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27 changes: 14 additions & 13 deletions util.py
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Original file line number Diff line number Diff line change
Expand Up @@ -23,25 +23,26 @@ def read_tree (bitreader):
Returns:
A Huffman tree constructed according to the given description.
'''

#dictionary mapping the bit sequence to the tree instance
tree_dict = {
'00' : huffman.TreeLeafEndMessage(),
'01' : lambda i: huffman.TreeLeaf(i),
'1' : lambda l, r: huffman.TreeBranch(l, r)
}

# check first bit
b1 = bitreader.readbit()
if b1 == 1:
left = read_tree(bitreader)
right = read_tree(bitreader)
tree = tree_dict['1'](left, right)
else:
if b1 == 1: # if first bit is 1, then it must be a branch
left = read_tree(bitreader) # apply this function again to each branch
right = read_tree(bitreader) # recursively because it is not a leaf
tree = tree_dict['1'](left, right) # (ie. not end of tree)
else: # else its either a endLeaf or valueLeaf
b2 = bitreader.readbit()
b = b1 + b2
if b == 0:
tree = tree_dict['00']
elif b == 1:
tree = tree_dict['01'](bitreader.readbits(8))
# print(tree)
return tree

def decompress (compressed, uncompressed):
Expand All @@ -56,14 +57,14 @@ def decompress (compressed, uncompressed):
output is written.

'''
bitstream = bitio.BitReader(compressed)
tree = read_tree(bitstream)
while True:
bitstream = bitio.BitReader(compressed) # gets bits from compressed
tree = read_tree(bitstream) # produce tree based on bit sequence
while True: # do final decoding of tree based on remaining bits
val = huffman.decode(tree, bitstream)
if val == None:
if val == None: # stop at Special End Leaf
break
else:
uncompressed.write(bytes([val]))
else: # write the stored values in the tree (ordered by bit sequence)
uncompressed.write(bytes([val])) # as a byte in uncompressed

def write_tree (tree, bitwriter):
'''Write the specified Huffman tree to the given bit writer. The
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