How do you sort custom types using sort.Interface in Go?

sort.Interface is one of Go's classic interface examples. Any type that implements three methods can be sorted by sort.Sort:

// sort.Interface definition:
type Interface interface {
    Len()  int
    Less(i, j int) bool
    Swap(i, j int)
}

// Custom type implementing sort.Interface
type Person struct{ Name string; Age int }
type ByAge []Person

func (b ByAge) Len()  int           { return len(b) }
func (b ByAge) Less(i, j int) bool  { return b[i].Age < b[j].Age }
func (b ByAge) Swap(i, j int)       { b[i], b[j] = b[j], b[i] }

people := []Person{
    {"Alice", 30}, {"Bob", 25}, {"Carol", 35},
}
sort.Sort(ByAge(people))  // convert slice to ByAge to satisfy interface
fmt.Println(people)       // [{Bob 25} {Alice 30} {Carol 35}]

// Modern approach: sort.Slice (no type definition needed)
sort.Slice(people, func(i, j int) bool {
    return people[i].Name < people[j].Name // sort by Name
})

// sort.SliceStable — preserves order of equal elements
sort.SliceStable(people, func(i, j int) bool {
    return people[i].Age < people[j].Age
})

// Reverse sorting: sort.Reverse wraps any sort.Interface
sort.Sort(sort.Reverse(ByAge(people)))

// slices.SortFunc (Go 1.21) — generic, type-safe, faster
import "slices"
slices.SortFunc(people, func(a, b Person) int {
    return strings.Compare(a.Name, b.Name) // -1, 0, +1
})

The sort.Interface example illustrates how interfaces decouple the sorting algorithm from the data type. sort.Sort knows nothing about Person — it only calls Len, Less, and Swap. This is the Strategy pattern in Go: the algorithm is the same, the comparison strategy is injected via the interface.

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