Add Sort Slice In Ascending Or Descending Order as a Go TIL

go/sort-slice-in-ascending-or-descending-order.md

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+# Sort Slice In Ascending Or Descending Order
+
+The [`slices.Sort`](https://pkg.go.dev/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`](https://pkg.go.dev/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`](https://pkg.go.dev/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.
+
+```go
+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`](https://pkg.go.dev/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.