struct.go

package check

import (
	"encoding/json"
	"fmt"
	"reflect"
)

// ParseStruct validates data against a schema, then maps the validated result
// into a Go struct. The target must be a pointer to a struct.
//
// Field mapping uses the "json" struct tag (or lowercase field name if no tag).
//
// Usage:
//
//	type User struct {
//	    Name  string   `json:"name"`
//	    Email string   `json:"email"`
//	    Age   int      `json:"age"`
//	    Tags  []string `json:"tags"`
//	}
//
//	var user User
//	err := check.ParseStruct(schema, data, &user)
func ParseStruct(schema Schema, data any, target any) error {
	// Validate first
	result, err := schema.Parse(data)
	if err != nil {
		return err
	}

	if result == nil {
		return nil
	}

	// Marshal the validated result to JSON, then unmarshal into the struct.
	// This leverages Go's built-in JSON struct mapping (json tags, type coercion).
	rv := reflect.ValueOf(target)
	if rv.Kind() != reflect.Ptr || rv.IsNil() {
		return fmt.Errorf("target must be a non-nil pointer to a struct")
	}
	if rv.Elem().Kind() != reflect.Struct {
		return fmt.Errorf("target must be a pointer to a struct, got pointer to %s", rv.Elem().Kind())
	}

	b, err := json.Marshal(result)
	if err != nil {
		return fmt.Errorf("internal: failed to marshal validated result: %w", err)
	}

	if err := json.Unmarshal(b, target); err != nil {
		return fmt.Errorf("failed to map result to struct: %w", err)
	}

	return nil
}