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@globby
Created March 4, 2014 00:35
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The Fletcher16, Fletcher32 and Fletcher64 algorithms
def Fletcher16(string):
a = map(ord,string)
b = [sum(a[:i])%255 for i in range(len(a)+1)]
return (sum(b) << 8) | max(b)
def Fletcher32(string):
a = map(ord,string)
b = [sum(a[:i])%65535 for i in range(len(a)+1)]
return (sum(b) << 16) | max(b)
def Fletcher64(string):
a = map(ord,string)
b = [sum(a[:i])%4294967295 for i in range(len(a)+1)]
return (sum(b) << 32) | max(b)
@al3rez
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al3rez commented Jun 25, 2018

Python 3.x version:

def fletcher32(string):
    a = list(map(ord,string))
    b = [sum(a[:i])%65535 for i in range(len(a)+1)]
    return (sum(b) << 16) | max(b)

@Anil9860
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Anil9860 commented Nov 27, 2020

I am using the above mentioned code, i should get 4 characters only. But i am getting "221145" Something like this.
Is this Hex data .
http://www.ozeki.hu/p_1614-crc-16-checksum-generator.html
above link, i am using to get Fletchers CRC.

@orangecms
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The above function returns the integer representation, not a string.

You can convert around, e.g., using Python's binascii.hexlify after conversion to bytes:

f = fletcher32("python")
f_bytes = f.to_bytes(4, byteorder='big')
binascii.hexlify(f_bytes)

see also https://docs.python.org/3/library/binascii.html#binascii.hexlify

Another neat web tool for all sorts of things is the GCHQ's CyberChef.
https://gchq.github.io/CyberChef/#recipe=Fletcher-32_Checksum() prints you the hexadecimal representation as well.

@orangecms
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For long inputs, this doesn't seem to work though. I compared the output against CyberChef's for the first paragraph from cipsum.com.

@orangecms
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@mrx23dot
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I think you have to apply modulo before summing.

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