54 lines
926 B
JavaScript
54 lines
926 B
JavaScript
(function (n) {
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var dctMemo = {};
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// Length only of hailstone sequence
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// n -> n
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function collatzLength(n) {
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var i = 1,
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a = n,
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lng;
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while (a !== 1) {
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lng = dctMemo[a];
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if ('u' === (typeof lng)[0]) {
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a = (a % 2 ? 3 * a + 1 : a / 2);
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i++;
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} else return lng + i - 1;
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}
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return i;
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}
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// Iterative version of range
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// [m..n]
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function range(m, n) {
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var a = Array(n - m + 1),
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i = n + 1;
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while (i--) a[i - 1] = i;
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return a;
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}
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// Fold/reduce over an array to find the maximum length
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function longestBelow(n) {
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return range(1, n).reduce(
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function (a, x) {
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var lng = dctMemo[x] || (dctMemo[x] = collatzLength(x));
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return lng > a.l ? {
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n: x,
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l: lng
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} : a
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}, {
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n: 0,
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l: 0
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}
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)
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}
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return [100000, 1000000, 10000000].map(longestBelow);
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})();
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