Add JSON schema support

schema/builder.go

@@ -0,0 +1,58 @@
+package schema
+
+import (
+	"context"
+	"reflect"
+
+	E "github.com/sagernet/sing/common/exceptions"
+)
+
+type Builder interface {
+	Context() context.Context
+	Describe(valueType reflect.Type) (*Node, error)
+	FlattenStruct(node *Node, structType reflect.Type) error
+	Define(name string, build func() (*Node, error)) (*Node, error)
+}
+
+type Describer interface {
+	DescribeSchema(builder Builder) (*Node, error)
+}
+
+type UnionVariant struct {
+	Value        any
+	StructType   reflect.Type
+	TypeOptional bool
+}
+
+func DiscriminatedUnion(builder Builder, discriminatorKey string, discriminatorRequired bool, variants []UnionVariant, buildBase func(variant *Node) error) (*Node, error) {
+	variantNodes := make([]*Node, 0, len(variants))
+	for _, variant := range variants {
+		variantNode := StrictObject()
+		if variant.TypeOptional {
+			stringValue, isString := variant.Value.(string)
+			if !isString {
+				return nil, E.New("optional discriminator requires a string value")
+			}
+			variantNode.Properties.Put(discriminatorKey, StringEnum(stringValue, ""))
+		} else {
+			variantNode.Properties.Put(discriminatorKey, &Node{Const: variant.Value})
+		}
+		if discriminatorRequired || !variant.TypeOptional {
+			variantNode.Required = append(variantNode.Required, discriminatorKey)
+		}
+		if buildBase != nil {
+			err := buildBase(variantNode)
+			if err != nil {
+				return nil, err
+			}
+		}
+		if variant.StructType != nil {
+			err := builder.FlattenStruct(variantNode, variant.StructType)
+			if err != nil {
+				return nil, err
+			}
+		}
+		variantNodes = append(variantNodes, variantNode)
+	}
+	return OneOf(variantNodes...), nil
+}