Function mode

packages/db-ivm/src/operators/groupBy.ts:329–368

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

 * @param valueExtractor Function to extract a value from each data entry
 */
export function mode<T>(
  valueExtractor: (value: T) => number = (v) => v as unknown as number,
): AggregateFunction<T, number, Map<number, number>> {
  return {
    preMap: (data: T) => {
      const value = valueExtractor(data)
      const frequencyMap = new Map<number, number>()
      frequencyMap.set(value, 1)
      return frequencyMap
    },
    reduce: (values: Array<[Map<number, number>, number]>) => {
      // Combine all frequency maps
      const combinedMap = new Map<number, number>()

      for (const [frequencyMap, multiplicity] of values) {
        for (const [value, frequencyCount] of frequencyMap.entries()) {
          const currentCount = combinedMap.get(value) || 0
          combinedMap.set(value, currentCount + frequencyCount * multiplicity)
        }
      }

      return combinedMap
    },
    postMap: (result: Map<number, number>) => {
      if (result.size === 0) return 0

      let modeValue = 0
      let maxFrequency = 0

      for (const [value, frequency] of result.entries()) {
        if (frequency > maxFrequency) {
          maxFrequency = frequency
          modeValue = value
        }
      }

      return modeValue
    },
  }
}

export const groupByOperators = {
  sum,