Initial data commit

Task/Run-length-encoding/JavaScript/run-length-encoding-4.js

@@ -0,0 +1,51 @@
+(() => {
+    'use strict';
+
+    // runLengthEncode :: String -> [(Int, Char)]
+    const runLengthEncoded = s =>
+        group(s.split('')).map(
+            cs => [cs.length, cs[0]]
+        );
+
+    // runLengthDecoded :: [(Int, Char)] -> String
+    const runLengthDecoded = pairs =>
+        pairs.map(([n, c]) => c.repeat(n)).join('');
+
+
+    // ------------------------TEST------------------------
+    const main = () => {
+        const
+            xs = 'WWWWWWWWWWWWBWWWWWWWWWWWWBBBWWWWW' +
+            'WWWWWWWWWWWWWWWWWWWBWWWWWWWWWWWWWW',
+            ys = runLengthEncoded(xs);
+
+        console.log('From: ', show(xs));
+        [ys, runLengthDecoded(ys)].forEach(
+            x => console.log('  ->  ', show(x))
+        )
+    };
+
+    // ----------------------GENERIC-----------------------
+
+    // group :: [a] -> [[a]]
+    const group = xs => {
+        // A list of lists, each containing only equal elements,
+        // such that the concatenation of these lists is xs.
+        const go = xs =>
+            0 < xs.length ? (() => {
+                const
+                    h = xs[0],
+                    i = xs.findIndex(x => h !== x);
+                return i !== -1 ? (
+                    [xs.slice(0, i)].concat(go(xs.slice(i)))
+                ) : [xs];
+            })() : [];
+        return go(xs);
+    };
+
+    // show :: a -> String
+    const show = JSON.stringify;
+
+    // MAIN ---
+    return main();
+})();