Sliding Windows

Grouping & Intervals covers the window that grows on the right and shrinks on a repeat. IBM's assessment asks for a few other kinds. A window can hold a fixed number of characters, a span of seconds that widens and narrows with the traffic, or a running product that has to be trimmed whenever it goes over a target. The loop looks the same in all three, and only the rule for moving the left edge changes.

These are the log-processing problems that run through IBM's prompts, the grouping, deduplicating and counting over a window. With intervals included they're close to 29% of IBM's coding, and any of them could arrive as a log. One looks for a pattern that keeps coming back in a string, one watches a gateway for callers over a rate, and one asks how many stretches of compounding rates land exactly on a target.

The IBM angle

On IBM's grader, window bugs are usually at the left edge. Move it one place too late or too early and the samples still pass while a boundary test fails. When the window is measured in seconds, two requests exactly t seconds apart belong in different windows, so your eviction test has to cover that pair. When the window holds a product, you divide out the left value on every overshoot, and a run of ones lets several starts share one product. Write the eviction condition as a single line, then trace the pair that lands exactly on it.

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From 328 IBM reports, forum threads behind logins and write-ups in Chinese.

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