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POReordering items with multiple order criteria
 

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    <p><strong>Scenario</strong>:</p> <ul> <li>list of photos</li> <li>every photo has the following properties <ul> <li><code>id</code></li> <li><code>sequence_number</code></li> <li><code>main_photo_bit</code></li> </ul></li> <li>the first photo has the <code>main_photo_bit</code> set to <code>1</code> (all others are <code>0</code>)</li> <li>photos are ordered by <code>sequence_number</code> (<em>which is arbitrary</em>)</li> <li>the main photo does not necessarily have the lowest <code>sequence_number</code> (before sorting)</li> </ul> <p>See the following table:</p> <pre><code>id, sequence_number, main_photo_bit 1 10 1 2 5 0 3 20 0 </code></pre> <p>Now you want to change the order by changing the sequence number and main photo bit.</p> <p><strong>Requirements after sorting:</strong></p> <ul> <li>the <code>sequence_number</code> of the first photo is not changed</li> <li>the <code>sequence_number</code> of the first photo is the lowest</li> <li>as less changes as possible</li> </ul> <p><strong>Examples:</strong></p> <p>Example #1 (second photo goes to the first position):</p> <pre><code>id, sequence_number, main_photo_bit 2 10 1 1 15 0 3 20 0 </code></pre> <p><em>This is what happened:</em></p> <ul> <li><em>id 1: new <code>sequence_number</code> and <code>main_photo_bit</code> set to <code>0</code></em></li> <li><em>id 2: old first photo (id 2) <code>sequence_number</code> and <code>main_photo_bit</code> set to <code>1</code></em></li> <li><em>id 3: nothing happens</em></li> </ul> <p>Example #2 (third photo to first position):</p> <pre><code>id, sequence_number, main_photo_bit 3 10 1 1 20 0 2 30 0 </code></pre> <p><em>This is what happened:</em></p> <ul> <li><em>id 1: new <code>sequence_number</code> bigger than first photo and <code>main_photo_bit</code> to <code>0</code></em></li> <li><em>id 2: new <code>sequence_number</code> bigger than newly generated second <code>sequence_number</code></em></li> <li><em>id 3: old first photo <code>sequence_number</code> and <code>main_photo_bit</code> set to <code>1</code></em></li> </ul> <p><strong>What is the best approach to calculate the steps needed to save the new order?</strong></p> <p><em>Edit:</em><br> The reason that I want as less updates as possible is because I want to sync it to an external service, which is a quite costly operation.<br> I already got a working prototype of the algorithm, but it fails in some edge cases. So instead of patching it up (which might work -- but it will become even more complex than it is already), I want to know if there are other (better) ways to do it.<br> In my version (in short) it orders the photos (changing <code>sequence_number</code>'s), and swaps the <code>main_photo_bit</code>, but it isn't sufficient to solve every scenario.</p>
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    1. COMaking sure I understand... `Input` to the problem = `1)` Ordered list of the photo records with their _previous_ values, but in the newly desired order. `2)` Value of the sequence number to be used for the first record (if that info is not available, we can always scan the list until we find the record which has the soon-to-be-flipped main_photo_bit on). `Output` = a list (ordered or not is TBD) of "Commands" to be applied to individual rows so that a) the new list follow the requirements _and_ b) the number of modified rows is the smallest one. Is that it ?
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    2. CO@mjv Yes, that's correct. You have the current ordered list, and you receive a new list from the user in the new desired order. Now you want to save the new order with all _requirements_ met
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    3. CO@Tim but can **all** photos change order at once? (or at least many at the same time) In your examples just one was selected and moved to the top (and my answer assumed that). But if many can change, then that alone might not work.
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