Algorithm of mergesort

main.go

package main

import "fmt"

func main() {
	var nums []int = []int{10, 7, 8, 1, 4, 3}
	topDownMergesort(nums)
	fmt.Println(nums)
}

func topDownMergesort(a []int) {
	var b []int = make([]int, len(a))
	copy(b, a)
	topDownSplitMerge(b, 0, len(b), a)
}

func topDownSplitMerge(b []int, idx1 int, idx2 int, a []int) {
	if idx2-idx1 <= 1 {
		return
	}

	var middle int = (idx2 + idx1) / 2

	topDownSplitMerge(a, idx1, middle, b)
	topDownSplitMerge(a, middle, idx2, b)
	topDownMerge(b, idx1, middle, idx2, a)
}

func topDownMerge(a []int, idx1 int, middle int, idx2 int, b []int) {
	i := idx1
	j := middle

	for k := idx1; k < idx2; k++ {
		if i < middle && (j >= idx2 || a[i] <= b[j]) {
			b[k] = a[i]
			i++
		} else {
			b[k] = a[j]
			j++
		}
	}
}

https://en.wikipedia.org/wiki/Merge_sort#Top-down_implementation_using_lists