Code shared from run.dlang.io. Run with '-unittest'

main.d

import std.stdio : writeln, toFile;
import std.datetime.stopwatch : StopWatch, AutoStart;
import asdf.serialization : deserialize, serializeToJson;
import std.file : readText;
import std.array: array;
import std.algorithm : copy, map;

enum TopN = 5;

struct Post
{
	string _id;
	string title;
	string[] tags;
}

struct RelatedPosts
{
	string _id;
	string[] tags;
	Post[TopN] related;
}

struct PostsWithSharedTags
{
	ulong post;
	uint sharedTags;
}

void main()
{
	auto jsonText = readText("posts.json");
	auto posts = deserialize!(Post[])(jsonText);
	int postsCount = cast(int) posts.length;

	ulong[][string] tagMap;

	auto sw = StopWatch(AutoStart.yes);

	foreach (i, post; posts)
		foreach (tag; post.tags)
			tagMap[tag] ~= i;
				
	auto mapPreparation = sw.peek.total!"usecs" * 1.0 / 1000;

	auto relatedPosts = new RelatedPosts[postsCount];
	auto taggedPostsCount = new uint[postsCount];

	foreach (k, post; posts)
	{
		taggedPostsCount[] = 0;

		PostsWithSharedTags[TopN] top5;
		Post[TopN] topPosts;

		foreach (tag; post.tags)
			foreach (idx; tagMap[tag])
				taggedPostsCount[0]++;

		taggedPostsCount[k] = 0;

		auto minTags = 0;
		foreach (j, count; taggedPostsCount)
		{
			if (count > minTags)
			{
				// Find position to insert
				auto pos = 4;
				while (pos >= 0 && top5[pos].sharedTags < count)
				{
					pos--;
				}
				pos++;

				// Shift and insert
				if (pos < 4)
				{
					copy(top5[pos .. 4], top5[(pos + 1) .. $]);
				}
				top5[pos] = PostsWithSharedTags(j, count);
				minTags = top5[4].sharedTags;
			}
		}

		topPosts = top5[].map!(a => posts[a.post]).array;

		relatedPosts[k] = RelatedPosts(
			post._id,
			post.tags,
			topPosts
		);
	}
	sw.stop();

	 writeln("Map preparation (w/o IO): ", mapPreparation, "ms");
	 writeln("Processing time (w/o IO): ", sw.peek.total!"usecs" * 1.0 / 1000, "ms");
}