September 2017 Update

Task/Hailstone-sequence/JavaScript/hailstone-sequence-9.js

@@ -0,0 +1,58 @@
+(() => {
+    const dctMemo = {};
+
+    // Length only of hailstone sequence
+    // collatzLength :: Int -> Int
+    const collatzLength = n => {
+        let i = 1;
+        let a = n;
+        let lng;
+
+        while (a !== 1) {
+            lng = dctMemo[a];
+            if ('u' === (typeof lng)[0]) {
+                a = (a % 2 ? 3 * a + 1 : a / 2);
+                i++;
+            } else return lng + i - 1;
+        }
+        return i;
+    };
+
+    // range :: Int -> Int -> Maybe Int -> [Int]
+    const range = (m, n, delta) => {
+        const blnUp = n > m,
+            d = blnUp ? (delta || 1) : -(delta || 1),
+            lng = Math.abs(Math.floor((blnUp ? n - m : m - n) / d) + 1),
+            a = Array(lng);
+        let i = lng;
+
+        while (i--) a[i] = (d * i) + m;
+        return a;
+    };
+
+    // longestBelow :: Int -> {Number::Int, Length:Int}
+    const longestBelow = n =>
+        range(1, n)
+        .reduce(
+            (a, x) => {
+                const lng = dctMemo[x] || (dctMemo[x] = collatzLength(x));
+
+                return lng > a.l ? {
+                    n: x,
+                    l: lng
+                } : a
+
+            }, {
+                n: 0,
+                l: 0
+            }
+        );
+
+    // TEST
+    // show :: a -> String
+    const show = x => JSON.stringify(x, null, 2);
+
+    return show(
+        [100000, 1000000, 10000000].map(longestBelow)
+    );
+})();