Data commit

Task/Van-Eck-sequence/JavaScript/van-eck-sequence-1.js

@@ -0,0 +1,81 @@
+(() => {
+    'use strict';
+
+    // vanEck :: Int -> [Int]
+    const vanEck = n =>
+        reverse(
+            churchNumeral(n)(
+                xs => 0 < xs.length ? cons(
+                    maybe(
+                        0, succ,
+                        elemIndex(xs[0], xs.slice(1))
+                    ),
+                    xs
+                ) : [0]
+            )([])
+        );
+
+    // TEST -----------------------------------------------
+    const main = () => {
+        console.log('VanEck series:\n')
+        showLog('First 10 terms', vanEck(10))
+        showLog('Terms 991-1000', vanEck(1000).slice(990))
+    };
+
+    // GENERIC FUNCTIONS ----------------------------------
+
+    // Just :: a -> Maybe a
+    const Just = x => ({
+        type: 'Maybe',
+        Nothing: false,
+        Just: x
+    });
+
+    // Nothing :: Maybe a
+    const Nothing = () => ({
+        type: 'Maybe',
+        Nothing: true,
+    });
+
+    // churchNumeral :: Int -> (a -> a) -> a -> a
+    const churchNumeral = n => f => x =>
+        Array.from({
+            length: n
+        }, () => f)
+        .reduce((a, g) => g(a), x)
+
+    // cons :: a -> [a] -> [a]
+    const cons = (x, xs) => [x].concat(xs)
+
+    // elemIndex :: Eq a => a -> [a] -> Maybe Int
+    const elemIndex = (x, xs) => {
+        const i = xs.indexOf(x);
+        return -1 === i ? (
+            Nothing()
+        ) : Just(i);
+    };
+
+    // maybe :: b -> (a -> b) -> Maybe a -> b
+    const maybe = (v, f, m) =>
+        m.Nothing ? v : f(m.Just);
+
+    // reverse :: [a] -> [a]
+    const reverse = xs =>
+        'string' !== typeof xs ? (
+            xs.slice(0).reverse()
+        ) : xs.split('').reverse().join('');
+
+    // showLog :: a -> IO ()
+    const showLog = (...args) =>
+        console.log(
+            args
+            .map(JSON.stringify)
+            .join(' -> ')
+        );
+
+    // succ :: Int -> Int
+    const succ = x => 1 + x;
+
+    // MAIN ---
+    return main();
+})();