Simple Aggregate Function

1import { type Reader } from "./core.js";
2
3/**
4 * `SimpleAggregateFunction(func, T)` is TRANSPARENT in RowBinary: the column
5 * already holds a finished value of the underlying type `T` (the partial
6 * aggregate of a "simple" function — sum / min / max / groupArrayArray / … — is
7 * just a value of `T`), so it is encoded byte-for-byte the same as `T`. Decode
8 * the inner `T` directly.
9 *
10 * Do NOT confuse it with `AggregateFunction(func, T)`, whose value is an opaque
11 * serialized aggregation STATE with a function-specific binary layout — see
12 * `./aggregateFunction.js`.
13 *
14 * Identity combinator, documentation only:
15 *
16 *   readSimpleAggregateFunction(readUInt64) === readUInt64
17 */
18export const readSimpleAggregateFunction = <T>(
19  readValue: Reader<T>,
20): Reader<T> => readValue;