Function median

packages/db-ivm/src/operators/groupBy.ts:281–322

(
  valueExtractor: (value: T) => number = (v) => v as unknown as number,
)

 * @param valueExtractor Function to extract a numeric value from each data entry
 */
export function median<T>(
  valueExtractor: (value: T) => number = (v) => v as unknown as number,
): AggregateFunction<T, number, Array<number>> {
  return {
    preMap: (data: T) => [valueExtractor(data)],
    reduce: (values: Array<[Array<number>, number]>) => {
      // Flatten all values, taking multiplicity into account
      const allValues: Array<number> = []
      for (const [valueArray, multiplicity] of values) {
        for (const value of valueArray) {
          // Add each value multiple times based on multiplicity
          for (let i = 0; i < multiplicity; i++) {
            allValues.push(value)
          }
        }
      }

      // Return empty array if no values
      if (allValues.length === 0) {
        return []
      }

      // Sort values
      allValues.sort((a, b) => a - b)

      return allValues
    },
    postMap: (result: Array<number>) => {
      if (result.length === 0) return 0

      const mid = Math.floor(result.length / 2)

      // If even number of values, average the two middle values
      if (result.length % 2 === 0) {
        return (result[mid - 1]! + result[mid]!) / 2
      }

      // If odd number of values, return the middle value
      return result[mid]!
    },
  }
}

/**
 * Creates a mode aggregate function that computes the most frequent value in a group