util/reflect: import own path based interface lookup

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

util/reflect/path.go

@@ -0,0 +1,257 @@
+package reflect
+
+import (
+	"errors"
+	"fmt"
+	"reflect"
+	"strconv"
+	"strings"
+)
+
+const (
+	SplitToken     = "."
+	IndexCloseChar = "]"
+	IndexOpenChar  = "["
+)
+
+var (
+	ErrMalformedIndex    = errors.New("Malformed index key")
+	ErrInvalidIndexUsage = errors.New("Invalid index key usage")
+	ErrKeyNotFound       = errors.New("Unable to find the key")
+	ErrBadJSONPath       = errors.New("Bad path: must start with $ and have more then 2 chars")
+)
+
+func Lookup(i interface{}, path string) (reflect.Value, error) {
+	if path == "" || path[0:1] != "$" {
+		return reflect.Value{}, ErrBadJSONPath
+	}
+
+	if path == "$" {
+		return reflect.ValueOf(i), nil
+	}
+
+	if len(path) < 2 {
+		return reflect.Value{}, ErrBadJSONPath
+	}
+
+	return lookup(i, strings.Split(path[2:], SplitToken)...)
+}
+
+// Lookup performs a lookup into a value, using a path of keys. The key should
+// match with a Field or a MapIndex. For slice you can use the syntax key[index]
+// to access a specific index. If one key owns to a slice and an index is not
+// specificied the rest of the path will be apllied to evaley value of the
+// slice, and the value will be merged into a slice.
+func lookup(i interface{}, path ...string) (reflect.Value, error) {
+	value := reflect.ValueOf(i)
+	var parent reflect.Value
+	var err error
+
+	for i, part := range path {
+		parent = value
+
+		value, err = getValueByName(value, part)
+		if err == nil {
+			continue
+		}
+
+		if !isAggregable(parent) {
+			break
+		}
+
+		value, err = aggreateAggregableValue(parent, path[i:])
+
+		break
+	}
+
+	return value, err
+}
+
+func getValueByName(v reflect.Value, key string) (reflect.Value, error) {
+	var value reflect.Value
+	var index int
+	var err error
+
+	key, index, err = parseIndex(key)
+	if err != nil {
+		return value, err
+	}
+	switch v.Kind() {
+	case reflect.Ptr, reflect.Interface:
+		return getValueByName(v.Elem(), key)
+	case reflect.Struct:
+		value = v.FieldByName(key)
+	case reflect.Map:
+		kValue := reflect.Indirect(reflect.New(v.Type().Key()))
+		kValue.SetString(key)
+		value = v.MapIndex(kValue)
+	}
+
+	if !value.IsValid() {
+		return reflect.Value{}, ErrKeyNotFound
+	}
+
+	if index != -1 {
+		if value.Type().Kind() != reflect.Slice {
+			return reflect.Value{}, ErrInvalidIndexUsage
+		}
+
+		value = value.Index(index)
+	}
+
+	if value.Kind() == reflect.Ptr || value.Kind() == reflect.Interface {
+		value = value.Elem()
+	}
+
+	return value, nil
+}
+
+func aggreateAggregableValue(v reflect.Value, path []string) (reflect.Value, error) {
+	values := make([]reflect.Value, 0)
+
+	l := v.Len()
+	if l == 0 {
+		ty, ok := lookupType(v.Type(), path...)
+		if !ok {
+			return reflect.Value{}, ErrKeyNotFound
+		}
+		return reflect.MakeSlice(reflect.SliceOf(ty), 0, 0), nil
+	}
+
+	switch v.Kind() {
+	case reflect.Slice, reflect.Map:
+		break
+	default:
+		return reflect.Value{}, fmt.Errorf("unsuported kind for index")
+	}
+
+	index := indexFunction(v)
+	for i := 0; i < l; i++ {
+		value, err := lookup(index(i).Interface(), path...)
