Sort Slice In Ascending Or Descending Order

The slices.Sort function defaults to sorting a slice in ascending order. If we want to control the sort order, we have to do a little more work. We can reach for the slices.SortFunc function. This allows us to define a sort function and in that function we can control whether the sort order is ascending or descending.

Here I've defined SortItems which takes a list of items constrained by the cmp.Ordered interface (so things like int, string, uint64, etc.). It takes a direction (ASC or DESC) as a second argument. It does the directional sort based on that second argument.

import (
    "cmp"
    "fmt"
    "slices"
)

type Direction int

const (
    ASC Direction = iota
    DESC
)

func SortItems[T cmp.Ordered](items []T, dir Direction) {
    slices.SortFunc(items, func(i, j T) int {
        if dir == ASC {
            return cmp.Compare(i, j)
        } else if dir == DESC {
            return cmp.Compare(j, i)
        } else {
            panic(fmt.Sprintf("Unrecognized sort direction: %d", dir))
        }
    })
}

// items := []int{3,2,8,1}
// SortItems(items, ASC)
// // items => [1,2,3,8]
// SortItems(items, DESC)
// // items => [8,3,2,1]

Because slices.SortFunc expects a negative value, zero, or positive value to determine the sort order, we use cmp.Compare which returns those kinds of values. For ascending, we compare i to j. For descending, we swap them, comparing j to i to get the reverse sort order.