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copied!<p>There are a few potential issues, although I don't really see a single "smoking gun"... Anyway, here's what I've got. First:</p> <pre><code>sieve.map(x => if (x % seed == 0 && x > seed) 0 else x).filter(_ > 0) </code></pre> <p>could be written more concisely as:</p> <pre><code>sieve.filter(x => x <= seed || x % seed != 0) </code></pre> <p>Next, <code>upper</code> is unused in <code>filterPrimes</code> (this should have no effect on performance though).</p> <p>Third, don't use a <code>var</code> and a <code>for</code> loop if you want to really use a pure functional style, instead turn <code>filterPrimes</code> into a tail-recursive function. The compiler might be clever enough to optimize away the copies if you do it this way (although I wouldn't hold my breath).</p> <p>Finally, and probably most importantly, your <code>for</code> loop is wasting a huge amount of time filtering out values that have necessarily already been filtered. For example, it tries to filter multiples of 4 after already having filtered all multiples of 2. If you want to use this sieve algorithm efficiently, you need to choose your seeds from the remaining elements in the list.</p> <p>In other words, keep an <em>index</em> into the list, and determine the seed from the index, like:</p> <pre><code>iteration 0: 2 3 4 5 6 7 8 9 ... index: ^ iteration 1: 2 3 5 7 9 ... index: ^ iteration 2: 2 3 5 7 ... index: ^ </code></pre> <p>this avoids the duplicate effort. Also, you don't need to keep iterating until you get to <code>max</code>, I think you can actually stop when you get past <code>sqrt(max)</code>.</p>

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