Golang program for implementation of Merge Sort

package main

import (
	"fmt"
	"math/rand"
	"time"
)

func main() {

	slice := generateSlice(20)
	fmt.Println("\n--- Unsorted --- \n\n", slice)
	fmt.Println("\n--- Sorted ---\n\n", mergeSort(slice), "\n")
}

// Generates a slice of size, size filled with random numbers
func generateSlice(size int) []int {

	slice := make([]int, size, size)
	rand.Seed(time.Now().UnixNano())
	for i := 0; i < size; i++ {
		slice[i] = rand.Intn(999) - rand.Intn(999)
	}
	return slice
}
 
func mergeSort(items []int) []int {
    var num = len(items)
     
    if num == 1 {
        return items
    }
     
    middle := int(num / 2)
    var (
        left = make([]int, middle)
        right = make([]int, num-middle)
    )
    for i := 0; i < num; i++ {
        if i < middle {
            left[i] = items[i]
        } else {
            right[i-middle] = items[i]
        }
    }
     
    return merge(mergeSort(left), mergeSort(right))
}
 
func merge(left, right []int) (result []int) {
    result = make([]int, len(left) + len(right))
     
    i := 0
    for len(left) > 0 && len(right) > 0 {
        if left[0] < right[0] {
            result[i] = left[0]
            left = left[1:]
        } else {
            result[i] = right[0]
            right = right[1:]
        }
        i++
    }
     
    for j := 0; j < len(left); j++ {
        result[i] = left[j]
        i++
    }
    for j := 0; j < len(right); j++ {
        result[i] = right[j]
        i++
    }
     
    return
}