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October 5, 2016 16:43
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Implementation of the Lempel–Ziv–Storer–Szymanski algorithm in Python
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""" | |
Implementation of the Lempel–Ziv–Storer–Szymanski algorithm, ported from the | |
C implementation by Haruhiko Okumura | |
https://oku.edu.mie-u.ac.jp/~okumura/compression/lzss.c | |
Public Domain | |
""" | |
class LZSSBase(object): | |
def __init__(self, infile, outfile, EI=11, EJ=4, P=1, N=0, F=0, rless=0, init_chr=b' '): | |
self.infile = infile | |
self.outfile = outfile | |
self.EI = EI | |
self.EJ = EJ | |
self.P = P | |
self.N = N or (1 << self.EI) | |
self.F = F or ((1 << self.EJ) + 1) | |
self.rless = 0 | |
if isinstance(init_chr, int): | |
self.init_chr = init_chr | |
else: | |
self.init_chr = init_chr[0] | |
self.buffer = bytearray(self.N * 2) | |
class LZSSEncoder(LZSSBase): | |
def __init__(self, *args, **kwargs): | |
super().__init__(*args, **kwargs) | |
self.bit_buffer = 0 | |
self.bit_mask = 128 | |
self.codecount = 0 | |
self.textcount = 0 | |
def putbit1(self): | |
self.bit_buffer |= self.bit_mask | |
self.bit_mask >>= 1 | |
if self.bit_mask == 0: | |
self.outfile.write(bytes((self.bit_buffer,))) | |
self.bit_buffer = 0 | |
self.bit_mask = 128 | |
self.codecount += 1 | |
def putbit0(self): | |
self.bit_mask >>= 1 | |
if self.bit_mask == 0: | |
self.outfile.write(bytes((self.bit_buffer,))) | |
self.bit_buffer = 0 | |
self.bit_mask = 128 | |
self.codecount += 1 | |
def flush_bit_buffer(self): | |
if self.bit_mask != 128: | |
self.outfile.write(bytes((self.bit_buffer,))) | |
self.codecount += 1 | |
def output1(self, c): | |
self.putbit1() | |
mask = 256 | |
mask >>= 1 | |
while mask: | |
if c & mask: self.putbit1() | |
else: self.putbit0() | |
mask >>= 1 | |
def output2(self, x, y): | |
self.putbit0() | |
mask = self.N >> 1 | |
while mask: | |
if x & mask: self.putbit1() | |
else: self.putbit0() | |
mask >>= 1 | |
mask = (1 << self.EJ) >> 1 | |
while mask: | |
if y & mask: self.putbit1() | |
else: self.putbit0() | |
mask >>= 1 | |
def encode(self): | |
for i in range(0, self.N - self.F): | |
self.buffer[i] = self.init_chr | |
for i in range(self.N - self.F, self.N * 2): | |
bufferend = i | |
z = self.infile.read(1) | |
if len(z) == 0: break | |
self.buffer[i] = z[0] | |
self.textcount += 1 | |
r = self.N - self.F | |
s = 0 | |
while r < bufferend: | |
f1 = min(self.F, bufferend - r) | |
x = 0 | |
y = 1 | |
c = self.buffer[r] | |
for i in range(r - 1, s - 1, -1): | |
if self.buffer[i] == c: | |
for j in range(1, f1): | |
if self.buffer[i + j] != self.buffer[r + j]: break | |
if j > y: | |
x = i | |
y = j | |
if y <= self.P: | |
y = 1 | |
self.output1(c) | |
else: | |
self.output2(x & (self.N - 1), y - 2) | |
r += y | |
s += y | |
if r >= self.N * 2 - self.F: | |
for i in range(0, self.N): | |
self.buffer[i] = self.buffer[i + self.N] | |
bufferend -= self.N | |
r -= self.N | |
s -= self.N | |
while bufferend < self.N * 2: | |
z = self.infile.read(1) | |
if len(z) == 0: break | |
c = z[0] | |
self.buffer[bufferend] = c | |
bufferend += 1 | |
self.textcount += 1 | |
self.flush_bit_buffer() | |
return self.textcount, self.codecount | |
class LZSSDecoder(LZSSBase): | |
def __init__(self, *args, **kwargs): | |
super().__init__(*args, **kwargs) | |
self.bit_buffer = 0 | |
self.bit_mask = 0 | |
def getbit(self, n): | |
x = 0 | |
for i in range(0, n): | |
if self.bit_mask == 0: | |
z = self.infile.read(1) | |
if len(z) == 0: return None | |
self.bit_buffer = z[0] | |
self.bit_mask = 128 | |
x <<= 1 | |
if self.bit_buffer & self.bit_mask: | |
x += 1 | |
self.bit_mask >>= 1 | |
return x | |
def decode(self): | |
for i in range(0, self.N - self.F): | |
self.buffer[i] = self.init_chr | |
r = (self.N - self.F) - self.rless | |
while True: | |
c = self.getbit(1) | |
if c == None: break | |
if c: | |
c = self.getbit(8) | |
if c == None: break | |
self.outfile.write(bytes((c,))) | |
self.buffer[r] = c | |
r = (r + 1) & (self.N - 1) | |
else: | |
i = self.getbit(self.EI) | |
if i == None: break | |
j = self.getbit(self.EJ) | |
if j == None: break | |
for k in range(0, j + 2): | |
c = self.buffer[(i + k) & (self.N - 1)] | |
self.outfile.write(bytes((c,))) | |
self.buffer[r] = c | |
r = (r + 1) & (self.N - 1) | |
def encode(*args, **kwargs): | |
encoder = LZSSEncoder(*args, **kwargs) | |
return encoder.encode() | |
def decode(*args, **kwargs): | |
decoder = LZSSDecoder(*args, **kwargs) | |
return decoder.decode() |
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