Fletcher 16 (#9775)

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* Update fletcher16.py

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Christian Clauss <cclauss@me.com>

hashes/fletcher16.py

@@ -0,0 +1,36 @@
+"""
+The Fletcher checksum is an algorithm for computing a position-dependent
+checksum devised by John G. Fletcher (1934–2012) at Lawrence Livermore Labs
+in the late 1970s.[1] The objective of the Fletcher checksum was to
+provide error-detection properties approaching those of a cyclic
+redundancy check but with the lower computational effort associated
+with summation techniques.
+
+Source: https://en.wikipedia.org/wiki/Fletcher%27s_checksum
+"""
+
+
+def fletcher16(text: str) -> int:
+    """
+    Loop through every character in the data and add to two sums.
+
+    >>> fletcher16('hello world')
+    6752
+    >>> fletcher16('onethousandfourhundredthirtyfour')
+    28347
+    >>> fletcher16('The quick brown fox jumps over the lazy dog.')
+    5655
+    """
+    data = bytes(text, "ascii")
+    sum1 = 0
+    sum2 = 0
+    for character in data:
+        sum1 = (sum1 + character) % 255
+        sum2 = (sum1 + sum2) % 255
+    return (sum2 << 8) | sum1
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()