Under-full Huffman code tree")
numnodesatlevel //= 2
currentlevel -= 1
numnodesatlevel += 1
while currentlevel > 0:
if numnodesatlevel % 2 != 0:
raise ValueError("Under-full Huffman code tree")
numnodesatlevel //= 2
currentlevel -= 1
if numnodesatlevel < 1:
raise ValueError("Under-full Huffman code tree")
if numnodesatlevel > 1:
raise ValueError("Over-full Huffman code tree")
# Copy again
self.codelengths = list(codelengths)
elif tree is not None and symbollimit is not None and codelengths is None:
# Recursive helper method
def build_code_lengths(node, depth):
if isinstance(node, InternalNode):
build_code_lengths(node.leftchild , depth + 1)
build_code_lengths(node.rightchild, depth + 1)
elif isinstance(node, Leaf):
if node.symbol >= len(self.codelengths):
raise ValueError("Symbol exceeds symbol limit")
# Note: CodeTree already has a checked constraint that disallows a symbol in multiple leaves
if self.codelengths[node.symbol] != 0:
raise AssertionError("Symbol has more than one code")
self.codelengths[node.symbol] = depth
else:
raise AssertionError("Illegal node type")
if symbollimit < 2:
raise ValueError("At least 2 symbols needed")
self.codelengths = [0] * symbollimit
build_code_lengths(tree.root, 0)
else:
raise ValueError("Invalid arguments")
# Returns the symbol limit for this canonical Huffman code.
# Thus this code covers symbol values from 0 to symbolLimit-1.
def get_symbol_limit(self):
return len(self.codelengths)
# Returns the code length of the given symbol value. The result is 0
# if the symbol has node code; otherwise the result is a positive number.
def get_code_length(self, symbol):
if 0 <= symbol < len(self.codelengths):
return self.codelengths[symbol]
else:
raise ValueError("Symbol out of range")
# Returns the canonical code tree for this canonical Huffman code.
def to_code_tree(self):
nodes = []
for i in range(max(self.codelengths), -1, -1): # Descend through code lengths
assert len(nodes) % 2 == 0
newnodes = []
# Add leaves for symbols with positive code length i
if i > 0:
for (j, codelen) in enumerate(self.codelengths):
if codelen == i:
newnodes.append(Leaf(j))
# Merge pairs of nodes from the previous deeper layer
for j in range(0, len(nodes), 2):
newnodes.append(InternalNode(nodes[j], nodes[j + 1]))
nodes = newnodes
assert len(nodes) == 1
return CodeTree(nodes[0], len(self.codelengths))
# ---- Bit-oriented I/O streams ----
# A stream of bits that can be read. Because they come from an underlying byte stream,
# the total number of bits is always a multiple of 8. The bits are read in big endian.
class BitInputStream(object):
# Constructs a bit input stream based on the given byte input stream.
def __init__(self, inp):
# The underlying byte stream to read from
self.input = inp
# Either in the range [0x00, 0xFF] if bits are available, or -1 if end of stream is reached
self.currentbyte = 0
# Number of remaining bits in the current byte, always between 0 and 7 (inclusive)
self.numbitsremaining = 0
# Reads a bit from this stream. Returns 0 or 1 if a bit is available, or -1 if
# the end of stream is reached. The end of stream always occurs on a byte boundary.
def read(self):
if self.currentbyte == -1:
return -1
if self.numbitsremaining == 0:
temp = self.input.read(1)
if len(temp) == 0:
self.currentbyte = -1
return -1
self.currentbyte = temp[0] if python3 else ord(temp)
self.numbitsremaining = 8
assert self.numbitsremaining > 0
self.numbitsremaining -= 1
return (self.currentbyte >> self.numbitsremaining) & 1
# Reads a bit from this stream. Returns 0 or 1 if a bit is available, or raises an EOFError
# if the end of stream is reached. The end of stream always occurs on a byte boundary.
def read_no_eof(self):
result = self.read()
if result != -1:
return result
else:
raise EOFError()
# Closes this stream and the underlying input stream.
def close(self):
self.input.close()
self.currentbyte = -1
self.numbitsremaining = 0
# A stream where bits can be written to. Because they are written to an underlying
# byte stream, the end of the stream is padded with 0's up to a multiple of 8 bits.
# The bits are written in big endian.
class BitOutputStream(object):
# Constructs a bit output stream based on the given byte output stream.
def __init__(self, out):
self.output = out # The underlying byte stream to write to
self.currentbyte = 0 # The accumulated bits for the current byte, always in the range [0x00, 0xFF]
self.numbitsfilled = 0 # Number of accumulated bits in the current byte, always between 0 and 7 (inclusive)
# Writes a bit to the stream. The given bit must be 0 or 1.
def write(self, b):
if b not in (0, 1):
raise ValueError("Argument must be 0 or 1")
self.currentbyte = (self.currentbyte << 1) | b
self.numbitsfilled += 1
if self.numbitsfilled == 8:
towrite = bytes((self.currentbyte,)) if python3 else chr(self.currentbyte)
self.output.write(towrite)
self.currentbyte = 0
self.numbitsfilled = 0
# Closes this stream and the underlying output stream. If called when this
# bit stream is not at a byte boundary, then the minimum number of "0" bits
# (between 0 and 7 of them) are written as padding to reach the next byte boundary.
def close(self):
while self.numbitsfilled != 0:
self.write(0)
self.output.close()
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