Generics merge sort for ordered slices ([]int) inplace (WIP)

mergesort.go

package mergesort

import (
	"fmt"
	"testing"

	"golang.org/x/exp/constraints"
)

// based on https://github.com/TheAlgorithms/Go/blob/master/sort/mergesort.go

// Merge Perform merge sort on a slice
func Merge[T constraints.Ordered](items []T, left, right int) {

	if (right - left) < 2 {
		return
	}

	var middle = left + (right-left)/2
	Merge(items, left, middle)
	Merge(items, middle, right)
	merge(items, left, middle, right)
}

func merge[T constraints.Ordered](a []T, left, middle, right int) {

	var i = left   // left - middle
	var j = middle // middle - right

	for i < middle && j < right {
		if a[i] <= a[j] {
			i++
		} else {
			a[i], a[j] = a[j], a[i] // swap
			j++
		}
	}

	for i < middle {
		a[i], a[j] = a[j], a[i] // swap
		i++
		j++
	}
	for j < right {
		a[i], a[j] = a[j], a[i] // swap
		i++
		j++
	}

}

func TestMerge(t *testing.T) {

	v := []int{3, 2, 1, 5, 4}

	fmt.Println(v)
	Merge(v, 0, len(v))
	fmt.Println(v)
}