Merge remote-tracking branch 'arithmetic_coding/master'
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ac_encode.py
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251
ac_encode.py
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# Arithmetic coding compressor and decompressor for binary strings.
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# via: http://www.inference.org.uk/mackay/python/compress/ac/ac_encode.py
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# main page: http://www.inference.org.uk/mackay/python/compress/
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# this has been cleaned up (passes pycodestyle) and ported to python 3.
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# default prior distribution
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BETA0 = 1
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BETA1 = 1
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M = 30
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ONE = 1 << M
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HALF = 1 << (M - 1)
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QUARTER = 1 << (M - 2)
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THREEQU = HALF + QUARTER
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def clear(c, charstack):
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# print out character c, and other queued characters
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a = repr(c) + repr(1 - c) * charstack[0]
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charstack[0] = 0
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return a
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def encode(string, c0=BETA0, c1=BETA1, adaptive=True):
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assert c0 > 0
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assert c1 > 0
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b = ONE
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a = 0
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if not adaptive:
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p0 = c0 / (c0 + c1)
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ans = ""
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charstack = [0] # how many undecided characters remain to print
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for c in string:
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w = b - a
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if adaptive:
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cT = c0 + c1
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p0 = c0 / cT
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boundary = a + int(p0 * w)
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# these warnings mean that some of the probabilities
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# requested by the probabilistic model are so small
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# (compared to our integers) that we had to round them up
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# to bigger values.
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if boundary == a:
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boundary += 1
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print("warningA")
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if boundary == b:
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boundary -= 1
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print("warningB")
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if c == '1':
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a = boundary
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if adaptive:
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c1 += 1
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elif c == '0':
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b = boundary
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if adaptive:
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c0 += 1
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# ignore other characters
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while a >= HALF or b <= HALF: # output bits
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if a >= HALF:
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ans += clear(1, charstack)
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a -= HALF
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b -= HALF
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else:
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ans += clear(0, charstack)
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a *= 2
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b *= 2
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assert a <= HALF
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assert b >= HALF
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assert a >= 0
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assert b <= ONE
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# if the gap a-b is getting small, rescale it
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while a > QUARTER and b < THREEQU:
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charstack[0] += 1
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a *= 2
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b *= 2
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a -= HALF
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b -= HALF
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assert a <= HALF
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assert b >= HALF
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assert a >= 0
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assert b <= ONE
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# terminate
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if HALF - a > b - HALF:
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w = HALF - a
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ans += clear(0, charstack)
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while w < HALF:
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ans += clear(1, charstack)
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w *= 2
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else:
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w = b - HALF
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ans += clear(1, charstack)
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while w < HALF:
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ans += clear(0, charstack)
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w *= 2
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return ans
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def decode(string, N, c0=BETA0, c1=BETA1, adaptive=True):
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# must supply N, the number of source characters remaining.
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assert c0 > 0
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assert c1 > 0
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b = ONE
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a = 0
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model_needs_updating = True
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if not adaptive:
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p0 = c0 / (c0 + c1)
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ans = ""
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u = 0
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v = ONE
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for c in string:
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if N <= 0:
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break # out of the string-reading loop
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assert N > 0
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# (u,v) is the current "encoded alphabet" binary interval,
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# and halfway is its midpoint.
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# (a,b) is the current "source alphabet" interval,
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# and boundary is the "midpoint"
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assert u >= 0
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assert v <= ONE
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halfway = u + (v - u) / 2
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if c == '1':
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u = halfway
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elif c == '0':
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v = halfway
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# Read bits until we can decide what the source symbol was.
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# Then emulate the encoder's computations,
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# and tie (u,v) to tag along for the ride.
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while 1: # do-while
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if model_needs_updating:
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w = b - a
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if adaptive:
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cT = c0 + c1
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p0 = c0 / cT
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boundary = a + int(p0 * w)
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if boundary == a:
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boundary += 1
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print("warningA")
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if boundary == b:
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boundary -= 1
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print("warningB")
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model_needs_updating = False
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if boundary <= u:
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ans += "1"
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if adaptive:
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c1 += 1
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a = boundary
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model_needs_updating = True
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N -= 1
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elif boundary >= v:
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ans += "0"
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if adaptive:
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c0 += 1
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b = boundary
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model_needs_updating = True
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N -= 1
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else:
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# not enough bits have yet been read to know the decision.
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pass
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# emulate outputting of bits by the encoder,
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# and tie (u,v) to tag along for the ride.
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while a >= HALF or b <= HALF:
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if a >= HALF:
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a -= HALF
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b -= HALF
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u -= HALF
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v -= HALF
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a *= 2
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b *= 2
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u *= 2
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v *= 2
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model_needs_updating = True
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assert a <= HALF
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assert b >= HALF
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assert a >= 0
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assert b <= ONE
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# if the gap a-b is getting small, rescale it
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while a > QUARTER and b < THREEQU:
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a *= 2
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b *= 2
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u *= 2
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v *= 2
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a -= HALF
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b -= HALF
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u -= HALF
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v -= HALF
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# this is the condition for this do-while loop
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if not (N > 0 and model_needs_updating):
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break
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return ans
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def test():
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tests = [
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"1010",
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"111",
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"00001000000000000000",
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"1",
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"10",
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"01",
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"0",
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"0000000",
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"""
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00000000000000010000000000000000
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00000000000000001000000000000000
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00011000000
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""",
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]
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for s in tests:
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# an ugly way to remove whitespace and newlines from the test strings:
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s = "".join(s.split())
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N = len(s) # required for decoding later.
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print("original:", s)
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e = encode(s, c0=10, c1=1)
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print("encoded: ", e)
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ds = decode(e, N, c0=10, c1=1)
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print("decoded: ", ds)
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if ds != s:
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print("FAIL")
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else:
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print("PASS")
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print()
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if __name__ == '__main__':
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test()
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