September Morn Update

Task/CRC-32/Python/crc-32-3.py

@@ -0,0 +1,62 @@
+'''CRC-32 checksums for ascii strings'''
+
+from functools import (reduce)
+from itertools import (islice)
+
+
+# crc32 :: String -> Int
+def crc32(s):
+    '''CRC-32 checksum for an ASCII encoded string'''
+    def go(x):
+        x2 = x >> 1
+        return 0xedb88320 ^ x2 if x & 1 else x2
+    table = [
+        index(iterate(go)(n))(8)
+        for n in range(0, 256)
+    ]
+    return reduce(
+        lambda a, c: (a >> 8) ^ table[
+            (a ^ ord(c)) & 0xff
+        ],
+        list(s),
+        (0xffffffff)
+    ) ^ 0xffffffff
+
+
+# TEST ----------------------------------------------------
+# main :: IO ()
+def main():
+    '''Test'''
+    print(
+        format(
+            crc32('The quick brown fox jumps over the lazy dog'),
+            '02x'
+        )
+    )
+
+
+# GENERIC ABSTRACTION -------------------------------------
+
+# index (!!) :: [a] -> Int -> a
+def index(xs):
+    '''Item at given (zero-based) index.'''
+    return lambda n: None if 0 > n else (
+        xs[n] if (
+            hasattr(xs, "__getitem__")
+        ) else next(islice(xs, n, None))
+    )
+
+
+# iterate :: (a -> a) -> a -> Gen [a]
+def iterate(f):
+    '''An infinite list of repeated applications of f to x.'''
+    def go(x):
+        v = x
+        while True:
+            yield v
+            v = f(v)
+    return lambda x: go(x)
+
+
+if __name__ == '__main__':
+    main()