September 2017 Update

Task/Range-extraction/JavaScript/range-extraction-3.js

@@ -1 +1,55 @@
-"0-2,4,6-8,11,12,14-25,27-33,35-39"
+(() => {
+    'use strict';
+
+    // rangeFormat :: [Int] -> String
+    const rangeFormat = xs =>
+        splitBy((a, b) => b - a > 1, xs)
+        .map(rangeString)
+        .join(',');
+
+    // rangeString :: [Int] -> String
+    const rangeString = xs =>
+        xs.length > 2 ? (
+            [head(xs), last(xs)].map(show)
+            .join('-')
+        ) : xs.join(',')
+
+
+    // GENERIC FUNCTIONS
+
+    // Splitting not on a delimiter, but whenever the relationship between
+    // two consecutive items matches a supplied predicate function
+
+    // splitBy :: (a -> a -> Bool) -> [a] -> [[a]]
+    const splitBy = (f, xs) => {
+        if (xs.length < 2) return [xs];
+        const
+            h = head(xs),
+            lstParts = xs.slice(1)
+            .reduce(([acc, active, prev], x) =>
+                f(prev, x) ? (
+                    [acc.concat([active]), [x], x]
+                ) : [acc, active.concat(x), x], [
+                    [],
+                    [h],
+                    h
+                ]);
+        return lstParts[0].concat([lstParts[1]]);
+    };
+
+    // head :: [a] -> a
+    const head = xs => xs.length ? xs[0] : undefined;
+
+    // last :: [a] -> a
+    const last = xs => xs.length ? xs.slice(-1)[0] : undefined;
+
+    // show :: a -> String
+    const show = x => JSON.stringify(x);
+
+    // TEST
+    return rangeFormat([0, 1, 2, 4, 6, 7, 8, 11, 12, 14,
+        15, 16, 17, 18, 19, 20, 21, 22, 23, 24,
+        25, 27, 28, 29, 30, 31, 32, 33, 35, 36,
+        37, 38, 39
+    ]);
+})();