package arraysum

// SumWithForLoop uses a for loop to
// calculate the sum of the slice argument.
func SumWithForLoop(numbers []int) int {
	output := 0
	for _, v := range numbers {
		output += v
	}
	return output
}

// SumWithRecursion uses recursion to
// calculate the sum of the slice argument.
func SumWithRecursion(numbers []int) int {
	if len(numbers) == 0 {
		return 0
	}

	return numbers[0] + SumWithRecursion(numbers[1:])
}

// SumWithRecursion uses tail recursion to
// calculate the sum of the slice argument.
func SumWithTailRecursion(numbers []int) int {
	return SumWithSubTailCall(numbers, 0)
}

// SumWithSubTailCall is a sub function of
// SumWithTailRecursion. This function takes the
// first element of the array and adds it to the
// running total. It then removes said element and
// calls itself with the updated slice and the current
// running total.
// Recursion ends when there are no more elements in the
// slice. It returns the total sum back to the SubWithTailRecursion
// function.
func SumWithSubTailCall(numbers []int, runningTotal int) int {
	if len(numbers) == 0 {
		return runningTotal
	}

	runningTotal += numbers[0]
	return SumWithSubTailCall(numbers[1:], runningTotal)
}