FlatMap Over Map Reduce

Reports the .map(…​).reduce([], +) chain. .flatMap(…​) is equivalent and runs in a single pass.

.map(…​).reduce([], +) builds an intermediate , then concatenates the inner arrays one at a time. Array concatenation copies both operands, so the whole pipeline is O(n²) in the total element count. flatMap walks the input once and appends to a single output array — O(n).

Replace the chain with flatMap:

The rule only matches the exact (empty-array, +) reduce shape, so it leaves alone reductions into scalars (.reduce(0, +)) and non-empty initial values.