nested.go

package nestedslices

type out struct {
	data []int
}

func (o *out) Append(v int) {
	o.data = append(o.data, v)
}

func FlatSlice(in []interface{}) []int {
	o := out{}
	flatSlice(in, &o)
	return o.data
}

func flatSlice(in []interface{}, o *out) {
	for _, val := range in {
		v := val
		switch vt := v.(type) {
		case int:
			o.Append(vt)
		case []interface{}:
			flatSlice(vt, o)
		}
	}
}

nested_test.go

package nestedslices

import (
	"reflect"
	"testing"
)

func TestFlatSlice(t *testing.T) {
	tests := []struct {
		name string
		in   []interface{}
		want []int
	}{
		{"basic", []interface{}{1, 2, 3, 4}, []int{1, 2, 3, 4}},
		{"basic", []interface{}{[]interface{}{1, 2, 3}, 4}, []int{1, 2, 3, 4}},
		{"basic", []interface{}{[]interface{}{1, 2, []interface{}{3}}, 4}, []int{1, 2, 3, 4}},
		{"basic", []interface{}{[]interface{}{1, 2, []interface{}{3, 4, 5}}, 4}, []int{1, 2, 3, 4, 5, 4}},
		{"basic", []interface{}{[]interface{}{1, 2, []interface{}{[]interface{}{3, 3, 3}, 4, 5}}, 4}, []int{1, 2, 3, 3, 3, 4, 5, 4}},
	}
	for _, tt := range tests {
		t.Run(tt.name, func(t *testing.T) {
			if got := FlatSlice(tt.in); !reflect.DeepEqual(got, tt.want) {
				t.Errorf("FlatSlice() = %v, want %v", got, tt.want)
			}
		})
	}
}