Function count

packages/db-ivm/src/operators/groupBy.ts:169–183

(
  valueExtractor: (value: T) => any = (v) => v,
)

 * Creates a count aggregate function
 */
export function count<T>(
  valueExtractor: (value: T) => any = (v) => v,
): AggregateFunction<T, number, number> {
  return {
    // Count only not-null values (the `== null` comparison gives true for both null and undefined)
    preMap: (data: T) => (valueExtractor(data) == null ? 0 : 1),
    reduce: (values: Array<[number, number]>) => {
      let totalCount = 0
      for (const [nullMultiplier, multiplicity] of values) {
        totalCount += nullMultiplier * multiplicity
      }
      return totalCount
    },
  }
}

/**
 * Creates an average aggregate function