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Kotlin
groupBy and partition
`groupBy` returns a `Map<Key, List<T>>` keyed by what the lambda returns. `partition` is a special-case `groupBy` for booleans — returns `(matching, nonMatching)` Pair.
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SQL
ROW_NUMBER OVER PARTITION
Number rows within a partition — the most common "give me the Nth thing per group" pattern. Pair with a WHERE rn = 1 in an outer query to keep just the top row per group.
PHP
Partition Array Into Two Buckets
Split an array into [matches, non-matches] using a predicate callback. Single pass, preserves order, returns two lists.