+		if err != nil {
+			return reflect.Value{}, err
+		}
+
+		values = append(values, value)
+	}
+
+	return mergeValue(values), nil
+}
+
+func indexFunction(v reflect.Value) func(i int) reflect.Value {
+	switch v.Kind() {
+	case reflect.Slice:
+		return v.Index
+	case reflect.Map:
+		keys := v.MapKeys()
+		return func(i int) reflect.Value {
+			return v.MapIndex(keys[i])
+		}
+	}
+	return func(i int) reflect.Value { return reflect.Value{} }
+}
+
+func mergeValue(values []reflect.Value) reflect.Value {
+	values = removeZeroValues(values)
+	l := len(values)
+	if l == 0 {
+		return reflect.Value{}
+	}
+
+	sample := values[0]
+	mergeable := isMergeable(sample)
+
+	t := sample.Type()
+	if mergeable {
+		t = t.Elem()
+	}
+
+	value := reflect.MakeSlice(reflect.SliceOf(t), 0, 0)
+	for i := 0; i < l; i++ {
+		if !values[i].IsValid() {
+			continue
+		}
+
+		if mergeable {
+			value = reflect.AppendSlice(value, values[i])
+		} else {
+			value = reflect.Append(value, values[i])
+		}
+	}
+
+	return value
+}
+
+func removeZeroValues(values []reflect.Value) []reflect.Value {
+	l := len(values)
+
+	var v []reflect.Value
+	for i := 0; i < l; i++ {
+		if values[i].IsValid() {
+			v = append(v, values[i])
+		}
+	}
+
+	return v
+}
+
+func isAggregable(v reflect.Value) bool {
+	k := v.Kind()
+
+	return k == reflect.Map || k == reflect.Slice
+}
+
+func isMergeable(v reflect.Value) bool {
+	k := v.Kind()
+	return k == reflect.Map || k == reflect.Slice
+}
+
+func hasIndex(s string) bool {
+	return strings.Contains(s, IndexOpenChar)
+}
+
+func parseIndex(s string) (string, int, error) {
+	start := strings.Index(s, IndexOpenChar)
+	end := strings.Index(s, IndexCloseChar)
+
+	if start == -1 && end == -1 {
+		return s, -1, nil
+	}
+
+	if (start != -1 && end == -1) || (start == -1 && end != -1) {
+		return "", -1, ErrMalformedIndex
+	}
+
+	index, err := strconv.Atoi(s[start+1 : end])
+	if err != nil {
+		return "", -1, ErrMalformedIndex
+	}
+
+	return s[:start], index, nil
+}
+
+func lookupType(ty reflect.Type, path ...string) (reflect.Type, bool) {
+	if len(path) == 0 {
+		return ty, true
+	}
+
+	switch ty.Kind() {
+	case reflect.Slice, reflect.Array, reflect.Map:
+		if hasIndex(path[0]) {
+			return lookupType(ty.Elem(), path[1:]...)
+		}
+		// Aggregate.
+		return lookupType(ty.Elem(), path...)
+	case reflect.Ptr:
+		return lookupType(ty.Elem(), path...)
+	case reflect.Interface:
+		// We can't know from here without a value. Let's just return this type.
+		return ty, true
+	case reflect.Struct:
+		f, ok := ty.FieldByName(path[0])
+		if ok {
+			return lookupType(f.Type, path[1:]...)
+		}
+	}
+	return nil, false
+}

util/reflect/path_test.go

@@ -0,0 +1,34 @@
+package reflect
+
+import (
+	"testing"
+)
+
+func TestPath(t *testing.T) {
+
+	type Nested2 struct {
+		Name string
+	}
+	type Nested1 struct {
+		Nested2 Nested2
+	}
+	type Config struct {
+		Nested1 Nested1
+	}
+
+	cfg := &Config{
+		Nested1: Nested1{
+			Nested2: Nested2{
+				Name: "NAME",
+			},
+		},
+	}
+
+	v, err := Lookup(cfg, "$.Nested1.Nested2.Name")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if v.String() != "NAME" {
+		t.Fatalf("lookup returns invalid value: %v", v)
+	}
+}