intervals

Three decisions decide whether this is correct: the numeric type, how partial hours are handled, and what happens to a record that does not make sense. The implementation with its tests, the floating-point trap that costs real money, and the questions to ask before writing any of it.

The union is a sort and a scan. The intersection is the same sweep with a counter, and it is the one that goes wrong, because a single GPU reporting overlapping intervals will be counted twice and produce an answer that looks plausible. Both implementations, the trap, and the tests that catch it.

The arithmetic is one multiplication and the difficulty is entirely in the events that do not pair cleanly. Clipping to the window, an allocation still running when the period ends, a stop with no start, and why every anomaly must be reported rather than dropped.