Data commit

Task/Hailstone-sequence/JavaScript/hailstone-sequence-6.js

@@ -0,0 +1,53 @@
+(function (n) {
+
+  var dctMemo = {};
+
+  // Length only of hailstone sequence
+  // n -> n
+  function collatzLength(n) {
+    var i = 1,
+      a = n,
+      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;
+  }
+
+  // Iterative version of range
+  // [m..n]
+  function range(m, n) {
+    var a = Array(n - m + 1),
+      i = n + 1;
+    while (i--) a[i - 1] = i;
+    return a;
+  }
+
+  // Fold/reduce over an array to find the maximum length
+  function longestBelow(n) {
+
+    return range(1, n).reduce(
+      function (a, x) {
+
+        var lng = dctMemo[x] || (dctMemo[x] = collatzLength(x));
+
+        return lng > a.l ? {
+          n: x,
+          l: lng
+        } : a
+
+      }, {
+        n: 0,
+        l: 0
+      }
+    )
+  }
+
+  return [100000, 1000000, 10000000].map(longestBelow);
+
+})();