gotilty / gotil

package internal

import (

	"reflect"

	"github.com/gotilty/gotil/internal/errs"

)

// Reduce iterates given collection and returns the accumulated result of running each element

// the last param is initial value of accumulator.

// 	data := []int{5, 10}

// 	result, _ := Reduce(data, func(accumulator, val interface{}, i int) interface{} {

// 		return accumulator.(int) + val.(int)

// 	}, 0)

// 	// Output: 15

func Reduce(a interface{}, f func(accumulator interface{}, val interface{}, i int) interface{}, accumulator interface{}) (interface{}, error) {

	val := reflect.ValueOf(a)

	switch val.Kind() {

	case reflect.Slice, reflect.Array:

		length := val.Len()

		index := -1

		newSlice := copySlice(a)

		if IsAssigned(accumulator) && length > 0 {

			index++

			accumulator = newSlice.Index(index).Interface()

		}

		index++

		for index < length {

			rowVal := val.Index(index)

			accumulator = f(accumulator, rowVal.Interface(), index)

			index++

		}

		return accumulator, nil

	}

	return nil, errs.NewUnsupportedTypeError(val.Kind().String())

}