Support for const type validation

pkg/generator/schema_generator.go

@@ -376,6 +376,7 @@ func (g *schemaGenerator) generateDeclaredType(t *schemas.Type, scope nameScope)
 	return &codegen.NamedType{Decl: &decl}, nil
 }
 
+//nolint:gocyclo // todo: reduce cyclomatic complexity
 func (g *schemaGenerator) structFieldValidators(
 	validators []validator,
 	f codegen.StructField,
@@ -393,12 +394,23 @@ func (g *schemaGenerator) structFieldValidators(
 		validators = g.structFieldValidators(validators, f, v.Type, v.IsNillable())
 
 	case codegen.PrimitiveType:
-		if v.Type == schemas.TypeNameString {
+		switch {
+		case v.Type == schemas.TypeNameString:
 			hasPattern := len(f.SchemaType.Pattern) != 0
-			if f.SchemaType.MinLength != 0 || f.SchemaType.MaxLength != 0 || hasPattern {
+			if f.SchemaType.MinLength != 0 || f.SchemaType.MaxLength != 0 || hasPattern || f.SchemaType.Const != nil {
 				// Double escape the escape characters so we don't effectively parse the escapes within the value.
 				escapedPattern := f.SchemaType.Pattern
 
+				var constVal *string
+
+				if f.SchemaType.Const != nil {
+					if s, ok := f.SchemaType.Const.(string); ok {
+						constVal = &s
+					} else {
+						g.warner(fmt.Sprintf("Ignoring non string const value: %v", f.SchemaType.Const))
+					}
+				}
+
 				replaceJSONCharactersBy := []string{"\\b", "\\f", "\\n", "\\r", "\\t"}
 
 				replaceJSONCharacters := []string{"\b", "\f", "\n", "\r", "\t"}
@@ -413,19 +425,22 @@ func (g *schemaGenerator) structFieldValidators(
 					minLength:  f.SchemaType.MinLength,
 					maxLength:  f.SchemaType.MaxLength,
 					pattern:    escapedPattern,
+					constVal:   constVal,
 					isNillable: isNillable,
 				})
 			}
 
 			if hasPattern {
 				g.output.file.Package.AddImport("regexp", "")
 			}
-		} else if strings.Contains(v.Type, "int") || v.Type == float64Type {
+
+		case strings.Contains(v.Type, "int") || v.Type == float64Type:
 			if f.SchemaType.MultipleOf != nil ||
 				f.SchemaType.Maximum != nil ||
 				f.SchemaType.ExclusiveMaximum != nil ||
 				f.SchemaType.Minimum != nil ||
-				f.SchemaType.ExclusiveMinimum != nil {
+				f.SchemaType.ExclusiveMinimum != nil ||
+				f.SchemaType.Const != nil {
 				validators = append(validators, &numericValidator{
 					jsonName:         f.JSONName,
 					fieldName:        f.Name,
@@ -435,13 +450,34 @@ func (g *schemaGenerator) structFieldValidators(
 					exclusiveMaximum: f.SchemaType.ExclusiveMaximum,
 					minimum:          f.SchemaType.Minimum,
 					exclusiveMinimum: f.SchemaType.ExclusiveMinimum,
+					constVal:         f.SchemaType.Const,
 					roundToInt:       strings.Contains(v.Type, "int"),
 				})
 			}
 
 			if f.SchemaType.MultipleOf != nil && v.Type == float64Type {
 				g.output.file.Package.AddImport("math", "")
 			}
+
+		case v.Type == "bool":
+			if f.SchemaType.Const != nil {
+				var constVal *bool
+
+				if f.SchemaType.Const != nil {
+					if b, ok := f.SchemaType.Const.(bool); ok {
+						constVal = &b
+					} else {
+						g.warner(fmt.Sprintf("Ignoring non boolean const value: %v", f.SchemaType.Const))
+					}
+				}
+
+				validators = append(validators, &booleanValidator{
+					jsonName:   f.JSONName,
+					fieldName:  f.Name,
+					isNillable: isNillable,
+					constVal:   constVal,
+				})
+			}
 		}
 
 	case *codegen.ArrayType:

pkg/generator/validator.go

@@ -356,6 +356,7 @@ type stringValidator struct {
 	maxLength  int
 	isNillable bool
 	pattern    string
+	constVal   *string
 }
 
 func (v *stringValidator) generate(out *codegen.Emitter, format string) error {
@@ -393,6 +394,14 @@ func (v *stringValidator) generate(out *codegen.Emitter, format string) error {
 		}
 	}
 
