September Morn Update

Task/CRC-32/JavaScript/crc-32.js

@@ -0,0 +1,87 @@
+(() => {
+    'use strict';
+
+    const main = () =>
+        showHex(
+            crc32('The quick brown fox jumps over the lazy dog')
+        );
+
+    // crc32 :: String -> Int
+    const crc32 = str => {
+
+        // table :: [Int]
+        const table = map(
+            n => take(9,
+                iterate(
+                    x => (
+                        x & 1 ? z => 0xEDB88320 ^ z : id
+                    )(x >>> 1),
+                    n
+                )
+            )[8],
+            enumFromTo(0, 255)
+        );
+        return (
+            foldl(
+                (a, c) => (a >>> 8) ^ table[
+                    (a ^ c.charCodeAt(0)) & 255
+                ],
+                -1,
+                chars(str)
+            ) ^ -1
+        );
+    };
+
+    // GENERIC ABSTRACTIONS -------------------------------
+
+    // chars :: String -> [Char]
+    const chars = s => s.split('');
+
+    // enumFromTo :: Int -> Int -> [Int]
+    const enumFromTo = (m, n) =>
+        Array.from({
+            length: 1 + n - m
+        }, (_, i) => m + i);
+
+    // foldl :: (a -> b -> a) -> a -> [b] -> a
+    const foldl = (f, a, xs) => xs.reduce(f, a);
+
+    // id :: a -> a
+    const id = x => x;
+
+    // iterate :: (a -> a) -> a -> Gen [a]
+    function* iterate(f, x) {
+        let v = x;
+        while (true) {
+            yield(v);
+            v = f(v);
+        }
+    }
+
+    // map :: (a -> b) -> [a] -> [b]
+    const map = (f, xs) => xs.map(f);
+
+    // showHex :: Int -> String
+    const showHex = n =>
+        n.toString(16);
+
+    // take :: Int -> [a] -> [a]
+    // take :: Int -> String -> String
+    const take = (n, xs) =>
+        xs.constructor.constructor.name !== 'GeneratorFunction' ? (
+            xs.slice(0, n)
+        ) : [].concat.apply([], Array.from({
+            length: n
+        }, () => {
+            const x = xs.next();
+            return x.done ? [] : [x.value];
+        }));
+
+
+    // MAIN -------------
+    const result = main();
+    return (
+        console.log(result),
+        result
+    );
+})();