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ac_encode.py
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177
ac_encode.py
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## Arithmetic coding compressor and uncompressor for binary source.
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## This is a cleaned-up version of AEncode.py
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BETA0=1;BETA1=1 ## default prior distribution
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M = 30 ; ONE = (1<<M) ; HALF = (1<<(M-1))
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QUARTER = (1<<(M-2)) ; 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 = `c`+`(1-c)`*charstack[0]
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charstack[0]=0
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return a
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pass
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def encode (string, c0=BETA0, c1=BETA1, adaptive=1,verbose=0):
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b=ONE; a=0; tot0=0;tot1=0; assert c0>0; assert c1>0
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if adaptive==0:
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p0 = c0*1.0/(c0+c1)
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pass
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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*1.0/cT
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pass
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boundary = a + int(p0*w)
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if (boundary == a): boundary += 1; print "warningA"; pass # these warnings mean that some of the probabilities
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if (boundary == b): boundary -= 1; print "warningB"; pass # requested by the probabilistic model
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## are so small (compared to our integers) that we had to round them up to bigger values
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if (c=='1') :
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a = boundary
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tot1 += 1 ; if adaptive: c1 += 1.0 ; pass
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elif (c=='0'):
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b = boundary
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tot0 +=1 ; if adaptive: c0 += 1.0 ; pass
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pass ## 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 = ans + clear(1,charstack)
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a = a-HALF ;
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b = b-HALF ;
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else :
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ans = ans + clear(0,charstack)
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pass
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a *= 2 ; b *= 2
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pass
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assert a<=HALF; assert b>=HALF; assert a>=0; 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*a-HALF
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b = 2*b-HALF
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pass
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assert a<=HALF; assert b>=HALF; assert a>=0; assert b<=ONE
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pass
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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 = ans + clear(0,charstack)
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while ( w < HALF ) :
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ans = ans + clear(1,charstack)
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w *=2
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pass
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pass
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else :
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w = (b-HALF) ;
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ans = ans + clear(1,charstack)
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while ( w < HALF ) :
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ans = ans + clear(0,charstack)
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w *=2
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pass
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pass
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return ans
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pass
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def decode (string, N=10000, c0=BETA0, c1=BETA1, adaptive=1,verbose=0):
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## must supply N, the number of source characters remaining.
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b=ONE ; a=0 ; tot0=0;tot1=0 ; assert c0>0 ; assert c1>0
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model_needs_updating = 1
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if adaptive==0:
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p0 = c0*1.0/(c0+c1)
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pass
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ans=""
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u=0 ; v=ONE
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for c in string :
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if N<=0 :
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break ## 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, and halfway is its midpoint.
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## // (a,b) is the current "source alphabet" interval, and boundary is the "midpoint"
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assert u>=0 ; 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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else:
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pass
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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, and tie (u,v) to tag along for the ride.
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while (1): ## condition at end
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firsttime = 0
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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 ; p0 = c0 *1.0/cT
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pass
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boundary = a + int(p0*w)
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if (boundary == a): boundary += 1; print "warningA"; pass
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if (boundary == b): boundary -= 1; print "warningB"; pass
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model_needs_updating = 0
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pass
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if ( boundary <= u ) :
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ans = ans + "1"; tot1 +=1 ; if adaptive: c1 += 1.0 ; pass
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a = boundary ; model_needs_updating = 1 ; N-=1
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elif ( boundary >= v ) :
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ans = ans + "0"; tot0 +=1 ; if adaptive: c0 += 1.0 ; pass
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b = boundary ; model_needs_updating = 1 ; N-=1
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## // every time we discover a source bit, implement exactly the
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## // computations that were done by the encoder (below).
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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, 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 = a-HALF ; b = b-HALF ; u = u-HALF ; v = v-HALF
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pass
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else :
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pass
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a *= 2 ; b *= 2 ; u *= 2 ; v *= 2 ;
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model_needs_updating = 1
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pass
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assert a<=HALF; assert b>=HALF; assert a>=0; 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*a-HALF; b = 2*b-HALF ; u = 2*u-HALF ; v = 2*v-HALF
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pass
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if not (N>0 and model_needs_updating) : ## this is the "while" for this "do" loop
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break
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pass
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pass
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return ans
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pass
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def test():
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sl=["1010"]
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sl=["1010", "111", "00001000000000000000",\
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"1", "10" , "01" , "0" ,"0000000", \
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"000000000000000100000000000000000000000000000000100000000000000000011000000" ]
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for s in sl:
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print "encoding", s
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N=len(s)
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e = encode(s,10,1)
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print "decoding", e
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ds = decode(e,N,10,1)
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print ds
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if (ds != s) :
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print s
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print "ERR@"
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pass
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else:
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print "ok ---------- "
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pass
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pass
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if __name__ == '__main__': test()
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