I once needed to find several elements in a ~500,000 object array several times a second. And the criterion by which I could judge whether they were the object

I once needed to find several elements in a ~500,000 object array several times a second. And the criterion by which I could judge whether they were the objects I wanted was expensive to compute, and often complex -- involving looking at neighboring or seemingly unrelated objects, or the general program state. And in special cases, I needed to return a filtered slice of the array. There wasn't a library function to do what I needed, and that's a pretty serious understatement. I wrote my own libraries for the job, and still wound up writing one flavor or another of tweaked binary search about once a week.