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--- code:python_reedsolomon [2008/02/06 16:06] – new code laurenceb
+++ code:python_reedsolomon [2008/07/19 23:33] (current) – external edit 127.0.0.1
@@ Line 1: / Line 1: @@
+===== Main code (uses a c library for decoding, as the tx only has to encode, and we're assuming running binaries on the ground is easy) =====
+The decoder can be found at [[http://hathawaymix.org/Software/ReedSolomon|hathawaymix.org]]
 <code python>
 #!/usr/bin/python
@@ Line 81: / Line 85: @@
 				bb[j] = bb[j-1]
 			bb[0] = 0
+def setup_rs():
 # generate the Galois Field GF(2**mm)
-generate_gf()
+	generate_gf()
-print "Look-up tables for GF(2**%2d)\n",mm
-print "  i   alpha_to[i]  index_of[i]\n"
-for i in range(0,nn):
-	print i,alpha_to[i],index_of[i]
-print "\n\n"
 #compute the generator polynomial for this RS code
-gen_poly()
+	gen_poly()
-#for known data, stick a few numbers into a zero codeword. Data is in
-# polynomial form.
-# for example, say we transmit the following message (nothing special!)
-s=raw_input("enter string ")
-d=0
-for i in s:
-	data[d]=ord(i)
-	d=d+1
-for i in range(len(s),kk):
-	data[i]=0
+def encode_string(s):
+	d=0
+	for i in s:
+		data[d]=ord(i)
+		d=d+1
+	for i in range(len(s),kk):
+		data[i]=0
 # encode data[] to produce parity in bb[].  Data input and parity output
 #  is in polynomial form
+	encode_rs()
+# put the transmitted codeword, made up of data plus parity, in recd[] */
+	for i in range(0,nn-kk):
+		recd[i] = bb[i]
+	for i in range(0,kk):
+		recd[i+nn-kk] = data[i]
+#	print recd
+#	print len(recd)
+	s=''
+	for i in range(2*tt,len(recd)):   #pick all the human readable data
+		s+=chr(recd[i])
+	for i in range(2*tt):
+		s+=chr(recd[i])
+#	print s
+	return s
-encode_rs()
+def decode_string(s,corruption):    #corruption is a list of know corrupted chr positions
+	from reedsolomon import Codec
+	c=Codec(nn,kk,mm)
+	f=''
+	for i in range(2*tt):   #pick all the parity and put @ front
+		f+=s[len(s)+i-2*tt]
+	for i in range(2*tt,len(s)):  #copy over the data
+		f+=s[i-2*tt]
+	t=''
+	for i in range(len(f)):   #reverses the whole string
+		t+=f[len(f)-len(t)-1]
+	#print len(t)
+	#print t
+	t=c.decode(t,corruption)
+	del s
+	s=''
+	b=''
+        for i in t[0]:
+		b+=t[0][len(t[0])-len(b)-1]
+	for i in range(len(b)):
+		s+=b[i]
+	return s,t[1]
-# put the transmitted codeword, made up of data plus parity, in recd[] */
-for i in range(0,nn-kk):
-	recd[i] = bb[i]
-for i in range(0,kk):
-	recd[i+nn-kk] = data[i]
-print recd
-print len(recd)
-s=''
+</code>
-for i in range(len(recd)-1,-1,-1):
-	if i>40 or i<17:
+===== Usage example =====
-		s+=chr(recd[i])
-	else:
+<code python>
-		s+=" "
+import reed_solomon
+reed_solomon.setup_rs()
+s=reed_solomon.encode_string("hello world my name is Laurence and I am your supreme ruler !!!111!!111!")
 print s
-from reedsolomon import Codec
+t=s[0:9]+"df"+s[11:len(s)]
-c=Codec(nn,kk,mm)
+d = reed_solomon.decode_string(t,[])
-print c.decode(s,[86, 87, 88, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101])
+print d[0]
+print "Errors at: ",d[1]
 </code>
+===== produces: =====
+<code>
+$ python testcode.py
+hello world my name is Laurence and I am your supreme ruler !!!111!!111! dJ{,wOWOIUv?M
+         ]'tkN{edkM98
+hello world my name is Laurence and I am your supreme ruler !!!111!!111!
+Errors at:  [84, 85]
+</code>