The code to automatically calculate the sitemap chunk size is wrong

I'm not sure what the usefulness of this section of code in xmlsitemap_chunk_count() is:

+  $chunk_count = (integer)($link_count / $chunk_size);
+  if ($chunk_count != ($link_count / $chunk_size)) {
+    $chunk_count++;
+  }

The variable $chunk_count is getting assigned a value and then getting compared to the value again? Why not just do $chunk_count = ceil($link_count / $chunk_size);

The chunk size adjustment code in xmlsitemap_output() would be even better with some inline comments. It was hard for me to see what was happening there and why each calculation was needed.

Why change the default chunk size from 1000 to 50,000? But then leave the chunk-size-changing code in? Why not leave the default at 1000?

I wanted to try and compare this to how the chunk size is calculated in 6.x-2.x, but it seems it's not too different. Whenever the sitemap files are regenerated and the 'auto' chunk-size is enabled, we simply run this function once to reset and recalculate the chunk size value:

function xmlsitemap_get_chunk_size($reset = FALSE) {
  static $size;
  if (!isset($size) || $reset) {
    $size = xmlsitemap_var('chunk_size');
    if ($size === 'auto') {
      $count = max(xmlsitemap_get_link_count($reset), 1); // Prevent divide by zero.
      $size = min(ceil($count / XMLSITEMAP_MAX_SITEMAP_LINKS) * 1000, XMLSITEMAP_MAX_SITEMAP_LINKS);
    }
  }
  return $size;
}

I tested the function with a *lot* of different link counts and think I came up with a pretty good system that doesn't get too wacky:

Links: 500      	Chunk size: 1000  	Chunk count: 1   
Links: 750      	Chunk size: 1000  	Chunk count: 1   
Links: 1000     	Chunk size: 1000  	Chunk count: 1   
Links: 2500     	Chunk size: 1000  	Chunk count: 3   
Links: 5000     	Chunk size: 1000  	Chunk count: 5   
Links: 7500     	Chunk size: 1000  	Chunk count: 8   
Links: 10000    	Chunk size: 1000  	Chunk count: 10  
Links: 25000    	Chunk size: 1000  	Chunk count: 25  
Links: 50000    	Chunk size: 1000  	Chunk count: 50  
Links: 75000    	Chunk size: 2000  	Chunk count: 38  
Links: 100000   	Chunk size: 2000  	Chunk count: 50  
Links: 101000   	Chunk size: 3000  	Chunk count: 34  
Links: 102000   	Chunk size: 3000  	Chunk count: 34  
Links: 250000   	Chunk size: 5000  	Chunk count: 50  
Links: 500000   	Chunk size: 10000 	Chunk count: 50  
Links: 750000   	Chunk size: 15000 	Chunk count: 50  
Links: 1000000  	Chunk size: 20000 	Chunk count: 50  
Links: 2500000  	Chunk size: 50000 	Chunk count: 50  
Links: 5000000  	Chunk size: 50000 	Chunk count: 100 
Links: 7500000  	Chunk size: 50000 	Chunk count: 150 
Links: 10000000 	Chunk size: 50000 	Chunk count: 200 
Links: 25000000 	Chunk size: 50000 	Chunk count: 500 
Links: 50000000 	Chunk size: 50000 	Chunk count: 1000

So we start out with a 1000 minimum chunk size and we focus more on increasing the chunk size and keeping a lower chunk count, because it means fewer files to be generated, and fewer large SQL queries to execute. We don't reach the maximum chunk size until the site has 2.5 million links, which is probably around the high-end of sites using the module.

So in general, I prefer to make larger chunk size adjustments like earnie has proposed.

Currently the 6.x-2.x module will not pass the 50,000 links per chunk, but will go on and create more chunks than the 1000 specification limit. Note that there is already a warning in http://project.davereid.net/api/function/xmlsitemap_requirements/xmlsitemap that says to reduce the number of links, and I've already add another requirements check for more than 50 mil links locally, but just need to commit.