+	if v.constVal != nil {
+		out.Printlnf(`if %s%s%s != "%s" {`, checkPointer, pointerPrefix, value, *v.constVal)
+		out.Indent(1)
+		out.Printlnf(`return fmt.Errorf("field %%s: must be equal to %%s", "%s", "%s")`, fieldName, *v.constVal)
+		out.Indent(-1)
+		out.Printlnf("}")
+	}
+
 	if v.minLength == 0 && v.maxLength == 0 {
 		return nil
 	}
@@ -432,6 +441,7 @@ type numericValidator struct {
 	exclusiveMaximum *any
 	minimum          *float64
 	exclusiveMinimum *any
+	constVal         any
 	roundToInt       bool
 }
 
@@ -445,6 +455,14 @@ func (v *numericValidator) generate(out *codegen.Emitter, format string) error {
 		pointerPrefix = "*"
 	}
 
+	if v.constVal != nil {
+		out.Printlnf(`if %s%s%s != %v {`, checkPointer, pointerPrefix, value, v.constVal)
+		out.Indent(1)
+		out.Printlnf(`return fmt.Errorf("field %%s: must be equal to %%v", "%s", %v)`, v.jsonName, v.constVal)
+		out.Indent(-1)
+		out.Printlnf("}")
+	}
+
 	if v.multipleOf != nil {
 		if v.roundToInt {
 			out.Printlnf(`if %s %s%s %% %v != 0 {`, checkPointer, pointerPrefix, value, v.valueOf(*v.multipleOf))
@@ -527,6 +545,42 @@ func (v *numericValidator) valueOf(val float64) any {
 	return val
 }
 
+type booleanValidator struct {
+	jsonName   string
+	fieldName  string
+	isNillable bool
+	constVal   *bool
+}
+
+func (v *booleanValidator) generate(out *codegen.Emitter, unmarshalTemplate string) error {
+	value := getPlainName(v.fieldName)
+	fieldName := v.jsonName
+	checkPointer := ""
+	pointerPrefix := ""
+
+	if v.isNillable {
+		checkPointer = fmt.Sprintf("%s != nil && ", value)
+		pointerPrefix = "*"
+	}
+
+	if v.constVal != nil {
+		out.Printlnf(`if %s%s%s != %t {`, checkPointer, pointerPrefix, value, *v.constVal)
+		out.Indent(1)
+		out.Printlnf(`return fmt.Errorf("field %%s: must be equal to %%t", "%s", %t)`, fieldName, *v.constVal)
+		out.Indent(-1)
+		out.Printlnf("}")
+	}
+
+	return nil
+}
+
+func (v *booleanValidator) desc() *validatorDesc {
+	return &validatorDesc{
+		hasError:            true,
+		beforeJSONUnmarshal: false,
+	}
+}
+
 func getPlainName(fieldName string) string {
 	if fieldName == "" {
 		return varNamePlainStruct

pkg/schemas/model.go

@@ -177,6 +177,7 @@ type Type struct {
 	AdditionalProperties *Type            `json:"additionalProperties,omitempty"` // Section 5.18.
 	Enum                 []interface{}    `json:"enum,omitempty"`                 // Section 5.20.
 	Type                 TypeList         `json:"type,omitempty"`                 // Section 5.21.
+	Const                interface{}      `json:"const,omitempty"`
 	// RFC draft-bhutton-json-schema-01, section 10.
 	AllOf []*Type `json:"allOf,omitempty"` // Section 10.2.1.1.
 	AnyOf []*Type `json:"anyOf,omitempty"` // Section 10.2.1.2.