index.js

module.exports = plugin;

function plugin(opts){
    opts = opts || {};
    opts.exclude = opts.exclude || [];
    opts.min_posts = opts.min_posts || 1
    opts.min_matches = opts.min_matches || 1
    var tags = opts.attribute || "tags"
    return function(files, metalsmith, done){
        setImmediate(done);
        var files2 = files;
        Object.keys(files).forEach(function(file){
            var post = files[file]
            if (typeof post[tags] !== "undefined") {
                var related = Object.keys(files2).map(file2=> {
                    var count = 0;
                    var post2 = files2[file2]
                    if (post !== post2) {
                        for (tag of post[tags]) {
                            if (typeof post2[tags] !== "undefined" && !opts.exclude.includes(tag) && post2[tags].includes(tag)) {
                                count++
                            }
                        }
                    }
                    return {file:file2, count:count}
                }).filter(match => {
                    return files[match.file] !== post && match.count !== 0 && match.count >= opts.min_matches})
                .sort((a,b)=> {
                    return a.count - b.count
                })
                if (related.length < opts.min_posts) {
                    return
                }
                if (typeof opts.max_posts !== "undefined") {
                    related = related.slice(0, opts.max_posts)
                }
                //get full file
                post.related = related.map(match=> {return files[match.file]})
            }
        })
    }
}